Chapter 7. Development and Structure of Skin

Skin is a complex organ that protects its host from its environment, at the same time allowing interaction with its environment. It is much more than a static, impenetrable shield against external insults. Rather, skin is a dynamic, complex, integrated arrangement of cells, tissues, and matrix elements that mediates a diverse array of functions: skin provides a physical permeability barrier, protection from infectious agents, thermoregulation, sensation, ultraviolet (UV) protection, wound repair and regeneration, and outward physical appearance (Table 7-1). These various functions of skin are mediated by one or more of its major regions—the epidermis, dermis, and hypodermis (Fig. 7-1; see also Fig. 6-1, Chapter 6). These divisions are interdependent, functional units; each region of skin relies upon, and is connected with, its surrounding tissue for regulation and modulation of normal structure and function at molecular, cellular, and tissue levels of organization (see Chapter 6).

Whereas the epidermis and its outer stratum corneum provide a large part of the physical barrier provided by skin, the structural integrity of skin as a whole is provided primarily by the dermis and hypodermis. Antimicrobial activities are provided by the innate immune system and antigen-presenting dendritic cells of the epidermis, circulating immune cells that migrate from the dermis, and antigen-presenting cells of the dermis (see Chapter 10). Protection from UV irradiation is provided in great measure by the most superficial cells of the epidermis. Inflammation begins with the keratinocytes of the epidermis or immune cells of the dermis, and sensory apparatus emanates from nerves that initially traverse the hypodermis to the dermis and epidermis, ending in specialized receptive organs or free nerve endings. The largest blood vessels of the skin are found in the hypodermis, which serve to transport nutrients and immigrant cells (see Fig. 6-1, Chapter 6). The cutaneous lymphatics course through the dermis and hypodermis, serving to filter debris and regulate tissue hydration. Epidermal appendages provide special protective or sensory functions. Skin also determines a person's physical appearance, influenced by pigmentation provided by melanocytes, with body contours, appearance of age, and actinic damage influenced by the epidermis, dermis, and hypodermis. The skin begins to be organized during embryogenesis, where intercellular and intracellular signals, as well as reciprocal cross talk between different tissue layers, are instrumental in regulating the eventual maturation of the different components of skin.

What follows is an integrated description of the major structural features of the skin and how these structures allow the skin to achieve its major functions, followed by a review of their embryologic origins. Also highlighted are illustrative cutaneous diseases that manifest when these functions are defective. Understanding the genetic and molecular bases of skin disease has confirmed, and in some cases revealed, the many factors and regulatory elements that play critical roles in skin function.

One of the most fundamental and visible features of skin is the stratified, cornified epidermis (Fig. 7-2). The epidermis is a continually renewing structure that gives rise to derivative structures called appendages (pilosebaceous units, nails, and sweat glands). The epidermis ranges in thickness from 0.4 to 1.5 mm, as compared with the 1.5- to 4.0-mm full-thickness skin. The majority of cells in the epidermis are keratinocytes that are organized into four layers, named for either their position or a structural property of the cells. These cells progressively differentiate from proliferative basal cells, attached to the epidermal basement membrane, to the terminally differentiated, keratinized stratum corneum, the outermost layer and barrier of skin (see Chapter 46). Intercalated among the keratinocytes at various levels are the immigrant resident cells—melanocytes, Langerhans cells, and Merkel cells. Other cells, such as lymphocytes, are transient inhabitants of the epidermis and are extremely sparse in normal skin. There are many regional differences in the epidermis and its appendages. Some of these differences are apparent grossly, such as thickness (e.g., palmoplantar skin vs. truncal skin, Fig. 7-3); other differences are microscopic.

Pathologic changes in the epidermis can occur as a result of a number of different stimuli: repetitive mechanical trauma (as in lichen simplex chronicus), inflammation (as in atopic dermatitis and lichen planus), infection (as in verruca vulgaris), immune system activity and cytokine abnormalities (as in psoriasis, Fig. 7-4), autoantibodies (as in pemphigus vulgaris and bullous pemphigoid), or genetic defects that influence differentiation or structural proteins [as in epidermolysis bullosa (EB) simplex, epidermolytic ichthyosis and other ichthyoses, and Darier disease].

Layers of the Epidermis

Basal Layer

The keratinocyte is an ectodermally derived cell and is the primary cell type in the epidermis, accounting for at least 80% of the total cells. The ultimate fate of these cells is to contribute the components for the epidermal barrier as the stratum corneum. Thus, much of the function of the epidermis can be gleaned from the study of the structure and development of the keratinocyte.

Keratinocyte differentiation (keratinization) is a genetically programed, carefully regulated, complex series of morphologic changes and metabolic events whose endpoint is a terminally differentiated, dead keratinocyte (corneocyte) that contains keratin filaments, matrix protein, and a protein-reinforced plasma membrane with surface-associated lipids (see Chapter 46).

Keratins are a family of intermediate filaments and are the hallmark of all epithelial cells, including keratinocytes.1,2 They serve a predominantly structural role in the cells. Fifty-four different functional keratin genes have been identified in humans—34 epithelial keratins and 17 hair keratins.3 The coexpression of specific keratin pairs is dependent on cell type, tissue type, developmental stage, differentiation stage, and disease condition (Table 7-2). Furthermore, the critical role of these molecules is underscored by the numerous manifestations of disease that arise because of mutations in these genes (see Table 7-2). Thus, knowledge of keratin expression, regulation, and structure provides insight into epidermal differentiation and structure.

The basal layer (stratum germinativum) contains mitotically active, columnar-shaped keratinocytes that attach via keratin filaments (K5 and K14) to the basement membrane zone at hemidesmosomes (see Chapter 53), attach to other surrounding cells through desmosomes, and that give rise to cells of the more superficial, differentiated epidermal layers. Membrane-bound vacuoles that contain pigmented melanosomes are transferred from melanocytes by phagocytosis.4 The pigment within melanosomes contributes to the overall skin pigmentation perceived macroscopically.5 The basal layer is the primary location of mitotically active cells of the epidermis. Cell kinetic studies suggest that the basal layer cells exhibit different proliferative potentials (stem cells, transit amplifying cells, and postmitotic cells), and in vivo and in vitro studies suggest that there exist long-lived epidermal stem cells (see Chapter 45).6,7 Because basal cells can be expanded in tissue culture and used to reconstitute sufficient epidermis to cover the entire skin surface of burn patients,8,9 such a starting population is presumed to contain long-lived stem cells with extensive proliferative potential, located within the basal epidermal layer (at the base of epidermal proliferating units) and the hair follicle bulge.10–13

The second type of cell, the transit amplifying cells of the basal layer, arises as a subset of daughter cells produced by the infrequent division of stem cells, either by symmetric or asymmetric cell division.14 These cells provide the bulk of the cell divisions needed for stable self-renewal and are the most common cells in the basal compartment. These cells subsequently give rise to the third class of epidermal basal cells, the postmitotic cells that undergo terminal differentiation. In humans, the normal transit time for a basal cell, from the time it loses contact with the basal layer to the time it enters the stratum corneum, is at least 14 days. Transit through the stratum corneum and subsequent desquamation require another 14 days. These periods of time can be altered in hyperproliferative or growth-arrested states.

