Structure, Function, and Immunology of The Skin

STRUCTURE, FUNCTION, AND IMMUNOLOGY OF THE SKIN

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The skin is largest organ of the body in humans. It is the outer cover that humans present to the environment. As such, it has unique functions of protection from and perception of that environment. Its role in how others perceive an individual is also of great importance. It is divided into two inseparable layers, the epidermis and the dermis, and contains a multitude of cell types and supporting structures, all more or less essential to its function.

Its protective role resides in its ability to maintain homeostasis with retention of water vapor and modulation of temperature, to exclude noxious organisms and chemicals, to generate immune responses to those agents that are allowed to enter, and to produce pigment and keratin to protect against ubiquitous solar damage.

The function of the skin as an immune organ is one that has only been recently realized. This function is important in homeostasis and likewise in the production of a multitude of immunologically mediated disease states.

The epidermis is a self-replicating structure that renews itself about every 30 days. The keratinocyte, an ectodermally derived cell, constitutes about 80% of the cells in the epidermis. Other cells in the epidermis are immigrant cells and include Langerhans' cells, Merkle cells and melanocytes. The epidermis is composed chiefly of two layers, the stratum malpighii and the stratum corneum. The stratum malpighii is further divided into three layers: (1) a single layer of cells termed the basal layer (stratum germinativum), from which keratinocytes replicate, (2) the spinous layer, which is a layer two of three cells thick, and (3) the granular layer, which is also two or three cells thick. The stratum corneum is the outermost layer of the skin and is composed of corneocytes, which are basically anucleate, nonviable keratinocytes that are organized into geometric stacks embedded in a lipid-rich intercellular matrix. It is the stratum corneum that provides the skin's important barrier functions, such as protecting the body against water loss and against entrance of microorganisms, toxins, and allergens.

The keratinocyte is a nucleated epithelial cell composed of a cytoskeleton of keratins, which are structural proteins essential in maintaining the integrity of the epidermis. Thirty different types of keratins have been discovered in human cells, with molecular weights ranging from 40 to 70 kD. Keratins are expressed in all epithelial cells and are differentially expressed in the skin, hair, nail, mucosa, and gastrointestinal tract. Keratin proteins arrange into polypeptides called "filaments". The filaments are 10 nm in diameter and belong to a larger family of intermediate filaments. Their diameter is greater than actin microfilaments and smaller than microtubules. In epithelial cells such as keratinocytes, keratin filaments extend in a weblike pattern from a perinuclear ring, course through the cytoplasm of the cell, and attach to the cell membrane by inserting into desmosomal plates that anchor adjacent keratinocytes. They provide a cytoskeletal or supporting structure to the cell. Keratins are known to be obligate heteropolymers, which means that two different types of keratins similar in molecular weight, one acidic (type 1) and one basic (type 2), must be co-expressed to form filament polypeptides. Additionally, as keratinocytes migrate from the basal layer to the stratum corneum, they move through different states of differentiation that are characterized by the expression of biochemically distinct but morphologically similar keratin proteins. Thus basala cells are characterized by the paired expression of the lower-molecular-weight keratins 5 and 14, whereas spinous, granular, and cornified cells are characterized by the paired expression of the higher-weight keratins 1 and 10. Genetic defects in specific keratins have been identified in clinically distinct disorders of keratinization. Within the past few years, specific mutations have been found in keratin genes in genodermatoses such as epidermolysis bullosa simplex, epidermolytic hyperkeratosis, epidermal nevi of the epidermolytic hyperkeratotic subtype, palmoplantar keratoderma, and pachyonychia congenita.

Keratinocytes of the basal layer are attached to the basement membrane zone by hemidesmosomes and to each other by desmosomes. Stem cells are replicating cells of the basal layer that replenish keratinocytes, which ultimately are sloughed from the stratum corneum. They represent only a small portion of cells of the basal layer and can be identified functionally because they retain ³H-thymidine longer than other basal cells do. The basal cell migrates to upper layers of the epidermis, changing its state of differentiation as it moves through the spinous, granular, and corneal layers. It takes 12 to 14 days for a cell of the basal layer to migrate to the stratum corneum and another 14 days for a cornified cell of the stratum corneum to desquamate. Certain pathologic states such as wound healing and psoriasis may alter differentiation and decrease transit time. Stimulatory growth factors include keratinocyte growth factor, epidermal growth factor, and transforming growth factor-alpha, which is overexpressed in psoriasis. Inhibitory growth factors such as transforming growth factor-beta 1 decrease proliferation. Factors that influence both proliferation and differentiation include keratinocyte-derived cytokines (interleukins) and vitamin A and its related retinoid compounds. Retinoic acid stimulates proliferation of keratinocytes at low concentrations but is inhibitory at higher concentrations.

The spinous layer is so termed because early skin biologists noted that cells of this layer have a prickly or spiny appearance under the microscope, which is attributable to numerous desmosomes around the cell periphery. Desmosomes contain the proteins desmocollin, desmoplakin, and desmoglein, the last of which is in the family of cadherin, or calcium-dependent, cell adhesion molecules. Gap junctions in the desmosome allow for intercellular communication between keratinocytes, which influence cell metabolism, growth, and differentiation. Clinically distinct blistering disorders of the skin are associated with genetic abnormalities or antibodies against desmosomal proteins. For example, pemphigus vulgaris is caused by a circulating antibody to desmoglein 3, a 130-kD protein of the desmosome, whereas pemphigus foliaceus is caused by an antibody to desmoglein 1, a 160-kD protein of the desmosome. Bullous pemphigolid is caused by antibodies directed against bullous pemphigoid antigens 1 and 2, which are well characterized components of the hemidesmosomes of the basal layer.

As keratinocytes enter the granula layer, major differentiation events become evident and markers of differentiation are expressed. The first differentiation event is keratinization. Keratins 1 and 10, which initially commence synthesis in the spinous layer, become the predominant keratins produced and organize around a scaffolding of keratins 5 and 14 produced in the basal layer. This augmented cytoskeleton imparts great structural stability to the keratinocyte.

The second major differentiation-associated event is keratohyalin deposition. The most prominent morphologic feature of keratinocytes of the granular layer is protein-rich granules called keratohyalin granules. These granules contain profilaggrin, which is a precursor molecule to filaggrin. Filaggrin, or filament-aggregating protein, facilitates the alignment and aggregation of keratin filaments into tightly packed bundles in the cornified cell. Filaggrin hydrolysis is a tightly regulated process by which profilaggrin is dephosphorylated and proteolytically cleaved first into oligomeric, dimeric, and then monomeric subunits and ultimately into amino acids in the outer layers of the stratum corneum. Free amino acids contribute to the water-holding properties of the stratum corneum. Defects in profilaggrin synthesis have been detected in dermatologic diseases characterized by decreased amounts of water in the stratum corneum, which clinically manifests as crackling, scaling and flaking of the skin. Ichthyosis vulgaris is a fairly common autosomal dominant disorder in which profilaggrin synthesis is decreased or absent in the epidermis. This disorder presents early in life and is characterized by fine white epidermal scales associated with chapping and hyperlinearity of the palms, keratosis pilaris and atopy.

The third differentiation event is formation of the cornified envelope, sometimes referred to as cell cornification. The proteins involucrin, keratolinin, loricrin, and pancornulins are precursor proteins synthesized in the granular layer. They ultimately become cross-linked by the enzyme transglutaminase. This results in formation of a rigid structural ectoskeleton that serves as a scaffolding for insertion of keratin filaments and a barrier to the external milieu. Two disorders demonstrate the importance of the cornified envelope in epidermal homeostasis. Vohwinkel syndrome is an autosomal dominant disorder characterized by palmoplantar keratoderma, pseudoainhum (keratotic constriction bands around digits that leads to autoamputation), star-shaped, salmon-colored keratotic plaques on the dorsum of the hands, and occasional generalized ichthyosis. A mutation in the loricrin gene has recently been identified in this disorder. A second disorder, lamellar ichthyosis, is an autosomal dominant disorder associated with a collodion membrane at birth (a platelike armor) and large platelike scales after infancy. Abnormalities in this disorder are caused by genetic abnormalities in translugaminase.

Lastly, lamellar granules, in contrast to the protein-rich keratohyalin granules, form in the granular layer and ultimately contribute their lipid contents to intercellular domains of the stratum corneum. Lamellar granules are made of disks of lipid bilayers consisting of glycosphingolipids, phospholipids, and cholesterol, as well as a variety of hydrolases and glycosidases that catalyze the transformation of glycosphingolipids and phospholipids into ceramides and free fatty acids in the stratum corneum. In the process of differentiation and cornification, these lamellar granules fuse with the plasma membrane and extrude their contents into the extracellular space. The lipid contents of the lamellar granules act as a type of intercellular glue that constitutes the lipid permeablilty barrier of the stratum corneum.

