INTRODUCTION
The skin is the largest constantly renewable organ in the body. It is a complex tissue from which specialized structures such as hair, glands, and nails develop during embryogenesis. Several of the structures renew throughout life and recapitulate morphological processes and molecular and cellular events that are established during embryogenesis.
The skin is composed of the underlying dermis of mesodermal embryonic origin that is separated from the multilayered overlaying epidermis of ectodermal origin by a basement membrane. The epidermis is primarily composed of cells termed keratinocytes, due to the presence of the epidermal characteristic intermediate filaments, the keratins. Other cell types present in the skin are the Langerhans cells, melanocytes, and the sensory Merkel cells. The epidermis is organized into layers that correlate with stages of keratinocyte differentiation. During the differentiation process, proliferative keratinocytes of the basal layer divide and move upward, differentiating to spinous cells. These cells then differentiate to granular cells and ultimately culminate in a cornified, anucleated, nonviable cell of the stratum corneum. Stratification is concurrent with the layer-specific expression of structural and enzymatic markers. The cornified cell is the product of covalent cross-links of cornified envelope precursors by Ca++-dependent transglutaminases and attachment of lipid molecules. The cornified layer provides the protective and water barrier functions between the body and the environment.1,2 The endpoint of epidermal differentiation in both fetus and adult is keratinization, but there are differences in how this is accomplished. In the adult, keratinization requires a relatively short time. A cell that leaves the basal layer moves into the lowest cornified cell layer in about 14 days. The fetal epidermis requires 22–24 weeks to keratinize. Some of the properties of adult skin are established during embryogenesis and are maintained throughout life. Fetal skin has additional, unique characteristics outlined below.
SKIN DEVELOPMENT
During early embryogenesis, the conceptus undergoes gastrulation, a process that leads to the establishment of the three primary embryonic layers: the ectoderm, mesoderm, and endoderm. Ectoderm and endoderm are formed at 10–12 days estimated gestational age (EGA) and the mesoderm (origin of the dermis) forms around 18–19 days EGA. It is shortly after that the epidermis originates from the ectodermal cells covering the embryo.
Epidermis starts as a one-cell layer epithelium, which differentiates and stratifies during development to ultimately form the skin, a water-impermeable structure that protects us from dehydration and infection. The gestational period can be divided into distinct embryonic/fetal developmental stages—the embryonic period (0–60 days), the early fetal period (2–5 months), and the late fetal period (5–9 months) (Fig. 1). In epidermal cells, each of these stages is characterized by specific biochemical and immunohistochemical markers for morphogenetic and differentiation processes.3
Embryonic and fetal epidermis is characterized by the presence of the periderm, a transient epidermal layer of the developing skin. It is the outermost layer of the embryonic and fetal epidermis (Fig. 2). Periderm is ectodermal in origin and contains a single population of cells. Similarities in the cytoplasmic keratins, cell surface morphology, and antigens of the periderm support the hypothesis that it is the first layer of the skin, which divides and gives rise to the single layer of the epidermis.4–8 The periderm persists as a single layer of cells that remains on the surface of the developing skin until keratinization of cells in the underlying epidermal layers is complete.9 At that point, the periderm is sloughed in sheets and individual cells from the skin surface are shed into the amniotic fluid (Fig. 2). Loss of a certain number of periderm cells occurs throughout the second trimester, as determined from the composition of cells in the amniotic fluid.4
FIG. 1: Schematic summary of embryonic and fetal developmental stages and timing through gestation. The initiation of specific landmarks of epidermal differentiation are indicated (epidermal stratification, follicular keratinization, and interfollicular keratinization). Timing of different techniques utilized for prenatal testing are highlighted.
FIG. 2: Schematic diagram of the distinct stages of epidermal development during gestation. Epidermis develops from a single layer of undifferentiated ectoderm (embryonic stage) to form a multilayered differentiated epidermis (late fetal stage). The gestation-specific periderm layer appears in the embryonic stage, with blebbing and final shedding occurring before birth. The intermediate epidermal layer appears at the onset of epidermal stratification and differentiation in the embryonic/fetal transition stage. From the early fetal stage onward, the epidermis continues to undergo proliferation and differentiation programs that ultimately culminate in the formation of the cornified layer. The protective stratum corneum functions as a protective barrier against infection and dehydration by the time of birth.