Spinous Layer

The shape, structure, and subcellular properties of spinous cells correlate with their position within the midepidermis. They are named for the spine-like appearance of the cell margins in histologic sections. Suprabasal spinous cells are polyhedral in shape with a rounded nucleus. As these cells differentiate and move upward through the epidermis, they become progressively flatter and develop organelles known as lamellar granules (see Section “Granular Layer”). Spinous cells also contain large bundles of keratin filaments, organized around the nucleus and inserted into desmosomes peripherally.

Spinous cells retain the stable K5/K14 keratins that are produced in the basal layer and only synthesize new messenger RNA (mRNA) for these proteins in hyperproliferative disorders. Instead, new synthesis of the K1/K10 keratin pair occurs in this epidermal layer. These keratins are characteristic of an epidermal pattern of differentiation and thus are referred to as the differentiation-specific or keratinization-specific keratins. However, in hyperproliferative conditions such as psoriasis, actinic keratoses, and wound healing, synthesis of K1 and K10 mRNA and protein is downregulated, and the synthesis and translation of messages for K6 and K16 are favored. Correlated with this change in keratin expression is a disruption of normal differentiation in the subsequent granular and cornified epidermal layers (see Sections “Granular Layer” and “Stratum Corneum”). mRNA for K6 and K16 are present throughout the epidermis normally, but the message is only translated on stimulation of proliferation.

The “spines” of spinous cells are abundant desmosomes, calcium-dependent cell surface modifications that promote adhesion of epidermal cells and resistance to mechanical stress (see Chapters 46 and 53).15 Although desmosomes are related to adherens junctions, the latter associate with actin microfilaments at cell–cell interfaces, via a distinct set of cadherins (e.g., E-cadherin) and intracellular catenin adapter molecules. That the desmosomes are integral mediators of intercellular adhesion is clearly demonstrated in diseases in which these structures are disrupted, by genetic disorders, autoantibodies, or bacterial proteases (Table 7-3).16,17

The importance of calcium as a mediator of adhesion is well illustrated in the cases of two conditions that exhibit characteristic epidermal dyscohesion: (1) Darier disease (keratosis follicularis) and (2) Hailey–Hailey disease (benign chronic pemphigus) (see Chapter 51).18 Both of these diseases are caused by mutations in genes that regulate calcium transport, SERCA2 in Darier disease and ATP2C1 in Hailey–Hailey disease.

Lamellar granules are also formed in this layer of epidermal cells (Fig. 7-5). These secretory organelles deliver precursors of stratum corneum lipids into the intercellular space (see Chapter 47). Genetic diseases demonstrate the importance of steroid and lipid metabolism for sloughing of cornified cells—in recessive X-linked ichthyosis, for example, mutation of steroid sulfatase results in a retention hyperkeratosis (see Chapter 49).19

Granular Layer

Named for the basophilic keratohyalin granules that are prominent within cells at this level of the epidermis, the granular layer is the site of generation of a number of the structural components that will form the epidermal barrier, as well as a number of proteins that process these components (see Fig. 7-2).20,21 Keratohyalin granules (see Fig. 7-5) are composed primarily of profilaggrin, keratin filaments, and loricrin. It is in this layer that the cornified cell envelope begins to form, with the conversion of profilaggrin to filaggrin. After aggregation with keratin to form macrofilaments, filaggrin is degraded into molecules such as urocanic acid and pyrrolidone carboxylic acid, which contribute to hydration of the stratum corneum and help filter UV radiation. Loricrin is a cysteine-rich protein that forms the major protein component of the cornified envelope. Upon its release from keratohyalin granules, loricrin binds to desmosomal structures and is subsequently cross-linked to the plasma membrane by tissue transglutaminases (TGMs, primarily TGMs 3 and 1) to form the cornified cell envelope.

Mutations in the TGM1 gene have been shown to be the basis of some cases of lamellar ichthyosis.22,23 Another form of ichthyosis, ichthyosis vulgaris, is caused by mutations in the gene encoding filaggrin.24,25 Loricrin abnormalities result in a form of Vohwinkel syndrome with ichthyosis and pseudoainhum, as well as the disease progressive symmetric keratodermia.26–28 These findings emphasize the importance of proper formation of the cornified envelope in normal epidermal keratinization.

The final stage of granular cell differentiation into a corneocyte involves the cell's own programed destruction, during which process almost all cellular contents are destroyed, with the exception of the keratin filaments and filaggrin matrix.20

Stratum Corneum (See Chapter 47)

Complete differentiation of granular cells results in stacked layers of anucleate, flattened cornified cells that form the stratum corneum. It is this layer that provides mechanical protection to the skin and a barrier to water loss and permeation of soluble substances from the environment.21,29 The stratum corneum barrier is formed by a two-compartment system of lipid-depleted, protein-enriched corneocytes surrounded by a continuous extracellular lipid matrix. These two compartments provide somewhat segregated but complementary functions that together account for the “barrier activity” of the epidermis. Regulation of permeability, desquamation, antimicrobial peptide activity, toxin exclusion, and selective chemical absorption are all primarily functions of the extracellular lipid matrix. On the other hand, mechanical reinforcement, hydration, cytokine-mediated initiation of inflammation, and protection from UV damage are all provided by the corneocytes.

Nonkeratinocytes of the Epidermis

Melanocytes are neural crest-derived, pigment-synthesizing dendritic cells that reside primarily in the basal layer (see Chapter 72).30 The function of melanocytes has been highlighted by disorders in melanocyte number or function. The classic dermatologic disease, vitiligo, is caused by the autoimmune depletion of melanocytes.31 Causes of other disorders of pigmentation are found in various defects in melanogenesis, including melanin synthesis, melanosome production, and melanosome transport and transfer to keratinocytes (see Chapters 72 and 75). Regulation of melanocyte proliferation and homeostasis is under intensive study as well as a means to understanding melanoma (see Chapter 124).32 Keratinocyte–melanocyte interactions are critical for melanocyte homeostasis and differentiation, influencing proliferation, dentricity, and melanization.