As cells progress and form the granular layer to the stratum corneum, keratinocytes lose their nuclei through an apoptotic mechanism initiated by the activation of a calcium-dependent endonuclease. Further degradation is carried out nuclei and organelles by hydrolytic enzymes. Keratin filaments become more loosely organized, and such looseness enchances the water binding capacity of the outer stratum corneum. Desquamation is secondary to enzymatic changes in cornified cell surface lipids and proteolytic degradation of desmosomes. It is estmated that in normal skin 0,5 to 1 g of horny material is lost daily.

The color of human skin in its normal state and in disease is produced by four biochromes. Two of them are in the dermis, oxygenated (red) and deoxygenated (purple) hemoglobin, and two of them are in the epidermis, carotenoids (yellow), and melanin (brown). The latter is the most important determinant of skin color. It is produced in the melanocyte and transferred to keratinocytes through melanocytic dendrites. The melanocyte is a cell of neural crest origin that migrates into the skin and resides in the suprabasillar area of the epidermis, where it appears as a dendritic "clear" cell on routine histologic examination. The production and distribution of melanin by melanocytes is determined by genetic factors and by response to environmental factors, particularly solar radiation.

Melanocytes are exocrine cells that have arborizing nervelike dendrites. Derived from the neural crest they migrate to various body sites, including the eye, the ear, and, most notably, the skin.

The number of melanocytes in the skin varies among individuals and among body sites. Generally, homever, racial differences in skin color are not attributable to differences in the number of melanocytes in the skin. There are roughly 2000 melanocytes per square millimeter in facial and forearm skin and 1000 per square millimeter in other body sites. The number of melanocytes in the skin decreases with aging, a reduction of 8% to 10% per decade of life. The loss of melanocytes in the hair follicle with aging is responsible for the graying of hair with increase in age. Generally speaking, the number of melanocytes is greater in chronically sun-exposed skin compared with nearby covered skin sites.

The major function of melanocytes is the production of melanin. This process occurs within specialized organelles called melanosomes. Melanosomes are spherical or ellipsoidal structures containing tyrosinase, the enzyme responsible for most of the process of production of melanin, and melanosomal proteins. The melanosome undergoes four developmental stages as it becomes melanized: in stage I the melanosome is a vacuole containing an amorphous proteinaceous material; in stage II organized lamellae and microvesicles are apparent; in stage III tyrosinase becomes activated and melanin is deposited on the lamellae; and finally in stage IV the melanosome becomes fully melanized and electron dense.

In the process of melanization for normal skin color (constitutive) or after solar exposure (facultative) the stage IV melanosomes move from the perikaryon into the dendritic processes and are transferred to and incorporated into keratinocytes. There melanosomes are gradually degraded and melanin is finally shed along with keratinocytes in the stratum corneum.

The darker skin of certain ethnic groups like African Americans compared with the lighter skin of white European Americans has more numerous melanosomes, larger melanosomes, and more stage IV melanosomes. The melanosomes in darker-skinned individuals are singly dispersed in keratinocytes, whereas they are found in complexes of two or three in lighter-skinned individuals. Overall, darker skin has higher levels of melanin in all layers of the epidermis than other shades of skin have.

Constitutive pigmentation of the skin, that present without the effect of solar radiation, is dependent on complex genetic factors. In mice over 150 genes at 60 loci affect this process.

Facultative pigmentation is dependent on light, hormones, and the genetically determined ability to tan.

Although exogenous alpha-melanocyte-stimulating hormone (a-MSH) and b-MSH have been reported to cause increased pigmentation in humans, only ACTH and b-lipotropin are produced in sufficient quantities to affect pigmentation in normal adults. And even they are not believed to play a role in normal constitutive pigmentation of human adults. Estrogen and progesterone, along with a-MSH from fetal sources, may be responsible for the pigmentary changes that occur during pregnancy.

Melasma, the patchy hyperpigmentation that occurs in women particularly during pregnancy ("mask of pregnancy") and while using oral contraceptives, is believed to be caused by the combined action of sex hormones and ultraviolet radiation.

The brown and black color of skin and the brown, black, yellow and red colors of the hair of humans are caused by melanins. The primary melanin in skin is eumelanin, a brown-black insoluble protein made up of indole-5,6-quinone units derived from tyrosine. Pheomelanin found primarily in hair and a to a far less degree in skin is a yellow-red alkali-soluble protein made up of 5-S-cysteinyldopa units also derived from tyrosine.

Tyrosinase is the primary enzyme responsible for melanin synthesis. It is produced in the Golgi apparatus and transported to the melanosome where it catalyzes the conversion of thyrosine to dopa and the conversion of dopa to dopaquinone. It is a bifunctional, copper-containing monoozygenase.

Dopaquinone is converted nonenzymatically to dopachrome. Dopachrome is then converted to 5,6-dihydroxyindole-2-caroxylic acid, which is polymerized into eumelanin. The final steps in the pathway are catalyzed by dopachrome tautomerase, metal ions, and tyrosinase.

In contrast, pheomelanins are produced by the addition of cysteine to dopaquinone to form cysdopa, cysdopaquinones, benzothiazinyl alanines, and pheomelanins.

Tanning is the facultative pigmentation that occurs in human skin when it is exposed to nonionizing radiation. The ability to exhibit this response is genetically determined. Very light complexioned white European Americans have little or no capacity to tan, whereas such individuals with constitutively darker complexions have a greater capacity to do so.

Tanning is believed to be a protective device. Sun exposure induces melanization of the skin, which protects it from the next exposure because melanin is a good, broad-spectrum absorber of ultraviolet radiation.

There are two basic types of tanning, and both may occur with the same exposure. Immediate pigment darkening is the development of color in the skin while the skin is being exposed. It is produced primarily by long-wave ultraviolet radiation — ultraviolet A (UVA, 320 to 400 nm). It is short-lived, fading almost immediately, and is attributable to photo-oxidation of preformed melanin. It is not associated with increased activity of tyrosinase, increased numbers of melanosomes, or melanocytic mitosis.

In contrast, delayed tanning is induced primarily by midwave ultraviolet radiation — ultraviolet B (UVB, 290 to 320 nm) —and to a lesser extent by UVA radiation. It occurs gradually, 2 or 3 days after exposure and lasts for weeks. It is caused by UV-induced mitosis of melanocytes, increased tyrosinase activity, increased numbers of melanosomes, and increased transfer of melanosomes to keratinocytes.

Basement membranes (BM) are highly specialized structures that lie at the interface of cells and their surrounding connective tissue. The cutaneous BM zone is located between the epidermis and dermis (dermoepidermal junction, DEJ). The DEJ separates these two distinct compartments of the skin and provides adhesion between them, thus affection the overall intergity of the skin. The DEJ excludes the transit of molecules based on size and charge, but it permits passage of migrating and invading cells under normal (i. e., melanocytes, Langerhans' cells) or pathologic (i. e., lymphocytes, tumor cells) conditions. It also supports the epidermis and influences the behavior of keratinocytes by modulating cell polarity, proliferation, migration, and differentiation. In addition to these functions, its role is important during ontogeny, wound repair, and remodelling of the tissue.

Ultrastructurally DEJ is divided into four zones: (1) the cell membrane of the basal keratinocyte, (2) the lamina lucida, a clear region seen on electron microscopy (EM), (3) the lamina densa, an electron-dense area seen on EM, and (4) the subbasal lamina.

Each of these zones consists of ubiquitous BM components. These include hemidesmosome, laminin, nidogen in the upper regions, type IV collagen and heparan sulfate proteoglycan predominantly in the lamina densa, and anchoring fibrils (type VII collagen) in the subbasal lamina densa.

Several of the molecules of BMs (type IV collagen, laminin, and heparan sulfate proteoglycan) are common to all BMs. Others are unique and tissue-specific.

In the skin both keratinocytes and fibroblasts contribute molecules to the BM zone. Hemidesmosomal plaque proteins, type IV collagen, type V collagen, laminin, heparan sulfate proteoglycan, nidogen (entactin), and type VII collagen are products of the basal keratinocyte. Fibroblasts of the papillary dermis contribute to type V collagen and fibronectin to the lamina densa and types I, III, and V collagens to the subbasal lamina densa.

Athough the exact function of many of these components has not yet been fully elucidated, abnormalities of several of these molecules have been associated with various cutaneous disorders.