The periderm cells cease to divide during the first trimester and do not appear to undergo any further steps in differentiation, but they continue to expand markedly in size and volume as the embryo/fetus grows. Characteristic changes in morphology (e.g., blebbing) and expression of markers characteristic of apoptotic cells [e.g., transglutaminases and deoxyribonucleic acid (DNA) fragmentation] suggest that the periderm undergoes a sequence of programmed cell death.10 The shape and surface characteristics of periderm cells are used to define stages of epidermal development.9 Monoclonal antibodies have been made that recognize only the periderm cells among the epidermal cell populations,11,12 some recognizing stage-specific periderm cells.
The function of the periderm is unknown, but it has been suggested that in early stages of development it protects the basal epidermal layer. The extensive microvilli and blebbing of the periderm cell surfaces that face the amniotic cavity suggest that these cells play a role in some type of exchange between the fetus and the amniotic fluid. Evidence for this function includes: (1) The observation that periderm cells share antigens in common with other absorptive fetal and adult epithelial cells; (2) The demonstration that periderm in the sheep fetus is involved in the uptake of substances from the amniotic fluid;13 and (3) The observation that the plasma membrane of human periderm cells has the morphologic characteristics of epithelia involved in water transport.14,15 The periderm may also be a secretory epithelium, contributing material to the amniotic fluid.16
Development of Epidermal Appendages
In the mammalian embryo, the surface ectoderm envelopes the embryo during gastrulation and neurulation, forming a simple epithelium comprised of a single cell layer. During embryonic development and organ formation, a series of signals between epithelial ectodermal cells and underlying mesenchymal cells are the basis for the formation of the epidermal appendages.17 Skin appendages in mammals include hair, nail, and sweat, sebaceous and mammary glands.
The first morphologically distinguishable event is the thickening of the surface ectoderm to form a placode or anlage 5that subsequently invaginates to form a bud. Similar sets of intersecting signaling pathways are involved in patterning the epithelium for placode formation and controlling bud invagination. Despite initial similarities, subsequent events in appendage development are different.17,18 Findings in the last decade have helped elucidate the molecular and cellular mechanisms controlling early steps in the development of these organs and human genetic diseases that affect appendage morphogenesis.18 Disruption in the signalling that leads to anomalies in epithelial/mesenchymal-derived organs are features of a group of human pathological conditions defined as ectodermal dysplasias (EDs).19 In the last two decades, the molecular basis of some EDs has been characterized with the identification of mutations in transcription factors (i.e., p63, DLX3, MSX1) and effectors of signalling pathways (i.e., ectodysplasin, NF-κB) with important roles during developmental organogenesis.20,21
Specialized Nonkeratinocyte Cells within the Skin
The epidermis has contributions from cells derived not only from the surface ectoderm but from other embryological origins such as neural crest. The three major nonkeratinocyte cell types present in the skin are Langerhans cells, melanocytes, and Merkel cells. All three cell types are detected within the epidermis by the end of the embryonic stage and can be easily distinguished by their nuclear morphology, cytoplasmic density, orientation in the tissue, and antigenic properties.