Merkel cells are slow-adapting type I mechanoreceptors located in sites of high-tactile sensitivity (see Chapter 120).33 They are present among basal keratinocytes in hairy skin and in the glabrous skin of the digits, lips, regions of the oral cavity, and the outer root sheath of the hair follicle. Keratin 20 is restricted to Merkel cells in the skin and thus may be the most reliable molecular marker. Ultrastructurally, Merkel cells are easily identified by the membrane-bounded, dense-core granules that collect opposite the Golgi and proximal to an unmyelinated neurite (Fig. 7-6). These granules contain neurotransmitter-like substances and markers of neuroendocrine cells, including Met-enkephalin, vasoactive intestinal peptide, neuron-specific enolase, and synaptophysin. Although increasingly more is being learned about the normal function of Merkel cells, they are of particular clinical note because Merkel cell-derived neoplasms are particularly aggressive and difficult to treat (see Chapter 120).

Langerhans cells are dendritic antigen-processing and antigen-presenting cells in the epidermis (see Chapter 10).34 Although they are not unique to the epidermis, they form 2% to 8% of the total epidermal cell population, mostly found in a suprabasal position. The cytoplasm of the Langerhans cells contains characteristic small rod- or racket-shaped structures called Langerhans cell granules or Birbeck granules (Fig. 7-7). Langerhans cells principally function to sample and present antigens to T cells of the epidermis. Because of these functions, they are implicated in the pathologic mechanisms underlying allergic contact dermatitis, cutaneous leishmaniasis, and human immunodeficiency virus infection. Langerhans cells are reduced in the epidermis of patients with certain conditions, such as psoriasis, sarcoidosis, and contact dermatitis; they are functionally impaired by UV radiation, especially UVB.

Because of their effectiveness in antigen presentation and lymphocyte stimulation, dendritic cells and Langerhans cells have become prospective vehicles for tumor therapy and tumor vaccines. These cells are loaded with tumor-specific antigens, which will then stimulate the host immune response to mount an antigen-specific, and therefore tumor-specific, response.

The dermal–epidermal junction (DEJ) is a basement membrane zone that forms the interface between the epidermis and dermis (see Chapter 53).35,36 The major functions of the DEJ are to attach the epidermis and dermis to each other and to provide resistance against external shearing forces. It serves as a support for the epidermis, determines the polarity of growth, directs the organization of the cytoskeleton in basal cells, provides developmental signals, and serves as a semipermeable barrier.

The DEJ can be subdivided into three supramolecular networks: (1) the hemidesmosome-anchoring filament complex, (2) the basement membrane itself, and (3) the anchoring fibrils. The critical role of this region in maintaining skin structural integrity is revealed by the large number of mutations in DEJ components that cause blistering diseases of varying severity, covered in detail in Chapter 62. These bullous diseases are grouped according to the level of the cleavage within the DEJ—the most superficial, EB simplex, involves basal keratinocyte cleavage. Junctional EB occurs within the lamina lucida and lamina densa regions. Dystrophic EB is the deepest level of blistering, within the sublamina densa/anchoring filaments. Chapter 53 provides a detailed discussion of the DEJ networks.

The dermis is an integrated system of fibrous, filamentous, diffuse, and cellular connective tissue elements that accommodates nerve and vascular networks, epidermally derived appendages, and contains many resident cell types, including fibroblasts, macrophages, mast cells, and transient circulating cells of the immune system (see Figs. 6-9 and 6-14). The dermis makes up the majority of skin and provides its pliability, elasticity, and tensile strength. It protects the body from mechanical injury, binds water, aids in thermal regulation, and includes receptors of sensory stimuli. The dermis interacts with the epidermis in maintaining the properties of both tissues, collaborates during development in the morphogenesis of the DEJ and epidermal appendages (see Section “Development of Skin Appendages”), and interacts in repairing and remodeling skin after wounding.

The dermis is arranged into two major regions: (1) the upper papillary dermis and (2) the deeper reticular dermis. These two regions are readily identifiable on histologic section, and they differ in their connective tissue organization, cell density, and nerve and vascular patterns. The papillary dermis abuts the epidermis, molds to its contours, and is usually no more than twice its thickness (see Fig. 6-9). The reticular dermis forms the bulk of the dermal tissue. It is composed primarily of large-diameter collagen fibrils, organized into large, interwoven fiber bundles, with branching elastic fibers surrounding the bundles (see Fig. 6-14). In normal individuals, the elastic fibers and collagen bundles increase in size progressively toward the hypodermis. The subpapillary plexus, a horizontal plane of vessels, marks the boundary between the papillary and reticular dermis. The lowest boundary of the reticular dermis is defined by the transition of fibrous connective tissue to adipose connective tissue of the hypodermis.

Fibrous Matrix of the Dermis

The connective tissue matrix of the dermis is comprised primarily of collagenous and elastic fibrous tissue.37,38 These are combined with other, nonfibrous connective tissue molecules, including finely filamentous glycoproteins, proteoglycans (PGs), and glycosaminoglycans (GAGs) of the “ground substance.” 39

Collagen forms the bulk of the acellular portion of the dermis, accounting for approximately 75% of the dry weight of skin, and providing both tensile strength and elasticity. (For details regarding the polypeptide structure and distribution of collagens, see Chapter 63.) The periodically banded, interstitial collagens account for the greatest proportion of collagen in adult dermis (type I, 80% to 90%; type III, 8% to 12%; and type V, <5%). Type VI collagen is associated with fibril and in the interfibrillar spaces. Type IV collagen is confined to the basal lamina of the DEJ, vessels, and epidermal appendages. Type VII collagen forms anchoring fibrils at the DEJ.

Elastic connective tissue (see Chapter 63) is a complex molecular mesh, extending from the lamina densa of the DEJ throughout the dermis and into the connective tissue of the hypodermis.38 Elastic fibers return the skin to its normal configuration after being stretched or deformed. They are also present in the walls of cutaneous blood vessels and lymphatics and in the sheaths of hair follicles. Mutations in elastin, the elastic fiber matrix component, cause the disease cutis laxa. Elastic fibers are normally located between bundles of collagen fibers, although in certain pathologic conditions, such as Buschke–Ollendorff syndrome, both elastic and collagen fibers become assembled within the same bundle. The importance of the elastic fiber network is clearly seen in the number of multisystem diseases that arise because of mutations in components of this network. The defect underlying pseudoxanthoma elasticum (PXE) is a mutation in ABCC6, a member of the large adenosine triphosphate-dependent transmembrane transporter family. Thus, this disease that is characterized by loss of skin elasticity and calcified elastic fibers is unlikely a primary defect in elastic tissue, but rather a metabolic disorder with secondary involvement of elastic fibers.40–42 In addition to genetic mutations, solar radiation and aging also contribute to elastic fiber damage.43

Filamentous and Diffuse Matrix Components of the Dermis (See Chapter 63)

The fibrous and cellular matrix elements are embedded within more amorphous matrix components, which also are found in basement membranes.44–46 PGs are large molecules consisting of a core protein that determines which GAGs will be incorporated into the molecule. The PG/GAG complex can bind water up to 1,000 times its own volume and have roles in regulation of water binding and compressibility of the dermis, as well as increasing local concentrations of growth factors through binding (e.g., basic fibroblast growth factor). They also link cells with the fibrillar and filamentous matrix, influencing proliferation, differentiation, tissue repair, and morphogenesis.