The skin diseases that result from disruption of the structures in the DEJ clinically present with skin fragility and blistering. The prototype of these skin diseases is epidermolysis bullosa (EB), a group of heritable mechanobullous disorders. EB can be divided into three major types based on the level of tissue separation as established by electron microscopy: (1) EB simplex (EBS), (2) junctional EB (JEB), (3) dystrophic EB (DEB). In EBS the tissue separation occurs within the basal keratinocytes, whereas in JEB this separation is within DEJ at the level of the lamina lucida. In the dystrophic forms of EB the tissue separation occurs below the lamina densa at the level of anchoring fibrils.

Hemidesmosomes (HD) are located at the plasma membrane of the basal keratinocytes. They anchor the epidermis firmly to the lamina densa through the anchoring filaments. Five major components of the hemidesmosomes have been identified, consisting of polypeptides with molecular masses of 500, 230, 200, 180 and 120 kD and designated HD1 to HD5 respectively. HD2 and HD4 are known to be identical to the 230-kD bullous pemphigoid anitgen (BPAg1) and 180-kD bullous pemphigoid antigen (BPAg2, COL17A1) respectively. HD3 and HD5 correspond to the b4 and a6 subunits of a6b4 integrin.

Bullous Pemphigoig Antigen 1 (BPAg1). BPAg1 (HD2) is a 230 kD intracellular noncollagenous protein located at the hemidesmosomal inner plaque. It is believed to play a role in cell-matrix adhesion. Antibodies to BPAg1 have been shown to be present in patients with bullous pemphigoid (BP), an acquired blistering skin disease characterized by tense bullae on the trunk and extremities, mostly affecting the elderly. However, whether autoantibodies against BPAg1 are an epiphenomenon or the cause of BP is controversial.

Bullous Pemphigoid Antigen 2 (BPAg2). BPAg2 (HD4) is a 180-kD collagenous protein located at the HD complex. Unlike BPAg1 it has both intracellular and extracellular domains. The collagen-like domain of BPAg2 is extracellular. This protein contains a series of collagen-like repeats (Gly-X-Y) at its –COOH terminus and has been classified as type XVII collagen (COL17A1).

Antibodies against BPAg2 are believed to play a role in the pathogenesis of herpes gestationis, a blistering skin disease occuring during pregnancy. In addition to this, recently, mutations in BPAg2 gene have been identified in a rare form of EB known as generalized atrophic benign epidermolysis bullosa (GABEB), a subset of nonlethal JEB.

a6b4 Integrin. Integrins are transmembrane glycoprotein receptors that mediate cell-matrix or cell-cell adhesion and transduce signals that regulate gene expression and cell growth. These heterodimeric molecules consist of noncovalently linked a and b subunits. Different combinations of a and b polypeptides form complexes that vary in their ligand-binding specificities. In the human epidermis, basal keratinocytes express integrins a2b1, a3b1, and a6b4. Heterodimers a2b1 and a3b1 are located primarily at the lateral surface of basal keratinocytes, a location suggestive of a role in cell-cell interaction, whereas a6b4 is restricted to the ventral surface opposed to the BM zone, suggestive of its role in cell-matrix adhesion.

The protein a6b4 intergin is a basal keratinocyte-specific intergrin that contributes to the anchoring of basal keratinocytes to the underlying BM. In a subtype of nonlethal JEB, characterized by blistering of the skin and pyloric atresia, mutations in the b4 integrin have recently been demonstrated.

Plectin and Hemidesmosome 1. Plectin is a large cytoskeleton-associated protein that is widely distributed throughout the stratified squamous epithelium, muscle, and brain. Recent findings suggest that plectin and hemidesmosome 1 (HD1) are the same protein. In the skin, plectin is located at the inner hemidesmosomal plaque. Together with BPAg1, it plays an important role in the attachment of the intracellular cytoskeleton (intermediate filaments) to the HD. Mutations of the plectin gene have been identified in patients with an unusual recessive form of epidermolysis bullosa associated with muscular dystrophy (epidermolysis bullosa simplex–muscular dystrophy (EBS-MD). Interestingly, knockout mice targeted against BPAg1 also developed skin fragility and muscular dystrophy. The HD in these mice lacked the hemidesmosomal inner plaque and thus had a complete separation of the HD from the intermediate filament network. These recent findings give further support to the suggestion that the hemidesmosomal inner plaque (BPAg1, plectin) is vital for the attachment of the intermediate filaments to the HD.

The lamina lucida is an electron-lucent region seen on electron microscopy that separates the HD from the lamina densa. It appears to be the weakest zone of the DEJ. It separates easily with heat and suction, with treatment with salt solutions, and with proteolytic enzymes. The structures in this region include the anchoring filaments, laminin 5, K-laminin, laminin 1, fibronectin, and nidogen (entactin). Spanning across this zone are the anchoring filaments. These filaments are thin, threadlike structures, 2 to 4 nm in diameter, that connect HDs to the lamina densa. It is not known whether anchoring filaments insert nto the lamina densa or into anchoring fibrils.

The anchoring filaments have been shown to consist of laminin 5 (previously known as kalinin, nicein, epiligrin), a distinct member of the laminin family of proteins. Laminin 5 consits of three subunit polypeptides, the a3, b3, and g2 chains encoded by the genes LAMA3, LAMB3, and LAMC2 respectively. It has a Y-shaped configuration. Mutations in the laminin 5 genes have been identified in patients with both the Herlitz type and non-Herlitz type JEB. In JEB the tissue cleavage occurs at the lamina lucida. In addition to laminin 5 mutations in JEB, mutations in other genes have been identified.

Nidogen is a sulfated noncollagenous glycoprotein that has an affinity for laminin. It is a single-chain 150-kD glycoprotein with globular ends that forms a dumbbell configuration. K-laminin (laminin 6) is another noncollagenous glycoprotein localized to lamina lucida. It is composed of three chains (a3, b1, g2) forming a Y configuration and complexes with nidogen and laminin 5.

Laminin 1 is a noncollagenous glycoprotein composed of three chains (a1, b1, g1) forming an "asymmetric cross" configuration. It associates with type IV collagen and nidogen and plays a role in cell attachment.

This electron-dense zone consists mainly of type IV collagen. It is a nonbanded, network-forming collagen synthesized by keratinocytes that provides structural support and flexibility to this layer. Type IV collagen is also found in the subbasal lamina densa within the anchoring plaques. In addition to type IV collagen, heparan sulfate proteoglycan and laminin are also components of the lamina densa. Heparan sulfate proteoglycan is predominantly a lamina densa constituent but may also be present within the lamina lucida and subbasal lamina densa. These molecules probably assist in regulating permeability by restricting the cationic macromolecules.

Anchoring fibrils are broad (20 to 60 nm), elongated, flexible, fibrillar structures that originate at the lamina densa and extend into the dermis. They either end freely in the matrix and insert into electron-dense structures termed anchoring plaques or loop back into the lamina densa. Anchoring fibrils are mainly if not exclusively composed of type VII collagen (COL7A1). Type VII collagen is a nonfibrillar collagen composed of three identical a1 (VII) chains. The gene has been localized to chromosome 3p21.1. It consists of 118 exons, the largest number of exons in any gene published so far. The NC1 domain (N-terminus of type VII collagen) specifically binds to type IV collagen of lamina densa and anchoring plaques.

The anchoring fibrils are abnormal in morphology, reduced in number, or entirely absent in patients with dystrophic epidermolysis bullosa (DEB). Furthermore, mutations in type VII collagen gene have been indentified in both the dominant and recessive forms of DEB.

In addition to DEB (a genodermatosis), type VII collagen has been found to play a role in the pathogenesis of epidermolysis bullosa acquisita (EBA) and bullous systemic lupus erythematosus (bullous SLE), both acquired blistering disorders. EBA is a rare bullous dermatosis with adult onset, clinically resembling DEB. On the other hand, bullous SLE develops as a manifestation of systemic lupus erythematosus, usually seen in second or third decades. Anti-basement membrane antibodies have been identified in both diseases. Recent studies have demonstrated that the antibodies are formed against (290 and 145 kD) type VII collagen.

The dermis supports the epidermis and is composed of a fibrous connective tissue (collagen and elastin) in close association with the ground substance. It is divided into two layers on the basis of differences in connective tissue density and arrangement: (1) the papillary dermis, which underlies the epidermis; and (2) the reticular dermis, which lies between the papillary dermis and the subcutaneous fat.