Langerhans cells function as antigen-presenting dendritic cells and are evident in embryonic epidermis as early as 43 days EGA.22 The appearance of Langerhans cells before the onset of bone marrow function and recent studies have determined the contributions of yolk sac and fetal liver hematopoiesis to the formation of Langerhans cells. Langerhans cell precursors are identified in the embryonic epidermis based on the expression of CD45, CD1c, HLA-DR, and ATPase.22,23 The expression of langerin and CD1a, present in adult human epidermal Langerhans cells, is detectable only after 11 weeks EGA.22–24 The density of these cells is low (65 cells/mm2) in the epidermis during the first two trimesters of pregnancy, after which their numbers increase several fold in the third trimester and after birth.22 Langerhans cell granules are evident in the cytoplasm around 80 days EGA, suggesting that the cells already may be involved in antigen processing and presentation.23,24
The neural crest-derived melanocytes, responsible for pigmentation, migrate into the embryonic epidermis around 50 days EGA. Although they do not contain melanosomes or produce distinguishable quantities of pigment until mid-pregnancy, they are easily detected using the HMB-45 monoclonal antibody that recognizes an antigen common to melanoblasts and melanomas. Even at this early age, melanocytes are dendritic, high in density (~1,000 cells/mm2), and distributed uniformly throughout the epidermal tissue.25 Melanocytes reach a remarkably high density (~3,000 cells/mm2) in the fetal epidermis at about 80 days EGA.25 Melanosomes first appear in the cytoplasm of melanocytes around 65 days EGA depending on the region of the body. The eyelids, external auditory meatus, and labial mucosa appear to have melanin-producing melanocytes before they are evident in the skin in other regions,26 but the cells are present only transiently in these structures. Melanocytes begin to transfer melanosomes to keratinocytes in the fifth month of gestation. Studies of HMB-45 immunostained epidermal sheets have revealed that the density of immunopositive cells decreases toward birth, probably reflecting the growth of the fetus. The total cutaneous numbers at this time appear to be similar to the total numbers of the melanocytes in newborn epidermis.27
The third cell type, the mechanoreceptor Merkel cells have an epidermal origin.28–33 Adult Merkel cells are easily recognized by characteristic neuropeptides, but in the fetal epidermis, detection of these cells relies on the presence of keratins specific to Merkel cells (K8, K18, and K20). Merkel cells are known to be plentiful in adult palmar skin. Merkel cells have been demonstrated at 8 weeks EGA in fetal palmar and plantar skin,29,32 although the density of cells at this stage was low and quite variable.32 No other morphologic markers (e.g., dense-core granules) are apparent at this stage. By 80–90 days EGA, the density was as great as 1,400 cells/mm2 and by 22–24 weeks EGA increased to 1,700 cells/mm2.4,5,29,32 At both ages, the cells were organized specifically along the primary epidermal ridges. This is the expected location because Merkel cells in adult skin are commonly found in association with hair follicles and sweat glands and both of these appendages are beginning to form during this stage of fetal development. Merkel cells were first seen in hairy skin at 75 days EGA where they were located in the infundibulum and bulge regions of the developing hair pegs and bulbous hair pegs.32,34
Cytochemical evidence suggests that Merkel cells may also function as neuroendocrine cells and may serve as target structures for the ingrowth of nerve fibers or they may attract other cells such as the smooth muscle cells of the arrector pili muscle that are associated with nerve fibers in the skin.34,35
STAGES OF HUMAN SKIN DEVELOPMENT
Skin Development during the Embryonic Period
A single-layered epithelium covers the human embryo from gastrulation and a cuboidal ectoderm overlying undifferentiated mesenchyme is recorded by 5 weeks of development. By 40 days gestation, the skin consists of an epidermis, dermal-epidermal junction (DEJ), dermis, and subcutaneous connective tissue (Fig. 3). The epidermis includes basal and periderm layers. The periderm is a simple pavement epithelium composed of hexagonally shaped, microvilli-covered cells. Periderm and basal cells are similar morphologically; both contain large amounts of glycogen, few organelles, and small quantities of keratin filaments that are organized into fine networks and associated with desmosomes (Fig. 3). The species of keratins in periderm and basal cells are characteristic of simple and glandular epithelia K19, K18, and K8.4,5,36 Basal cells also contain the keratins K5 and K14 that are markers of the basal layer keratinocytes of adult epidermis.4,5,37 K18 is found in periderm cells and in Merkel cells. A planar, microfilamentous network is present internal to the plasma membrane of basal cells adjacent to the basement membrane. 6This assembly may promote adhesion and reinforce the cell before hemidesmosomes are organized in sufficient numbers to maintain the structural integrity of the dermal-epidermal interface.
FIGS. 3A TO C: Light, transmission, and scanning electron micrographs of the human embryonic skin. (A) Histology of the two-layered embryonic epidermis consists of glycogen-filled basal and periderm cells, where large vascular channels are detected. (B) Transmission electron micrograph of human embryonic skin. (C) Scanning electron micrograph of the periderm of human embryonic 70-day estimated gestational age (EGA), note the microvilli present in the surface correct magnifications for panels B and C.Courtesy: (Part A) Dr Karen Holbrook.