The major PGs in the adult dermis are chondroitin sulfates/dermatan sulfate, including biglycan, decorin, and versican; heparan/heparan sulfate PGs, including perlecan and syndecan; and chondroitin-6 sulfate PGs, which are components of the DEJ (see Chapter 63). Glycoproteins interact with other matrix components via integrin receptors. They facilitate cell migration, adhesion, morphogenesis, and differentiation. Fibronectin is synthesized by both epithelial and mesenchymal cells, and it covers collagen bundles and the elastic network. Vitronectin is present on all elastic fibers except for oxytalan. Tenascin is found around the smooth muscle of blood vessels, arrector pili muscles, and appendages such as sweat glands.

Cellular Components of the Dermis

Fibroblasts, macrophages, and mast cells are the regular residents of the dermis, mostly found around the papillary region and surrounding vessels of the subpapillary plexus (see Fig. 6-20), as well as in the reticular dermis between collagen fiber bundles. The fibroblast is a mesenchymally derived cell that migrates through the tissue and is responsible for the synthesis and degradation of fibrous and nonfibrous connective tissue matrix proteins and a number of soluble factors. Fibroblasts provide a structural extracellular matrix framework as well as promote interaction between epidermis and dermis by synthesis of soluble mediators. Studies of human fibroblasts indicate that even within a single tissue, phenotypically distinct populations exist, some of which relate to regional anatomical differences.47,48 These cells are also instrumental in wound healing and scarring, increasing their proliferative and synthetic activity during these processes.

The monocytes, macrophages, and dermal dendrocytes constitute the mononuclear phagocytic system of cells in the skin. Macrophages are derived from precursors in the bone marrow, differentiate into circulating monocytes, and then migrate into the dermis to differentiate. These cells are phagocytic; process and present antigen to immunocompetent lymphoid cells; are microbicidal, tumoricidal, secretory, and hematopoietic (see Chapter 10); and are involved in coagulation, atherogenesis, wound healing, and tissue remodeling.

Mast cells (see Chapter 149) are specialized secretory cells that, in skin, are present in greatest density in the papillary dermis, near the DEJ, in sheaths of epidermal appendages, and around blood vessels and nerves of the subpapillary plexus. The surface of dermal mast cells is coated with fibronectin, which probably assists in securing cells within the connective tissue matrix. Mast cells are secretory cells that are responsible for immediate-type hypersensitivity reaction in skin and are involved in the production of subacute and chronic inflammatory disease. They synthesize secretory granules composed of histamine, heparin, tryptase, chymase, carboxypeptidase, neutrophil chemotactic factor, and eosinophilic chemotactic factor of anaphylaxis, which are mediators in these processes. Mast cells can become hyperplastic and hyperproliferative in mastocytosis (see Chapter 149).

The dermal dendrocyte is a dendritic, highly phagocytic fixed connective tissue cell in the dermis of normal skin. Similar to many other bone marrow-derived cells, dermal dendrocytes express factor XIIIa and CD45, and they lack typical markers of fibroblasts. These cells are particularly abundant in the papillary dermis and upper reticular dermis, frequently in the proximity of vessels of the subpapillary plexus. Dermal dendrocytes function in the afferent limb of an immune response as antigen presenting cells (see Chapter 10). They are also likely the cells of origin of a number of benign fibrotic proliferative conditions in the skin, such as dermatofibromas and fibroxanthomas (see Chapter 66).

Blood Vessels (See Chapter 162)

The blood vessels of skin provide nutrition for the tissue and are involved in temperature and blood pressure regulation, wound repair, and numerous immunologic events.49 The microcirculatory beds in skin progress from arterioles to precapillary sphincters. Extending from the sphincters are arterial and venous capillaries, which become postcapillary venules, and finally, collecting venules. When compared with vasculature of other organs, the vessels of skin are adapted to shearing forces, as they have thick walls supported by connective tissue and smooth muscle cells. Special cells, known as veil cells, surround the cutaneous microcirculation, defining a domain for the vessels within the dermis while remaining separate from the vessel walls.

The rich vascular network of the skin is located at boundaries within the dermis and supplies the epidermal appendages (see Fig. 163-2). The vessels that supply the dermis branch from musculocutaneous arteries that penetrate the subcutaneous fat and enter the deep reticular dermis. At this point, they are organized into a horizontal arteriolar plexus. From this plexus, ascending arterioles extend toward the epidermis. These arterioles contain two layers of smooth muscle cells, as well as pericytes, a second type of contractile cell of the vessel wall. At the junction between the papillary and reticular dermis, terminal arterioles form the subpapillary plexus. Capillary loops then extend from the terminal arterioles of the plexus into the papillary dermis. At the apex of each capillary loop is the thinnest portion, allowing for transport of material out of the capillary. The descending limbs of capillary loops are venous capillaries that drain into venous channels of the subpapillary plexus. The postcapillary venules of the subpapillary plexus are responsive to histamine and are therefore often the sites of inflammatory cells during these responses.

Certain regions of skin, such as the palms and soles, contain direct connections between arterial and venous circulation as potential shunts around congested capillary beds. These sites consist of an ascending arteriole (a glomus body), which is modified by three to six layers of smooth muscle cells and has associated sympathetic nerve fibers.

In the adult, the cutaneous vasculature normally remains quiescent, in part due to inhibition of angiogenesis by factors such as thrombospondin. Pathogenic stimuli sometimes result in secondary angiogenesis, from tumors or during wounding. One of the key mediators of such angiogenesis is vascular endothelial growth factor (VEGF), often secreted by tumors or by keratinocytes (see Chapter 162).50,51

Numerous disorders can manifest themselves within the cutaneous vasculature. Leukocytoclastic vasculitis (cutaneous necrotizing venulitis) occurs within the venules in response to a number of potential pathogenic mechanisms (see Chapter 163). Stasis dermatitis, urticaria, polyarteritis nodosa, thrombosis, and thrombophlebitis all affect vessels in the skin, of different sizes, some by occlusion of vessels (vasculopathy) and others by inflammation of the vessels (vasculitis).

Lymphatics

The lymph channels of the skin regulate pressure of the interstitial fluid by resorption of fluid released from vessels and in clearing the tissues of cells, proteins, lipids, bacteria, and degraded substances.52,53 The vessels begin in blind-ending initial lymphatics in the papillary dermis. They drain into a horizontal plexus of larger lymph vessels located deep to the subpapillary venous plexus. A vertical system of lymphatics then carries fluid and debris through the reticular dermis to another deeper collecting plexus at the reticular dermis–hypodermis border. Lymph flow within the skin depends on movements of the tissue caused by arterial pulsations and larger scale muscle contractions and movement of the body, with backflow prevented by bicuspid-like valves within the vessels.