Collagen is a family of closely related yet genetically distinct proteins. A characteristic feature of these molecules is that they are composed of three chains and form a triple-helical structure. In general, the collagens have been divided into fibrillar and nonfibrillar polypeptides. The fibrillar collagens (types I, II, III, V, and XI) contain a triple-helical collagenous domain that consists of uninterrupted Gly-X-Y repeat sequence. The nonfibrillar collagens include FACIT collagens (fibril-associated collagens with interrupted triple helices), such as types IX, XII, and XIV, and other nonfibrillar collagens, such as type IV collagen. This group of collagens contains interruptions in the triple-helical collagenous domain of the molecule.

Currently there have been 11 different collagen types detected in the human skin. The dermis contains predominantly type I collagen (85% to 90%), type III collagen (8% to 11%), and type V collagen (2% to 4%). The high proportion of type I collagen contributes to the great tensile strength of the dermis and its ability to withstand deformation. In the fetus or neonatal skin, type I collagen represents a smaller proportion of the total collagen. In the dermis the broad bands of reticular collagen are type I, whereas the finer fibers (known as reticulin) of the papillary dermis are type III. Type IV collagen is present within the lamina densa of the basement membrane. Type V collagen is abundant in the papillary dermis and in the loose connective tissue sheaths surrounding nerves and vessels. Type VI collagen is also abundant within the dermis, where it is organized into 3-nm beaded filaments that are interwoven among the banded collagen fibrils and are within the diffuse matrix milieu between fiber bundles. As stated previously, type VII collagen is a major constituent of the anchoring fibril. Type VIII collagen is present as a minor component in the dermis, being primarily a product of endothelial cells. Types XII and XIV collagens belong the the group of FACIT collagens and are found ubiquitously distributed but in small amounts. Type XVII collagen, also known as 180-kD bullous pemphigoid antigen 2, is a transmembrane collagen present in the hemidesmosomes.

Abnormalities in some of the collagens of the dermis have been shown to play a role in some skin diseases. Osteogenesis imperfecta (OI), a hereditary disease in which the major clinical finding is bone fragility, results from mutations in the collagen I gene (COL1A1 or COL1A2). In patients with Ehlers-Danlos syndrome type IV, a hereditary disease in which the patients have a tendency for bleeding and bruising, absence of type III collagen has been identified.

Elastic fibers are responsible for the retractile properties of the skin. Elastic fibers in the dermis are composed of 10- to 12-nm thick microfibrils embedded in an amorphous protein called elastin. Oxytalan fibers, consisting of bundles of these microfibrils, are the elastic connective tissue components of the papillary dermis. They extend perpendicularly from the DEJ into the papillary dermis, where they merge with elaunin fibers, microfibrils with small amounts of cross-linked elastin, in the deep papillary and intermediate dermis. Elaunin fibers join with the mature fibers of the deeper reticular dermis. The latter fibers have the highest proportion of elastin of all elements of the elastic fiber system. Mature elastic fibers are flat, branching structures that have the appearance of broad rubber bands. The microfibrils of all layers have been shown to contain fibrillin, an immunologically distrinct microfibril-associated glycoprotein (MAGP).

The diseases associated with abnormalities of the elastic connective tissue include Williams syndrome, cutis laxa, Marfan syndrome, congenital arachnodactyly, pseudoxanthoma elasticum, Buschke-Ollendorff syndrome, Debarsy syndrome, and elastoderma.

Ground substance forms the milieu for the cellular and fibrous constituents. The major glycosaminoglycans (GAG) and proteoglycans in mammalian skin are hyaluronic acid, heparan sulfate proteoglycann, chondroitin 6-sulfate proteoglycan, chondroitin sulfate / dermatan sulfate, and keratan sulfate. Dermatan sulfate represents 30% to 40% of dermal proteoglycans. These are synthesized in the skin by fibroblasts and possibly by smooth muscle cells and mast cells.

Production of hair is unique to mammals. In most mammals, the major purpose of hair is heat conservation and protection from injury. In man, there is no evidence that complete loss of body hair produces any significant biologic impairment or reduction of life expectancy. Specialized hair, such as eyelashes and nasal hairs, still serve sensory and protective functions. However, the role of hair in social and sexual signalling has become much more important than its protective function. Loss of existing hair or hair growth in inappropriate sites is a cause of distress to individuals and a source of enormous commercial revenues.

There are three types of hair. The fetus is covered by soft, fine, lightly pigmented hairs called lanugo. After birth most of the body is covered by fine vellus hairs. Long, coarse, pigmented hairs on eyebrows, eyelashes, scalp, beard, axillae, and genital areas are called terminal hairs. Any hair follicle during its lifetime may generate all three types of hair. The structure of the follicles producing all three types of hair is the same.

There are approximately 100,000 hair follicles on the scalp. All follicles are formed before birth. The hair follicle begins as an epithelial bud overlying a collection of mesenchymal cells that are to become the follicular papilla. The epithelial bud, or primary hair germ, descends along with the papilla into the dermis to form the hair follicle. During this descent, three protuberances develop along the surface of the follicle and will eventually form the apocrine gland, the sebaceous gland, and the attachment of the arrector pili muscle. The latter protuberance is termed the "bulge". It is believed by some investigators that the bulge is the source of the stem cells that give rise to a new inferior segment of the hair follicle.

A mature hair follicle is divided into three parts. The infundibulum is the portion of the follicle from the ostium on the surface of the epidermis to the opening of the sebaceous duct. The infundibulum contains the hair shaft, sebum, desquamated epithelial cells, and many microorganisms, such as bacteria, Demodex species, and fungal spores. The isthmus is the short portion between the point of entry of the sebaceous duct and the attachment of the arrector pili muscle. The inferior segment consists of the bulb, which contains the matrix cells that surround the follicular papilla and the suprabulbar portion.

The hair follicle also consists of nine layers: the hair shaft, consisting of the medulla, cortex, and cuticle; the inner root sheath (IRS), consisting of the IRS cuticle, Huxley's layer, and Henle's layer; the outer root sheath; vitreous membrane; and the dermal fibrous root sheath.

Above and surrounding the papilla is the proliferative portion of the follicle known as the hair bulb. The follicular papilla is a flame-shaped collection of spindle cells in a fibrous stroma. It is connected to the surrounding dermis at its inferior portion and is surrounded on all other sides by the cells of the hair matrix. The matrix is composed of cuboidal, undifferentiated epidermal cells. Pigment is added to the newly formede cells from the melanocytes in the matrix. The cells overlying the top and sides of the follicular papilla keratinize at Adamson's fringe, the line where the bulb ends, to form the central hair shaft. Just above the hair bulb, the cells begin to keratinize without the formation of a granular layer, and the nuclei gradually disappear.

The hair shaft consists of three layers. In some but not all hairs, the center of the shaft contains the medulla. Fine hairs usually lack a medulla. Outside the medulla lies the hair cortex, which constitutes the bulk of the hair. The outermost layer of the hair shaft, the hair cuticle, contains flattened overlapping cells that point upward and interlock with the cells of the inner root sheath cuticle, which point downward. This interlocking keeps the hair firmly within the follicle.

Immediately external to the hair shaft are the three layers of the inner root sheath. The inner root sheath arises from the cells of the matrix. Outside the IRS cuticle lies Huxley's layer. The outermost layer and the first to keratinize is Henle's layer. It keratinizes through the formation of trichohyaline granules. The IRS coats and supports the hair shaft up to the level of the isthmus, where the IRS disintegrates and its cells desquamate into the lumen of the follicle. The hair shaft is haped as well as guided by the IRS. The cells of the IRS and the hair shaft move upward and out of the follicle together.

External to the IRS is the outer root sheath (ORS). The cells of the ORS are clear because of their content of large amounts of glycogen. The ORS is thickest at the isthmus and narrowest in the lower portion of the bulb. Below the isthmus, the ORS does not keratinize. At the level of the isthmus, where the IRS disappears, the ORS keratinizes withouth the formation of the granular layer (trichilemmal keratinization). In the infundibulum, the keratinization of the ORS switches to the epidermal mode of keratinization with the formation of the granular and cornified layers. These differences in keratinization can be seen microscopically when one is examining the pathologic condition of follicular cysts of isthmic origin (pilar and trichilemmal cysts), where the keratin is dense and compact and there is no granular layer, and those of infundibular origin (epidermoid cysts), where the granular layer is present and the keratin contents more closely resemble the stratum corneum.