(B) 3500×; (C) 2500×.
The epidermis is associated with a basement membrane at the DEJ at all embryonic and fetal ages. Ultrastructural images show the basal keratinocytes physically separated from the lamina densa by a clear zone known as the lamina lucida. Several molecules that are characteristic of all basement membranes (e.g., type IV collagen, laminin, heparan sulfate proteoglycan) regardless of the epithelia with which they are associated, are also present in the embryonic DEJ. Skin-specific antigens are added to various regions of the DEJ coincident with epidermal stratification at the embryonic-fetal transition of development.38 Early stages of hemidesmosome formation are evident morphologically, but these structures are sparse and incomplete in structure.39 Occasional strands of fine filamentous material (anchoring filaments) can be seen crossing the lamina lucida.39
The embryonic dermis is a loose network of mesenchymal cells with little intervening fibrous connective tissue matrix.40,41 The high water content and hyaluronic-acid rich environment promotes cell migration during all phases of active tissue morphogenesis.42 Types I, III, V, and VI collagens are detected in early embryonic skin, but the fiber bundles contain few fibrils and are associated primarily with the surfaces of fibroblastic cells and in the zone subjacent to the DEJ.36,40,41 The latter zone, sometimes called the compact mesenchyme, is also a site that contains sulfated proteoglycans and is rich in growth factors. At later stages, the collagens become distributed in accord with the adult pattern of deposition.40 Elastic fibers are not present in embryonic dermis, while microfibrillar structures are seen by electron microscopy.1
Small capillary-like vessels are evident in the embryonic dermis.43 They are sparse and do not appear to be organized into patterns. Nerve fibers are recognized by immunostaining embryonic skin with antibodies to the p75 low-affinity nerve growth factor receptor. Fine fibers in the dermis connect with large nerve trunks located at the dermal-subcutaneous junction. It is difficult to distinguish subcutaneous tissue from dermis in the embryonic skin because the cells are quite similar and adipose tissue is synthesized considerably later in development. There is frequently, however, a difference in the density and/or orientation of the mesenchymal cells in the two 7regions and large, dilated channels, organized in the plane of the skin form an arbitrary boundary (Fig. 3). In some regions of the body, there is also a greater density of fibrous connective tissue in the subcutaneous tissue compared with the dermis. There is no morphologic evidence that epidermal appendages have begun to form in embryonic skin.
Skin Development at the Early Fetal Stage
By the end of the second month gestation, hematopoiesis has switched from the extraembryonic sac to the bone marrow and remarkable changes in the structure and biochemistry of the skin occur during the embryo/fetal transition period (Fig. 1). From this time to the end of the first trimester, all regions of the skin acquire features that establish a template of adult skin.
The first obvious change in the structure of the skin in this transition stage is the stratification of the epidermis to basal, intermediate, and periderm cell layers (Figs. 2 and 4). The cells of the intermediate layer show little difference in morphology from the basal cells; they have high glycogen content (Fig. 4) and few organelles are positioned around the nucleus and at the cell borders. There are larger and more densely staining bundles of keratin filaments located in the peripheral cytoplasm where they are associated with desmosomes (Fig. 4). Intermediate cells stain with antibodies that recognize the differentiation-specific keratins K1 and K10, of the adult stratified keratinized epidermis.4,5 This indicates that as soon as the basal keratinocytes divide and a layer of intermediate cells is added, the tissue is “differentiated” in terms of the expression of adult epidermal keratins. Other markers of differentiation present in the intermediate layer cells are involucrin and loricrin.1 Once the intermediate layer is formed, the basal layer begins to lose some of its glycogen content and thus appears to begin to acquire more of the adult features. Glycogen is common in cells of fetal tissues where it may serve as an energy source; it occupies a significant volume of fetal keratinocytes and fibroblasts through the first and second trimester but diminishes in later stages of development toward birth.
The epidermis continues to stratify by additional cell layers during the second trimester of development (Fig. 4). All cells of the fetal epidermis, including the periderm, can divide during the first trimester, but around the end of the third month this ability appears to become restricted primarily to basal keratinocytes.44,45 Periderm cells at this stage have a large volume of cytoplasm and appear rounded. At later ages within the same period one or more blebs project from the surfaces of periderm cells (Figs. 2 and 4) and remain densely covered by microvilli.