Lymphatic vessels are often collapsed in skin and therefore are only seen with difficulty on histologic section. They are composed of a large lumen and a thinner wall than blood vessels. Molecular characterization of these vessels has identified Prox1, VEGFR-3, and LYVE-1 as specific markers of lymphatic character.53

Certain pathologic conditions involve or highlight the function of lymphatic vessels, such as lymphedema, lymphangioma circumscriptum, and stasis dermatitis. The importance of lymphatics in the progression and spread of cancer is also becoming more clear, as melanoma cells destroy endothelial cells of the initial lymphatics to gain entry to the lymph circulation, and recent studies have shown that tumors themselves can promote lymphangiogenesis as part of their early program on the way to metastasis.51,54 The discovery of the molecular defects in hereditary lymphedemas has implicated the VEGFR-3 and FoxC2 in lymphatic development. One of the most heavily studied lymphangiogenic molecules is VEGF-C (Chapter 162).

See Chapters 102 and 103.

The nerve networks of the skin contain somatic sensory and sympathetic autonomic fibers.55 The sensory fibers alone (free nerve endings) or in conjunction with specialized structures (corpuscular receptors) function as receptors of touch, pain, temperature, itch, and mechanical stimuli. The density and types of receptors are regionally variable, accounting for the variation in acuity at different sites of the body. Receptors are particularly dense in hairless areas such as the areola, labia, and glans penis. Sympathetic motor fibers are codistributed with the sensory nerves in the dermis until they branch to innervate the sweat glands, vascular smooth muscle, the arrector pili muscle of hair follicles, and sebaceous glands.

The nerves of skin branch from musculocutaneous nerves that arise segmentally from spinal nerves. The pattern of nerve fibers in skin is similar to the vascular patterns—nerve fibers form a deep plexus, then ascend to a superficial, subpapillary plexus.

Free nerve endings include the penicillate and papillary nerve fibers and are the most widespread sensory receptors in skin. In humans, they are ensheathed by Schwann cells and a basal lamina. Free nerve endings are particularly common in the papillary dermis.

The penicillate fibers are the primary nerve fibers found subepidermally in haired skin. These are rapidly adapting receptors that function in the perception of touch, temperature, pain, and itch. Because of overlapping innervation, discrimination tends to be generalized in these regions. On the other hand, free nerve endings present in nonhaired, ridged skin, such as the palms and soles, project individually without overlapping distribution and so are thought to function in fine discrimination.

Papillary nerve endings are found at the orifice of a follicle and are thought to be particularly receptive to cold sensation. Hair follicles also contain other receptors, slow-adapting receptors that respond to the bending or movement of hairs. Cholinergic sympathetic fibers en route to the eccrine sweat gland and adrenergic and cholinergic fibers en route to the arrector pili muscle are carried along with the sensory fibers in the hair basket.

Free nerve endings are also associated with individual Merkel cells. In haired skin, touch domes are associated with hair follicles. In palmoplantar skin, these complexes are found at the site where the eccrine sweat duct penetrates a glandular epidermal papilla.

Corpuscular receptors, both Meissner's and Pacinian, contain a capsule and inner core and are composed of both neural and nonneural components. The capsule is a continuation of the perineurium, and the core includes the nerve fiber surrounded by lamellated wrappings of Schwann cells. Meissner's corpuscles are elongated or ovoid mechanoreceptors located in the dermal papillae of digital skin and oriented vertically toward the epidermal surface (Fig. 7-8).

The Pacinian corpuscle lies in the deep dermis and subcutaneous tissue of skin that covers weight-bearing surfaces of the body. It has a characteristic capsule and lamellar wrappings (Fig. 7-9). Pacinian corpuscles serve as rapidly adapting mechanoreceptors that respond to vibrational stimuli.

The tissue of the hypodermis insulates the body, serves as a reserve energy supply, cushions and protects the skin, and allows for its mobility over underlying structures. It has a cosmetic effect in molding body contours. The boundary between the deep reticular dermis and the hypodermis is an abrupt transition from a predominantly fibrous dermal connective tissue to a primarily adipose subcutaneous one (see Fig. 6-1, Chapter 6). Despite this clear distinction anatomically, the two regions are still structurally and functionally integrated through networks of nerves and vessels and through the continuity of epidermal appendages. Actively growing hair follicles span the dermis and extend into the subcutaneous fat, and the apocrine and eccrine sweat glands are normally confined to this depth of the skin.

Adipocytes form the bulk of the cells in the hypodermis.56,57 They are organized into lobules defined by septa of fibrous connective tissue. Nerves, vessels, and lymphatics are located within the septa and supply the region. The synthesis and storage of fat continues throughout life by enhanced accumulation of lipid within fat cells, proliferation of existing adipocytes, or by recruitment of new cells from undifferentiated mesenchyme. The hormone leptin, secreted by adipocytes, provides a long-term feedback signal regulating fat mass. Leptin levels are higher in subcutaneous than omental adipose, suggesting a role for leptin in control of adipose distribution as well.

The importance of the subcutaneous tissue is apparent in patients with Werner syndrome (see Chapter 139), in which subcutaneous fat is absent in lesion areas over bone, or with scleroderma (see Chapter 157), where the subcutaneous fat is replaced with dense fibrous connective tissue. Such regions in Werner patients ulcerate and heal poorly. The skin of patients with scleroderma is taut and painful. In the hereditary and acquired lipodystrophies, loss of subcutaneous fat disrupts glucose, triglyceride, and cholesterol regulation, and causes significant cosmetic alteration, increasing the interest in possible hormonal therapy for these disorders (see Chapter 71).58 The subcutaneous tissue is involved in different inflammatory conditions (Chapter 70).

Significant advances in the understanding of the molecular processes responsible for the development of skin have been made over the last several years. Such advances increase the understanding of clinicopathologic correlation among some inherited disorders of skin and allow for the early diagnosis of such diseases. The developmental progression of various components of the skin is well documented, and a time line indicating the events that occur during embryonic and fetal development is provided (Table 7-4).59,60 Of note, the estimated gestational age (EGA) is used throughout this chapter; this system refers to the age of the fetus, with fertilization occurring on day 1. To avoid confusion, it should be pointed out that obstetricians and most clinicians define day 1 as the first day of the last menstrual period (menstrual age), in which fertilization occurs on approximately day 14. Thus, the two dating systems differ by approximately 2 weeks, such that a woman who is 14 weeks pregnant (menstrual age) is carrying a 12-week-old fetus (EGA).