The glassy, or vitreous, membrane is the acellular eosinophilic zone surrounding the follicle. It is analogous to and continuous with the epidermal basement membrane. Unlike the epidermal basement membrane, the vitreous layer is thicker and visible with routine stains. During catagen (involutional stage of the hair cycle; see below) the vitreous layer becomes thicker and corrugated in appearance. Surrounding the vitreous layer is the fibrous root sheath, the outermost layer of the hair follicle, composed of thick collagen bundles. This sheath is continuous with the dermal papilla below and blends with the papillary dermis above.

The growth of hair is cyclical. Each period of active growth of hair, known as anagen, alternates with a resting period, telogen. Between these two phases is the relatively short transitional or involutional phase, catagen. Both terminal and vellus hair go through all stages of the follicular cycle. The definitive length of the hair depends mainly on the duration of anagen and partly on the rate of growth, body site, and sex. The average lenght of anagen for scalp hair is 3 years, catagen 3 weeks, and telogen 3 months. At any one point, approximately 84% of scalp hairs are in anagen stage, 2% in catagen, and 14% in telogen. The hair follicles on the scalp are out of the phase with their neighbors. The daily hair growth rate is on the average 0,4 mm on the scalp. Every day, approximately 50 to 100 hairs are shed from the scalp and replaced by newly growing hairs. The duration of anagen for hair on the trunk, extremities, and eyebrows generally does not exceed 6 months. Anagen is also short for vellus hairs.

Anagen follicles are highly active metabolically, a characteristic that explains their sensitivity to nutritional deprivation and chemical injury. Only the inferior portion of the hair follicle is involved in the hair cycle. Anagen is the period of active hair growth. Eventually the anagen hair receives a signal to cease growth. The follicle enters catagen, when mitotic activity and melanin production in the hair bulb stop. The matrix cells retract from the papilla. Above the papilla, the follicular epithelium remains as a thin column with wavy and irregular sides. The vitreous membrane surrounding the papilla and the epithelial column is thickened and corrugated. This thin cord of epithelial cells retracts upward, followed by the follicular papilla. The epithelial cord shortens until it forms a small protrusion from the lowest portion of the remaining hair follicle, called the secondary hair germ. By the end of catagen, the follicle reaches one third of its former lenght. The lowest portion of the follicle now lies near the attachment of the arrector pili muscle. The papilla is located directly beneath the secondary hair germ. At this point the hair has reached telogen. At the beginning of telogen, the keratinized club hair and its epithelial coating are firmly attached to each other. As telogen progresses, this attachment weakens.

Cotsarelis et al have proposed that new anagen growth is initiated by the slow-cycling, undifferentiated stem cells that reside in the "bulge" region, at the point of attachment of the arrector pili muscle. When regrowth of ne hair begins, the secondary hair germ begins to elongate and envelop the follicular papilla. As the follicle grows downward, it enlarges and begins to produce a new hair shaft. As the new hair shaft grows, it pushes the old telogen hair out of the follicle.

Immunologic events may play an important role in the cycling of the hair follicle. Cells of the lower two thirds of the hair follicle (the cycling portion) do not express MHC class I antigens. In early catagen, there is an increase in the number of cells expressing class II MHC and macrophage markers in close association with follicles. The cellular resorption seen in catagen may not simply be attributable to random cell death but may in some way be mediated by immunocomponent cells acting on a recognizable cell surface marker that distinguishes between cells that survive catagen, such as the cells of the upper one third of the follicle that express MHC class I molecules, and cells that do not. The follicular infundibulum also contains many CD1a-positive Langerhans' cells, which may serve as a reservoir of antigen-presenting cells and are sequestered from damage by environmental agents, such as ultraviolet radiation.

The endocrine system does not directly initiate or curtail the activity of hair follicles. It can, however, accelerate or retard the onset of events. Nearly all human hair follicles are influenced by androgens, but they differ in the nature and extent of their response. Axillary and pubic hair requires lower levels of hormone than growth of hair on the face, trunk, or extremities does. Growth of hair on the scalp is paradoxically inhibited by testosterones in persons genetically predisposed to androgenetic alopecia. Follicular cycles can be accelerated by thyroxin or delayed by estradiol. Estrogens retard growth but prolong anagen. Hormones of pregnancy prolong anagen. Unlike previous reports, retinoids have not been found to affect hair follicle proliferation.

The sebaceous gland is a lipid-producing structure that develops in the fourth month of gestation from the middle bulge of the hair follicle. Sebaceous glands are distributed over the entire body surface, except for the palms, soles, and the dorsa of the feet. The density of sebaceous glands is highest on the face and scalp (400 to 900 glands/cm²) and lowest on the extremities (none to 50 glands/cm²). On the skin, sebaceous glands are associated with hair follicles. In the mucosa, sebaceous glands drain directly onto the surface. Ectopic sebaceous glands include Fordyce spots on the vermillion border of the lips, Tyson's glands on the glans penis, Montgomery tubercles on the breast, and meibomian glands on the eyelid.

Sebaceous glands are fully formed and functional by the third trimester under the influence of maternal hormones. After birth they decrease in size and are quiescent until adolescence, when, under the influence of androgens, they become active again. The earliest sign of adrenarche is increased sebaceous gland activity.

Sebaceous glands vary in size according to location. They are unilocular in areas where sebaceous glands are sparse and multilobular where such glands are numerous. A "sebaceous follicle" is a follicle with large and numerous sebaceous glands and a small vellus hair. When the ostium of a sebaceous follicle is plugged with corneocytes, a comedo results. This lesion is important in the pathogenesis of acne.

A sebaceous lobule consists of an outer layer of small cuboidal basaloid cells and a large central population of progressibely differentiated lipid-laden cells. A short duct connects the glands to the follicle at the junction of the follicular infundibulum and isthmus.

Differentiation occurs by accumulation of lipid droplets within the sebocytes. The volume of a sebocyte increases more than 100-fold as it accumulates lipid. In the last stages of differentiation, the nucleus and the organelles disappear. The cell ultimately ruptures, releasing its lipid contents (sebum) into the duct (holocrine secretion). Sebum production is a continuous process, dependent on the cellular proliferation and differentiation in the sebaceous glands. It takes 2 or 3 weeks for an undifferentiated sebocytes to reach the surface of the skin as sebum.

Lipid is produced in the sebocyte in the smooth endoplasmic reticulum and Golgi apparatus, where it is packaged as vacuoles. The vacuoles collect in the cytoplasm, eventually impringing on the nucleus, giving it a scalloped or notched appearance.

Although not completely characterized, the sebum composition includes triglycerides, wax esters, squalene, cholesterol, and cholesterol esters. Wax esters and squalene are unique to sebum and do not exist in epidermally derived lipids.

Gonadal androgens are the primary stimulus for sebum production in both men and women. Sebum production increases in adolescence under the influence of testosterone. Testosterone stimulates both cell division and intracellular lipid synthesis. Dehydroepiandrosterone, an adrenal angrogen, can also stimulate sebaceous glands but to a lesser degree than testosterone does. Growth hormone also exerts a stimulatory effect. Estrogens are powerful inhibitors of sebum secretion, acting primarily against lipid synthesis.

Sebum production is suppressed by isotretinoin, a retinoic acid derivative, and androgen receptor inhibitors, such as cyproterone acetate and spironolactone. In humans the sebaceous glands have no proved function.

Apocrine glands develop only in certain areas of the body. They are found in the axillae, areolae (mammary glands), periumbilical region, perineal and perianal areas, prepuce, scrotum, mons pubis, labia minora, and external auditory canals (ceruminous glands) and on the eyelids (Moll's glands). The largest and most numerous glands are found in the axillae. Apocrine glands are small and nonfunctional until puberty, at which time they enlarge and begin to secrete their products. They develop from the upper bulge of the hair follicles. The formation of the apocrine glands begins late in the fourth month of gestation and continues until late in embryonic life.

Histologically, apocrine glands consist of a coiled secretory portion located deep in the reticular dermis or subcutaneous fat and a straight duct that empties into the follicular canal in the infundibulum, above the opening of the sebaceous duct. The secretory portion of the apocrine gland has a single layer of secretory cells and an outer layer of myoepithelial cells. Except in the apical portion, the secretory cells contain in their cytoplasm large, PAS-positive, diastase-resistant granules. This PAS-positive material, like the secretion of the dark cells in eccrine glands, consists of sialomucin. In addition, apocrine granules frequently contain iron and lipofuscin. The apocrine glands discharge their contents by decapitation secretion, that is, by pinching off of the apical portion of the cell.

The function of the apocrine glands in unclear. The characteristic odor in human axillary sweat is caused by the violatile C6 to C11 short-chain fatty acids, with the most abundant being 3-methyl-2-hexanoic acid. This initially odorless apocrine secretion is formed at the skin surface by the action of bacterial enzymes.

The control of apocrine gland secretion is unclear. Gonadal hormones seem to play an initial role in the development and maintenance of the apocrine glands, but once developed they are relatively independent. Apocrine secretion occurs after both adrenergic and cholinergic stimulation.

The eccrine gland is the only true sweat gland in humans. In a hot environment or during physical exercise, evaporative cooling is the only means by which humans regulate their body temperature. Evaporation of 1 g of sweat water from the skin surface removes approximately 0,58 kcal of heat. The maximal sweating rate is 1,8 liters/hour. Inability to regulate the body temperature by evaporative heat loss leads to hyperthermia, heat exhaustion, or heat stroke. Eccrine glands are present everywhere in human skin except the vermillion border of the lips, the nail beds, the labia minora, the glans penis, and the inner aspect of the prepuce. They are most numerous on the palms and soles and in the axillae. Embryologically the eccrine glands derive from the surface ectoderm, independentt of the pilosebaceous units. Eccrine glands are first seen early in the fourth gestational month. They develop much earlier on the palms and soles than elsewhere. Near the end of the fifth month, they begin to appear over the remainder of the body. Approximately 3 million eccrine sweat units are present at birth, and no additional units are formed thereafter. The eccrine gland is composed of three segments: the secretory portion (which makes up half of the basal coil), the dermal duct, and the spiral intraepidermal duct (acrosyringium). The entire sweat gland is highly vascularized and innervated by unmyelinated sympathetic nerve fibers. The eccrine dermal duct is lined by a double row of basophilic cuboidal cells. Each duct enters the epidermis at the bottom of the rete ridge, where it widens and spirals through the epidermis to open onto the skin surface. The lumen of the entire eccrine duct is lined by an eosinophilic cuticle.

The secretory portion, located at the junction of the deep reticular dermis and the subcutaneous fat, is composed of an outer discontinuous layer of contractile myoepithelial cells and an inner layer of pyramidal secretory epithelial cells. The inner layer of secretory cells consists of two types of cells: clear cells and dark cells. Clear cells secrete abundant amounts of aqueous material together with glycogen. Clear cells join to one another by an extensive network of canaliculi lined by microvilli. These intercellular canaliculi convey the sweat from the secretory cells to the lumen of the gland (exocrine secretion). The initial sweat secreted is nearly isotonic but is modified by the reabsorption of NaCl in the excess of water by the eccrine ductal cells.

The eccrine sweat gland is regulated by the sympathetic nervous system, except that acetylcholine, not epinephrine, is the major postganglionic neurotransmitter controlling sweat secretion. Other universal sympathetic neurotransmitters found in the periglandular nerve endings include norepinephrine, vasoactive intestinal polypeptide (VIP), galanin, serotonin, calcitonin gene-related peptide, ATP, neuropeptides, and substance P. Eccrine sweat can be induced by ATP, norepinephrine, and VIP.

Dark sweat cells secrete sialomucin. They are also rich in interleukin-1 and prolactin. Stimulation of the dark cells induces c-myc and c-fos expression, indicating that the dark cell may perform paracrine-autocrine functions and may be involved in regulating cellular proliferation. Interleukin-1 may play a proinflammatory role in a variety of dermatoses, such as atopic dermatitis.

The Merkel cell is named after Friedrich Merkel, the medical student who first observed these cells within the epidermis. The Merkel cell is of neuroendocrine origin and is believed to function as a slow-adapting touch receptor by interdigitating with keratinocytes and transmitting signals via axons. The Merkel cell-axon complex has been identified on the palms and soles and in nail beds, oral mucosa, and the genital region. The Merkel cell is indistinguishable from melanocytes or Langerhans' cell on light microscopy. However, on electron microscopy they possess distinctive electron-dense bodies. Merkel cell carcinoma is an uncommon malignant neoplasm that is uniformly fatal.

The skin contains both somatic sensory and autonomic motor nerves. The sensory system within the skin mediates the sensations of itch, pain, temperature, light touch, discriminative touch, pressure, vibration, and proprioception. The autonomic motor nerves regulate vascular tone, pilomotor responses, and sweating. Peripheral nerves within the dermis follow the course of the superficial and deep vascular plexuses, from which neurovascular plexuses are derived.

Sensory receptors in the skin may be divided into those that are specialized, such as pacinian corpuscles, Meissner's corpuscles, and Kraus's mucocutaneous end organs, or those that are unspecialized, such as the sensory nerves of the skin.

Pacinian corpuscles populate weight-bearing surfaces and primarily function as mechanoreceptors to detect pressure. A myelinated axon supplies each corpuscle. The distal end of this myelinated axon is wrapped in connective tissue and forms concentric layers that terminate where a sensory terminal is located. Distortion of the lamellar organization of the pacinian corpuscle permits amplification or modification of action potential.

Meissner corpuscles are rapid adapters for the sensation of light touch. They are found on volar skin and are in highest concentration on fingertips. A Meissner corpuscle is composed of layers of modified Schwann cells among which a myelinated axon terminal interweaves.

Kraus's mucocutaneous end organs are located at all mucanocutaneous junctions, namely, the glans penis, prepuce, clitoris, labia minora, perianal region, eyelids, and lips. Kraus's organs cannot be routinely visualized by light microscopy but, upon silver staining, are composed of axons that possess a "ball-of-wool" appearance.

Unspecialized sensory nerve endings that supply the skin are either associated with hair follicles or not. They form a branching and overlapping pattern within the dermis. Hair follicles are supplied by myelinated sensory nerves. Unmyelinated axons terminate near the dermoepidermal junction.

Itch is defined as a sensation that provokes the desire to scratch. Itch is the most aggravating of all cutaneous symptoms. The neurologic mechanisms that lead to the sensation of pruritus are not well defined. However, the sensation of both pain and itch travel via the lateral spinothalamic tract in the spinal cord, leading to the suggestion that itching may represent a type of subthreshold pain. However, pruritic stimuli exert their effect at or near the epidermis, whereas pain is elicited from deep nerve terminuses within the skin, and this finding supports the hypothesis that itch and pain are transmitted by different populations of sensory neurons and terminate at different points within the central nervous system. How nerve endings within the skin serve as itch receptors is not known, but they well may contain receptors for chemicals known to induce itching, such as histamine. Many inflammatory skin diseases itch. Noteworthy examples include dermatitis herpetiformis, scabies, lichen planus, and Grover's disease.

The vasculature of the skin is composed of two plexuses: a superficial plexus and a deep plexus of arterioles and venules. These two plexuses travel parallel to the skin, and communicating vessels travel perpendicular to the skin. From the superficial plexus emanate capillary loops that travel to the tips of dermal papillae. Each capillary loop is composed of an ascending afferent arteriolar portion and a descending venular efferent portion. The venous segment empties into the superficial plexus and thereupon into communicating vessels of the dermis, the venules of the deep dermal plexus, and, finally, into the veins of the subcutaneous fat. Paralleling the blood supply of the skin is a lymphatic system that serves to allow Langerhans' cell to travel to the lymph node and permits the clearance of foreign substances such as immune complexes.

Many inflammatory skin diseases can be classified based upon two cardinal features: (1) the pattern of involvement of the vasculature of the skin by inflammatory cells and (2) the character of the inflammatory cell infiltrate. Thus some diseases predominantly involve the superficial plexus, whereas others involve both the superficial and deep plexuses. In psoriasis a superficial perivascular infiltrate of lymphocytes is present in association with other changes, particularly epidermal hyperplasia. Urticaria is characterized histologically by a sparse superficial perivascular infiltrate in which eosinophils and neutrophils may be found within a predominantly lymphocytic infiltrate. In contrast, lupus erythematosus is characterized by a dense superficial and deep perivascular infiltrate of lymphocytes only. In scleroderma there is a superficial and deep perivascular and interstitial pattern of inflammation composed of lymphocytes, neutrophils, eosinophils, and plasma cells. In sum, the anatomy of the cutaneous blood supply determines patterns of inflammation that define and distinguish inflammatory skin diseases. Finally, dilation of the vasculature is largely responsible for the red color seen in many inflammatory dermatitides.

The skin is an integral component of the immune system and in many ways could be considered the front line of immune defense. It is in the skin where viruses, bacteria, fungi, protozoa, and multicellular parasites can first encounter the cells composing the human body. The results of these encounters between pathogen and immune defense must be overwhelmingly successful for the host if survival is to be ensured. The catastrophic infections that ensue when large portions of the epidermis are lost, as in burn victims, can attest to the powerful capacity of the skin to protect the human host from external pathogens.