Conceptually, fetal skin development can be divided into three distinct but temporally overlapping stages, those of (1) specification, (2) morphogenesis, and (3) differentiation. These stages roughly correspond to the embryonic period (0–60 days), the early fetal period (2–5 months), and the late fetal period (5–9 months) of development, respectively. The earliest stage, specification, refers to the process by which the ectoderm lateral to the neural plate is committed to become epidermis, and subsets of mesenchymal and neural crest cells are committed to form the dermis. It is at this time that patterning of the future layers and specialized structures of the skin occurs, often via a combination of gradients of proteins and cell–cell signals. The second stage, morphogenesis, is the process by which these committed tissues begin to form their specialized structures, including epidermal stratification, epidermal appendage formation, subdivision between the dermis and subcutis, and vascular formation. The last stage, differentiation, denotes the process by which these newly specialized tissues further develop and assume their mature forms. Table 7-5 integrates specification, morphogenesis, and differentiation with skin morphology and genetic diseases.

For simplification and greater clarity, the stages of development of the epidermis—dermis and hypodermis, dermal–epidermal junction, and epidermal appendages—are presented sequentially.

Epidermis

Embryonic Development

During the third week after fertilization, the human embryo undergoes gastrulation, a complex process of involution and cell redistribution that results in the formation of the three primary embryonic germ layers: (1) ectoderm, (2) mesoderm, and (3) endoderm. Shortly after gastrulation, ectoderm further subdivides into neuroectoderm and presumptive epidermis. The specification of the presumptive epidermis is believed to be mediated by the bone morphogenetic proteins (BMPs). Later during this period, BMPs again appear to play a critical role, along with Engrailed-1 (En1), in specifying the volar versus interfollicular skin.61–63 By 6 weeks EGA, the ectoderm that covers the body consists of basal cells and superficial periderm cells.

The basal cells of the embryonic epidermis differ from those of later developmental stages. Embryonic basal cells are more columnar than fetal basal cells, and they have not yet formed hemidesmosomes. Although certain integrins (e.g., α6β4) are expressed in these cells, they are not yet localized to the basal pole of the cells. Before the formation of hemidesmosomes and desmosomes, intercellular attachment between individual basal cells appears to be mediated by adhesion molecules such as E- and P-cadherin, which have been detected on basal cells as early as 6 weeks EGA. Keratins K5 and K14, proteins restricted to definitive stratified epithelia, are expressed even at these early stages of epidermal formation.

At this stage, periderm cells form a “pavement epithelium.” These cells are embryonic epidermal cells that are larger and flatter than the underlying basal cells. Apical surfaces contact the amniotic fluid and are studded with microvilli. Connections between periderm cells are sealed with tight junctions rather than desmosomes. By the end of the second trimester, these cells are sloughed and eventually form part of the vernix caseosa. Like stratified epithelial cells, periderm cells express K5 and K14, but they also express simple epithelial keratins K8, K18, and K19.

Aplasia cutis (see Chapter 107) may reflect focal defects in either epidermal specification or development caused by somatic mosaicism, or mutations that occur postzygotically. However, the molecular defect for this disorder is not known. The fact that few genetic diseases have been described in which either epidermal specification or morphogenesis is defective likely reflects the fact that such defects would be incompatible with survival.

Early Fetal Development (Morphogenesis)

By the end of 8 weeks of gestation, hematopoiesis has switched from the extraembryonic yolk sac to the bone marrow, the classical division between embryonic and fetal development. By this time, the epidermis begins its stratification and formation of an intermediate layer between the two preexisting cell layers. The cells in this new layer are similar to the cells of the spinous layer in mature epidermis. Like spinous cells, they express keratins K1/K10 and the desmosomal protein desmoglein-3. The cells are still highly proliferative and, during this period of development, they evolve into a multilayer structure that will eventually replace the degenerating periderm.

Expression of the p63 gene plays a critical role in the proliferation and maintenance of the basal layer cells. Epidermal stratification does not occur in mice deficient for p63. In humans, although no null mutations have been isolated, partial loss of p63 function mutations have been identified in ankyloblepharon, ectodermal dysplasia, and cleft lip/palate syndrome (Hay–Wells syndrome) as well as ectrodactyly, ectodermal dysplasia, and cleft lip/palate syndrome (see Chapter 142).64–66 The preexisting basal cell layer also undergoes morphologic changes at this time, becoming more cuboidal and expressing new keratin genes, K6, K8, K19, and K6/K16, that are usually expressed in hyperproliferative tissues. The basal layer also begins to elaborate proteins that will ultimately anchor them to the developing basal lamina (see Section “Dermal–Epidermal Junction”), including hemidesmosomal proteins BPAG1, BPAG2, and collagens V and VII (see Chapters 53, 56, and 62).

Embryonic lines of ectodermal formation are revealed in mosaic disorders that follow the lines of Blaschko, including congenital, nevoid, and acquired conditions.67–69 Molecular demonstration of genetic mosaicism has been reported for a number of X-linked disorders, as well as epidermal nevi in epidermolytic hyperkeratosis.70

Late Fetal Development (Differentiation)

Late fetal development reveals the further specialization and differentiation of keratinocytes in the epidermis. It is at this time that the granular and stratum corneal layers are formed, and the rudimentary periderm is sloughed. Keratinization of the surface epidermis is a process of keratinocyte terminal differentiation, which begins at 15 weeks EGA. The granular layer becomes prominent, and important structural proteins are elaborated in the basal layer cells. The hemidesmosomal proteins plectin and a6b4 integrin are expressed and correctly localized at this time. Mutations in these genes result in various bullous genodermatoses (reviewed in Chapter 62). The more superficial cells undergo further terminal differentiation, and the keratin-aggregating protein filaggrin is expressed at this time.

The formation of the cornified envelope is a late feature of differentiating keratinocytes, and it relies on a number of different modifications to create an impermeable barrier. Enzymes such as transglutaminase, LEKTI (encoded by the gene SPINK-5), phytanoyl coenzyme A reductase, fatty aldehyde dehydrogenase, and steroid sulfatase are all important in the elaboration of the cornified envelope and mature lipid barrier, and defects in these enzymes can lead to abnormal epidermal barrier formation (see Chapter 49).

Specialized Cells Within the Epidermis

The three major nonepidermal cell types—(1) melanocytes, (2) Langerhans cells, and (3) Merkel cells—can be detected within the epidermis by the end of the embryonic period. Melanocytes are derived from the neural crest, a subset of neuroectoderm cells. Pigment mosaicism (formerly called hypomelanosis of Ito and linear and whorled hypermelanosis) (see Chapter 75) following the lines of Blaschko may reflect the migratory paths of melanoblasts, or alternatively, mosaic defects in pigment transfer from melanocytes to keratinocytes. The founders of each melanoblast clone originate at distinct points along the dorsal midline, traversing ventrally and distally to take up residence in the epidermis.