The mechanisms of skin defense can be broadly divided into two categories: innate (or nonadaptive) immune responses and adaptive immune responses.

The innate immune response of the skin is first and foremost the barrier function of the skin. The most differentiated layer of the epidermis is the stratum corneum, and it is here that the initial interface with the environment takes place. The physical barrier composed of the extracellular lipids and keratinized cells of the stratum corneum is complete, allowing only water and selective micromolecules to pass through. In addition it has been suggested that the various lipid components of the stratum corneum, including sebaceous lipids, glycosphingolipids, and free fatty acids, have potent antimicrobial effects. The resident microflora of the skin is also protective through competition with pathogenic microorganisms. Finally, antibacterial antibodies, primarily IgA, are secreted onto the surface of the skin by sweating and by sebum secretion.

The adaptive component of cutaneous immune defense is multifaceted and complex and contains the full repertoire of immune mechanisms employed by the immune system in the other organ systems. This type of immune defense is antigen-specific and is amplified upon reexposure to the same antigen. Thus specificity and memory are key features of the immune system of the skin as well and are mediated by the various cellular components of the epidermis and dermis. These include Langerhans' cells and other antigen-presenting cells, resident and migratory lymphocytes, keratinocytes, endothelial cells, and others working in concert to provide active immune surveillance and defense within the skin itself. This is known as skin-associated lymphoid tissue, or "SALT", analogous to the conjunctiva-, bronchus-, and gastrointestine-associated lymphoid tissues. The full range of immune response is thus available in the skin, including types I to IV hypersensitivity reactions. We will endeavor to illustrate the dynamic nature of adaptive cutaneous immunology and autoimmunity in the following sections. Atopic dermatits, allergic contact dermatitis, urticaria and angioedema, and drug hypersensitivity deserve chapters of their own and are covered elsewhere in this text.

Langerhans' cells are the major antigen-presenting cells in the skin. They contain characteristic intracytoplasmic tennis racket-shaped organelles called Birbeck granules, whose precise role is not yet clear, though they may be involved in antigen processing, presentation, or both. These cells are derived from bone marrow but enter the embryonic epidermis during the first trimester and continue to increase in number throughout gestation. In the adult they account for only 2% to 8% of the epidermal cells but are the only cells that express substantial levels of MHC class II antigens in normal, noninflamed epidermis. In addition to class II MHC antigens, Langerhans' cells are characterized by numerous cell surface antigens and membrane-associated proteins, including CD1a, CD4, CD45, S100, T200, Fc receptors for IgE, C3 receptors, ATPase, ICAM 1, CD11/CD18, and LFA-3. Langerhans' cells are not stable; rather, they exhibit considerable dynamism, migrating in and out of the epidermis and modifying both their cell surface phenotype and critical aspects of their function, depending on their location, state of maturation, and the cytokine milieu. The chief function of Langerhans' cells is to process and present antigens encountered in the epidermis to naive T cells and thus to initiate an adaptive immune response.

Other antigen-presenting cells that may play a role in the skin and its regional lymph nodes include dermal dendrocytes (dermis), veiled cells (lymphatics), follicular dendritic cells (nodes), monocytes and macrophages, and B cells.

Circulating skin-homing lymphocytes are indispensable to the cutaneous immune response. In normal human skin, all extravascular lymphocytes are of the T-cell type. Most of these T cells express the alpha-beta receptor. The perivascular and periadnexal areas of the dermis contain 90% of cutaneous T lymphocytes. Most of dermal lymphocytes are in the activated state, expressing HLA-DR and IL-2 (interleukin-2_ receptors. These dermal T lymphocytes constitute an approximately equal number of CD4 helper/induced and CD8 suppressor/cytotoxic lymphocytes. The fact that the relative lymphocyte ratios within the skin differs from the peripheral blood supports the concept of lymphocyte "homing" to the skin.

Migration between peripheral blood and the skin is mediated by an array of cell adhesion molecules on lymphocytes, endothelial cells, and keratinocytes and is cytokine mediated. These include the cutaneous lymphocyte-associated antigen (CLA) and lymphocyte function-associated antigen 1 (LFA-1) on lymphocytes, intercellular adhesion molecule 1 (ICAM-1) on keratinocytes and endothelial cells, and endothelial cell-leukocyte adhesion molecule 1 (ELAM-1), as well as P-selectin and E-selectin on endothelial cells. When there is local cutaneous-specific antigenic stimulation or even nonspecific trauma or inflammation, local cytokines are produced, including interleukin-1 (IL-1), interferon-gamma (IFN-g) and tumor necrosis factor (TNF-a). These cytokines up-regulate the selectins and immunoglobulin superfamily molecules (ICAM-1 and ELAM-1) so that migrating memory and effector lymphocytes with receptors for these molecules are locally recruited by binding and diapedesis at local dermis and epidermis to initiate T cell-mediated immune response.

Gamma-delta T cells (also known as dendritic epidermal T cells in murine skin) express a T cell receptor utilizing the gamma and delta genes rather than alpha and beta genes as with most other T cells. Many ot these cells lack both the CD4 and CD8 receptors ("double-negative" T cells), and their function in the skin remains speculative. The diversity of the TCRs expressed on these cells is limited compared with other T cells, and it has been hypothesized that they may participate in an early warning system of cutaneous immune surveillance and cytotoxicity in which non-MHC restricted mechanisms predominate. It is believed that g/d T cells can recognize antigens presented in the context of the CD1a antigen (including nonprotein antigens) on Langerhans' cells, mycobacterial antigens, and various mammalian heat-shock proteins. Heat-shock proteins are a highly conserved group of molecules that can be expressed by many cell types after a variety of insults, including heat, hypoxia, ionizing and nonionizing radiation, viral infection, and malignant transformation. Thus it seems likely these unique T cell subsets provide an early defense against a variety of insults through a limited set of antigenic stimuli, such as heat-shock proteins.

The predominant cell of the epidermis is the keratinocyte. These cells are not simple structural components but are immunologically active. Keratinocytes may play a role in initiating cell-mediated immune responses in the skin by cytokine release and adhesion-molecule expression.

Keratinocytes constitutively synthesize interleukin-1. This cytokine may be secreted under the influence of other cytokines or released by direct cell injury. Keratinocytes thus store IL-1 for subsequent release under the appropriate stimulus for initiation of local and perhaps systemic immune response. Target cells of IL-1 include other keratinocytes, T lymphocytes, neutrophils, macrophages, fibroblasts, and smooth muscle cells. Through autocrine and paracrine actions, IL-1 may up-regulate the expression of its receptors on target cells and induce keratinocytes to become "activated". Activated keratinocytes synthesize and secrete IL-1 and an array of other cytokines to augment and shape immunologic response.

Activated keratinocytes may also affect the flux of lymphocytes and other migrating immune cells into and out of the skin by the selective expression of various adhesion molecules, such as ICAM-1. As discussed previously, the interaction of keratinocyte adhesion molecules and lymphocyte receptors such as ICAM-1– LFA-1 conjugate formation is integral to the trafficking of lymphocytes to the skin. Such interaction has been observed in f variety of inflammatory dermatoses, such as allergic contact dermatitis, psoriasis, and lichen planus and in cutaneous malignancies such as cutaneous T cell lymphoma (CTCL).

In addition to cytokine and adhesion molecule expression, keratinocytes may directly participate in cell-mediated immune responses by the expression of class II MHC molecules. In normal, noninflammed epidermis, keratinocytes express only class I MHC antigens. In inflamed skin, however, infiltrating T lymphocytes secrete IFN-g, which is capable of inducing transient expression of class II MHC antigens on the surface of keratinocytes. The role of these class II antigen-bearing keratinocytes has yet to be ascertained.

Mast cells, eosinophils, and basophils are present in the skin. Their roles in cutaneous immunologic function range from simple bystanders in inflammatory dermatoses of all types to infiltrating malignancies. They are discussed in detail elsewhere in this text.

Melanocytes are only recently emerging as active players in cutaneous immune response. Like keratinocytes, these pigment-producing cells can also synthesize and respond to a variety of biologic response modifiers.

Melanocytes produce several cytokines that may mediate dermal and epidermal inflammation. They can express IL-1, interleukin 3 (IL-3), interleukin 6 (IL-6), interleukin 8 (IL-8), monocyte chemotactic and activating factor, granulocyte-macrophage colony-stimulating factor, TNF-a, and TGF-b. Specific cytokines and neuropeptides acting on melanocytes can augment melanocyte expression and secretion of these inflammatory mediators.

Melanocytes do not constitutively express adhesion molecules but can be induced to do so by IFN-g, TNF-a, TNF-b, IL-1a, IL-6, and IL-7 in vitro. Cytokines may also have other autocrine and paracrine effects on melanocytes, altering their growth, differentiation, and their ability to synthesize melanin.

Future studies will help elucidate the specific roles of the melanocyte in cutaneous immunology.

As previously discussed in this text, CD4+ T lymphocytes can be subdivided into Th1 and Th2 by virtue of their distinctive cytokine profiles, which mediate cell-mediated and humoral immunity respectively. The balance between Th1 and Th2 cytokines has enormous implications for the clinical manifestations of cutaneous disease, be it inflammatory, infectious, or neoplastic. The cytokine state is not static but can vary toward either the Th1 or Th2 pole depending on a multitude of factors, including the nature and size of the antigenic insult, the genetic capacity of the host immune system to respond to the antigen, and the role of environmental influences such as concomitant infections and disease and ultraviolet radiation.

One of the clearest examples of this concept is the clinical spectrum of leprosy, a disease caused by the organism Mycobacterium leprae. At one pole, patients with tuberculoid leprosy display high cell-mediated immunity with few skin lesions containing few organisms and with a predominance of Th1 cytokines (IL-2 and interferon-g). At the opposite pole, lepromatous leprosy shows low cell-mediated immunity with many cutaneous lesions containing high numbers of bacilli and with a predominance of Th2 cytokines (IL-4 and IL-10). As mentioned, the cytokine state is dynamic and immune changes alter the clinical manifestations in the form of "reactional states". Reversal reactions are naturally occuring delayed type hypersensitivity responses to M. leprae and are associated with clearance of bacilli from lesions and "upgrading" toward the tuberculoid pole. Downgrading reactions also occur and represent the opposite phenomenon. In the near future, cutaneous administration of either IL-2 or interferon-g may be utilized therapeutically to manipulate the Th1-Th2 balance toward a more favorable immune state in affected individuals.

Almost all cutaneous disease can be measured in terms of predominant Th1 or Th2 cytokine status. Some relevant examples include contact dermatitis, which is Th1 mediated, and atopic dermatitis, now well known to be Th2 mediated. Leishmaniasis infection displays a clinical and immunologic spectrum almost identical to that of leprosy. Immunotherapy for these and other cutaneous immunologically mediated diseases is on the horizon.

The various adhesion molecule components of the desmosomes and hemidesmosomes discussed in detail elsewhere in this chapter are now known to be antigenic targets in a diverse group of autoimmune cutaneous diseases. This group of disorders is characterized by specific antibodies directed at adhesion molecule epitopes ith subsequent binding by the antibody to its target. Type II and type III reactions then ensue, with the final pathway among all these diseases being loss of cell-to-cell or cell-to-basement membrane zone adhesion and formation of either intraepidermal or subepidermal spaces. Clinically this phenomenon manifests as blisters, and hence this group of diseases is referred to as the autoimmune blistering diseases. What prompts the production of autoantibodies in genetically susceptible persons is not well understood, though the importance of certain specific HLA loci is unquestionable.

Examples of autoimmune blistering diseases include pemphigus, pemphigoid (bullous pemphigoid), herpes gestationis, linear IgA disease, dermatitis herpetiformis, cicatricial pemphigoid, and paraneoplastic pemphigus. Although the clinical intricacies and molecular biology of all the autoimmune diseases cannot be discussed in detail here, a screening of the antigen targets in pemphigus and pemphigoid as examples of intraepidermal and subepidermal blistering diseases respectively should illuminate the general characteristics of this group of autoimmune cutaneous diseases.

Pemphigus is of two distinct major subtypes: pemphigus vulgaris (PV) and pemphigus foliaceus (PF). The PV antigen is a 160-kD desmosomal protein, desmoglein 3. The PF antigen is a 160-kD desmosomal protein, desmoglein 1. Both these entities result in intraepidermal blisters. This is consistent with the location of the target antigens to the interkeratinocyte cell-cell junction, the desmosome. With desmosome-dependent adhesion disrupted, fluid accumulates, and clear, fragile, flaccid blisters form, commonly on the trunk and oral mucous membranes.

Pemphigoid (bullous pemphigoid) is an example of subepidermal blistering disease. There are two target antigens: bullous pemphigoid antigen 1 (BPAg1), a 230-kD hemidesmosomal protein, and bullous pemphigoid antigen 2 (BPAg2), a 180-kD hemidesmosomal protein. Binding of antibodies to these targets produces an inflammatory reaction with subsequent loss of adhesion of the keratinocyte to the underlying basement membrane zone. Fluid accumulates under this area to form tense blisters, often on the lower extremities.

Although the autoimmune blistering diseases are characterized by the production of antibodies to one or a few target antigens, lupus erythematosus can be thought of as a state of nonspecific B-cell stimulation in which a variety of autoantibodies are formed against many different antigens and especially many directed against cell nuclear components. Genetic defects are not well delineated in lupus, though they exist. Various stimuli, including ultraviolet radiation, infection, drugs, or others initiate or unmask the impaired homeostasis between T cells and B cells, resulting in the hyperactive B-cell state. The wide variety of antibodies produced, both specific and nonspecific, results in multisystem disease.

The American College of Rheumatology has defined the clinical criteria for systemic lupus erythematosus as four or more of the following: malar rash, discoid rash, photosensitivity, oral ulcers, arthritis, serositis, renal disorder, neurologic disorder, hematologic disorder, immunologic disorder, and the presence of antinuclear antibody. Lupus can also be confined exclusively to the skin with the formation of characteristic, initially erythematous but eventually hypopigmented, atrophic plaques known as "discoid" plaques in chronic discoid lupus erythematosus. Progression from purely cutaneous to systemic disease is rare (approximately 1% to 5%). Chronic cutaneous and systemic lupus erythematosus likely represent poles of a clinical spectrum with subacute cutaneous lupus and possibly Sjögren's syndrome being intermediate forms of the same underlying pathomechanism.

The field of photoimmunology concerns the study of the effects of nonionizing radiation on the immune function of animals and humans. It is a relatively new area of investigation that developed as the result of seminal studies by Kripke et al in the 1970s. In those studies the investigators showed that irradiation of mouse skin with ultraviolet radiation (UVR) resulted in a suppression of the animal's ability to reject transplanted skin tumors.

Although most of the studies were done in mice, studies in humans have likewise shown that UVB irradiation of skin was capable of suppressing the induction of contact allergy. Interestingly, suppression of contact allergy by UVB irradiation occured in 40% of normal individuals but in 95% of individuals who had previously been treated for UV-induced skin cancer. This findind indicated that photoimmunosuppression might play a role in the occurence of skin cancer in humans.

In animal models photoimmunosuppression has also been shown to down-regulate the response of the animal to numerous infectious agents, including herpes simplex virus, Candida albicans, Leishmania major, and Mycobacterium tuberculosis.

The mechanism by which UVR induces immunosuppression appears to be multifactorial and involves many cellular components of the skin. Irradiation of the skin of mice and humans has been shown to damage Langerhans' cells. These cells showed loss of dendricity, adenosine triphosphatase activity, major histocompatibility complex class II molecules activity, and the ability to activate T cells. It has been reasoned that this damage to structure and loss of junctions of Langerhans' cells would interfere with the presentation of antigens to T cells and might allow for presentation by other antigen-presenting cells, which might lead to toleration rather than sensitization. Other studies suggest that the inability of irradiated Langerhans' cells to stimulate T cells might apply only to Th1 cells, leaving the cells capable of activating the Th2 arm of the immune response and, furthermore, that the Th2 cytokine production might inhibit the Th1 response, leading to anergy to antigens presented to irradiated animals.

The keratinocyte has also been shown to participate in UVR-induced immunosuppression by generating soluble factors that are immunosuppressive. The two cytokines implicated in photoimmunosuppression are IL-10 and tumor necrosis factor-a. Antibodies directed at the former ablate UVR-induced suppression. The latter has been shown to induce changes in Langerhans' cells mimicking the effects of UVR when injected into skin.

The molecular basis of the response has also been extensively studied. Various researchers have reported that the initial event in the complex cascade of photoimmunosuppression may be UVR-induced DNA damage, UVR-induced alteration of signal-transduction systems in various cell types, or UVR-induced isomerization of trans-urocanic acid to the cis form. All three may in fact play a role in this complex response. The final effect of UVR exposure on immune function in humans is still ill defined but will certainly prove to be of significance.