Melanocytes are first seen within the epidermis at 50 days EGA. Melanocytes express integrin receptors in vivo and in vitro and may use these to migrate to the epidermis during embryonic development. Migration, colonization, proliferation, and survival of melanocytes in developing skin depend on the cell surface tyrosine kinase receptor, c-kit, and its ligand, stem cell factor.71,72 Melanin becomes detectable between 3 and 4 months EGA, and by 5 months, melanosomes begin to transfer pigment to keratinocytes. Many genetic disorders of pigmentation have been characterized and are presented in detail in Chapters 73, 75, and 143. In the adult, a pool of melanocyte precursor cells resides in the upper permanent portion of the hair follicle, capable of producing mature melanocytes.71,73,74

Langerhans cells, another immigrant population, are detectable by 40 days EGA. They begin to express CD1 on their surface and to produce their characteristic Birbeck granules by the embryonic–fetal transition. By the third trimester, most of the adult numbers of Langerhans cells will have been produced.75

Merkel cells, as described earlier in the chapter (see Section “Nonkeratinocytes of the Epidermis”), reside in the epidermis. They are first detectable in the volar epidermis of the 11- to 12-week EGA human fetus. The embryonic derivation of this population of cells is controversial, as there is experimental evidence supporting both in situ differentiation of Merkel cells from epidermal ectoderm as well as migration from the neural crest.33,76

Dermal and Subcutaneous Development

The origin of the dermis and subcutaneous tissue is more diverse than that of the epidermis, which is exclusively ectodermally derived. The embryonic tissue that forms the dermis depends on the specific body site.77,78 Dermal mesenchyme of the face and anterior scalp is derived from neural crest ectoderm. The limb and ventral body wall mesenchyme is derived from the lateral plate mesoderm. The dorsal body wall mesenchyme derives from the dermomyotomes of the embryonic somite. LIM homeobox transcription factor 1b (Lmx1B) and Wnt7a are important in the specification of the dorsal limb.79–81 En1 and BMPs, on the other hand, specify the volar (ventral) limb mesenchyme (see Table 7-5).66,80

The embryonic dermis, in contrast to the mature dermis, is cellular and amorphous, with few organized fibers. The mature dermis contains a complex mesh of collagen and elastic fibers embedded in a matrix of PGs, whereas the embryonic mesenchyme contains a large variety of pluripotent cells in a hydrated gel that is rich in hyaluronic acid. These mesenchymal cells are thought to be the progenitors of cartilage-producing cells, adipose tissue, dermal fibroblasts, and intramembranous bone. Dermal fibers exist as fine filaments but not thick fibers. The protein components of the future elastin and collagen fibers are synthesized during this period but not assembled. At this point, there is no obvious separation between cells that will become musculoskeletal elements and those that will give rise to the skin dermis.

Proteus syndrome, exhibits focal defects in multiple tissues, probably and is the result of genetic mosaicism affecting genes important in this process caused by AKT1 associated activating mutations.81a Rarely is the mutation found in peripheral blood cells demonstrating the importance of studying affected tissues (see Chapter 118). Mutations causing a global defect in this process would likely be incompatible with life.

The superficial mesenchyme becomes distinct from the underlying tissue by the embryonic–fetal transition (about 60 days EGA). By 12–15 weeks, the reticular dermis begins to take on its characteristic fibrillar appearance in contrast to the papillary dermis, which is more finely woven. Large collagen fibers continue to accumulate in the reticular dermis, as well as elastin fibers, beginning around midgestation and continuing until birth. By the end of the second trimester, the dermis has changed from a nonscarring tissue to one that is capable of forming scars. As the dermis matures, it also becomes thicker and well organized, such that at birth, it resembles the dermis of the adult, although it is still more cellular.

Many well-known clinical syndromes and molecules have been discovered that affect this final stage of dermal differentiation. These diseases include dystrophic EB (see Chapter 62), Marfan syndrome, Ehlers–Danlos syndrome, cutis laxa, PXE, hereditary hemorrhagic telangiectasia, and osteogenesis imperfecta (see Chapter 137).

Specialized Components of the Dermis

Blood Vessels and Nerves

Cutaneous nerves and vessels begin to form early during gestation, but they do not evolve into those of the adult until a few months after birth. The process of vasculogenesis requires the in situ differentiation of the endothelial cells at the endoderm–mesoderm interface. Originally, horizontal plexuses are formed within the subpapillary and deep reticular dermis, which are interconnected by groups of vertical vessels. This lattice of vessels is in place by 45–50 days EGA.

At 9 weeks EGA, blood vessels are seen at the dermal–hypodermal junction. By 3 months, the distinct networks of horizontal and vertical vessels have formed. By the fifth month, further changes in the vasculature derive from budding and migration of endothelium from preexisting vessels, the process of angiogenesis. Depending on the body region, gestational age, and presence of hair follicles and glands, this pattern can vary with blood supply requirements.

Defects in vascular development have been described (see Chapter 172). In the Klippel–Trénaunay syndrome, unilateral cutaneous vascular malformations develop, with associated venous varicosities, edema, and hypertrophy of associated soft tissue and bone. In Sturge–Weber syndrome, many cutaneous capillary malformations are seen in the lips, tongue, nasal, and buccal mucosae. Some familial defects in vascular formation result from mutations in the gene encoding Tie-2 receptor tyrosine kinase. Capillary malformations seen in hereditary hemorrhagic telangiectasia have been linked to mutations in transforming growth factor-β-binding proteins—endoglin, and activin receptor-like kinase 1.

Lymphatics

Accumulating evidence suggests that lymphatics originate from endothelial cells that bud off from veins. The pattern of embryonic lymphatic vessel development parallels that of blood vessels. Recent studies have identified new genes that appear to be specific for some of the earliest lymphatic precursors. LYVE-1 and Prox-1 are genes considered to be critical for earliest lymphatic specification, whereas VEGF-R3 and SLC may be important in later lymphatic differentiation.53

Nerves

The development of cutaneous nerves parallels that of the vascular system in terms of patterning, maturation, and organization. Nerves of the skin consist of somatic sensory and sympathetic autonomic fibers, which are predominantly small and unmyelinated. As these nerves develop, they become myelinated, with associated decrease in the number of axons. This process may continue as long as puberty.

Subcutis

As mentioned in Section “Specialized Components of the Dermis,” by 50–60 days EGA, the hypodermis is separated from the overlying dermis by a plane of thin-walled vessels. Toward the end of the first trimester, the matrix of the hypodermis can be distinguished from the more fibrous matrix of the dermis. By the second trimester, adipocyte precursors begin to differentiate and accumulate lipids. By the third trimester, fat lobules and fibrous septae are found to separate the mature adipocytes. The molecular pathways that define this process are currently an area of intense investigation. Although few regulators important in embryonic adipose specification and development have been identified, several factors critical for preadipocyte differentiation have been demonstrated, including leptin, a hormone important in fat regulation, and the peroxisome proliferator-activated receptor family of transcription factors.57

Dermal–Epidermal Junction

The dermal–epidermal junction is an interface where many inductive interactions occur that result in the specification or differentiation of the characteristics of the dermis and epidermis. This zone includes specialized basement membrane, basal cell extracellular matrix, the basal-most portion of the basal cells, and the superficial-most fibrillar structures of the papillary dermis. Both the epidermis and dermis contribute to this region.

As early as 8 weeks EGA, a simple basement membrane separates the dermis from the epidermis and contains many of the major protein elements common to all basement membranes, including laminin 1, collagen IV, heparin sulfate, and PGs. Components specific to the cutaneous basement membrane zone, such as proteins of the hemidesmosome and anchoring filaments, are first detected at the embryonic–fetal transition. By the end of the first trimester, or around the time of late embryonic development, all basement membrane proteins are in place. The α6 and β4 integrin subunits are expressed earlier than most of the other basement membrane components. However, they are not localized to the basal surface until 9.5 weeks EGA, coincident with the time that the hemidesmosomal proteins are expressed and hemidesmosomes are first observed. At the same time, anchoring filaments (laminin-332) and anchoring fibrils (collagen VII) begin to be assembled. The actual synthesis of collagen VII can be detected slightly earlier, at 8 weeks EGA.

Many congenital blistering disorders have been demonstrated to be a result of defects in proteins of the DEJ (for details, see Chapters 53 and 62). The severity of the disease, plane of tissue separation, and involvement of noncutaneous tissues depend on the proteins involved and the specific mutations. These genes are important candidates for prenatal testing.

Skin appendages, which include hair, nails, and sweat and mammary glands, are composed of two distinct components: (1) an epidermal portion, which produces the differentiated product, and (2) the dermal component, which regulates differentiation of the appendage. During embryonic development, dermal–epidermal interactions are critical for the induction and differentiation of these structures (Fig. 7-10). Disruption of these signals often has profound influences on development of skin appendages. Hair differentiation serves as a paradigm for appendageal development, because it is the appendage that has been studied most intensely.82,83

Hair (See Chapter 86)

Dermal signals are initially responsible for instructing the basal cells of the epidermis to begin to crowd at regularly spaced intervals, starting between days 75 and 80 on the scalp. This initial grouping is known as the follicular placode or anlage. From the scalp, follicular placode formation spreads ventrally and caudally, eventually covering the skin. The placodes then signal back to the underlying dermis to form a “dermal condensate,” which occurs at 12–14 weeks EGA. This process is thought to be a balance of placode promoters and placode inhibitors.83 Wnt family signaling molecules are proposed to promote placode formation, whereas BMP family molecules are postulated to inhibit follicle formation. Subsequent reciprocal signaling between the epidermal and dermal components of the appendage result in its ultimate development and maturation.

In addition to the widened bulge at the base, two other bulges form along the length of the developing follicle, termed the bulbous hair peg. The uppermost bulge is the presumptive sebaceous gland, whereas the middle bulge serves as the site for insertion of the arrector pili muscle. This middle bulge is also the location of the multipotent hair stem cells, which are capable of differentiating into any of the cells of the hair follicle, and also have the potential to replenish the entire epidermis, as has been seen in cases of extensive surface wounds or burns.

By 19–21 weeks EGA, the hair canal is completely formed and the hairs on the scalp are visible above the surface of the fetal epidermis. They continue to lengthen until 24–28 weeks, at which time they complete the first hair cycle (see Chapter 86). With subsequent hair cycles, hairs increase in diameter and coarseness. During adolescence, vellus hairs of androgen-sensitive areas mature to terminal-type hair follicles.

Sebaceous Glands (See Chapter 79)

Sebaceous glands mature during the course of follicular differentiation. This process begins between 13 and 16 weeks EGA, at which point the presumptive sebaceous gland is first visible as the most superficial bulge of the maturing hair follicle. The outer proliferative cells of the gland give rise to the differentiated cells that accumulate lipid and sebum. After they terminally differentiate, these cells disintegrate and release their products into the upper portion of the hair canal. Sebum production is accelerated in the second and third trimesters, during which time maternal steroids cause stimulation of the sebaceous glands. Hormonal activity is once again thought to influence the production of increased sebum during adolescence, resulting in the increased incidence in acne at this age.

Nail Development (See Chapter 89)

Presumptive nail structures begin to appear on the dorsal digit tip at 8–10 weeks EGA, slightly earlier than the initiation of hair follicle development. The first sign is the delineation of the flat surface of the future nail bed. A portion of ectoderm buds inward at the proximal boundary of the early nail field, and gives rise to the proximal nail fold. The presumptive nail matrix cells, which differentiate to become the nail plate, are present on the ventral side of the proximal invagination. At 11 weeks, the dorsal nail bed surface begins to keratinize. By the fourth month of gestation, the nail plate grows out from the proximal nail fold, completely covering the nail bed by the fifth month. Mutations in p63 affect nail development in syndromes such as ankyloblepharon, ectodermal dysplasia, and cleft lip/palate syndrome, as well as ectrodactyly, ectodermal dysplasia, and cleft lip/palate syndrome. Functional p63 is required for the formation and maintenance of the apical ectodermal ridge, an embryonic signaling center essential for limb outgrowth and hand plate formation. Wnt7a is thought to be important for dorsal limb patterning, and thus nail formation. In contrast to follicular development, Shh is not required for nail plate formation. Also similar to follicular differentiation, LMX1b and MSX1 are important for nail specification; LMX1b and MSX1 are mutated in nail–patella syndrome and Witkop syndrome, respectively.84–86 Hoxc13 appears to be an important homeodomain-containing gene for both follicular and nail appendages, at least in murine models.87

Eccrine and Apocrine Sweat Gland Development (See Chapter 83)

Eccrine glands begin to develop on the volar surfaces of the hands and feet, beginning as mesenchymal pads between 55 and 65 days EGA. By 12–14 weeks EGA, parallel ectodermal ridges are induced, which overlay these pads. The eccrine glands arise from the ectodermal ridge. By 16 weeks EGA, the secretory portion of the gland becomes detectable. The dermal duct begins around week 16, but the epidermal portion of the duct and opening are not complete until 2 weeks EGA.

Interfollicular eccrine and apocrine glands, in contrast, do not begin to bud until the fifth month of gestation. Apocrine sweat glands usually bud from the upper portion of the hair follicle. By 7 months EGA, the cells of the apocrine glands become distinguishable.

Although not much is known with regard to the molecular signals responsible for the differentiation of these structures, the EDA, EDAR, En1, and Wnt10b genes have been implicated. Hypohidrotic ectodermal dysplasia results from mutations in EDA or the EDAR (see Chapter 142).