Changes in the structure and composition of the DEJ parallel epidermal stratification. The basal keratinocytes synthesize the components of the hemidesmosomes, anchoring filaments, and anchoring fibrils. Fibroblasts of the papillary dermis contribute collagens and components of elastic fibrils to the connective tissue matrix of the sublamina densa.2 Hemidesmosomes are now complete in structure and anchoring filaments and anchoring fibrils organize in relation to these sites (Fig. 4).39 It has been suggested from studies of chick cornea that there may be an interdependent relationship between hemidesmosomes and anchoring fibrils during morphogenesis.46 However, in humans, patients with junctional epidermolysis bullosa (EB) who lack hemidesmosomes do not lack anchoring fibrils,47 and vice-versa in the case of individuals with recessive dystrophic EB who do not have anchoring fibrils but whose hemidesmosomes assemble normally.48 Several of the antigens that correlate with these structures are expressed in the DEJ at the time of the embryonic fetal transition.49,50
During the remainder of the first trimester, the mesenchymal cells differentiate into fibroblastic cells and the dermis becomes less cellular and watery as more fibrous connective tissue accumulates in the interstitial spaces. By the end of this period, papillary and reticular regions can be distinguished on the basis of differences in cell density and orientation, fibril diameter, and fiber bundle size.40,51 A decrease in hyaluronic acid parallels the transition of the dermis from a cellular to a fibrous tissue; the water content of the dermis still remains >80%. Like adult dermis, the proteoglycan is composed largely of sulfated molecules.
Around 70 days EGA, the basic pattern of the adult vasculature is evident in fetal dermis.43 Horizontal plexuses are present within the dermis (subpapillary plexus) and separate the dermis from the subcutaneous tissue. The structure of the vessel wall is simple, making identification of vascular segments difficult. Nerve patterns are also well developed by the mid- to later first trimester and generally follow the vascular pattern.
Thus, the beginning of the fetal period is characterized by epidermal stratification and the histogenesis of various tissues of the skin. Epidermal stratification coincides with the onset of tooth, nail, and follicle morphogenesis, the addition of antigens and adhesive structures at the DEJ, regionalization of the dermis via the organization of fibrous extracellular matrix and patterns of nerves and vessels, and the expression of surface and cytoplasmic markers by melanocytes and Langerhans cells. Merkel cells are established in both hairy and glabrous skin.
There can be a hiatus of as much as 10 weeks between keratinization of the epidermal appendages and keratinization of the interfollicular epidermis. Keratinization of the follicle begins around 15 weeks EGA. At the beginning of the second trimester, the follicle is an elongated cord of cells consisting of a cellular core and an outer cellular layer. In general, the core cells are similar to intermediate layer epidermal cells and the cells of the external layer are more like cells of the basal epidermal layer. The hair peg grows and differentiates to become the bulbous hair peg (a structure named for the terminal bulb of the follicle), around 12–14 weeks EGA when primordia of the sebaceous gland, bulge, and the apocrine gland are recognized.52,53 Lipid is synthesized in the sebaceous gland around 15 weeks EGA. Simultaneously smooth muscle cells of the arrector pili muscle grow toward and attach to the follicle in the region of the bulge.54 The apocrine gland grows as a short cord of cells that originates from the infundibular portion of the follicle; apocrine glands persist and continue to develop only in specialized regions of the body. The mechanisms for establishing these precisely positioned structures are unknown. The terminus of the bulbous hair peg has a concavity within which cells of the dermal papilla are sequestered.8
FIGS. 4A TO G: Light, transmission, and scanning electron micrographs of early fetal human skin. (A) Histology of normal human skin at the embryonic fetal transition stage 76-day estimated gestational age (EGA), and (B) at the early fetal stage 107-day EGA, with identification of the intermediate layer after stratification starts. (C) The intermediate layer cells contain abundant glycogen (arrows) and keratin filaments are evident at the periphery of the cell. (D) Note the presence of complete hemidesmosomes and anchoring fibril-like structures (arrows) in the dermal epidermal junction of skin from a 70-day EGA fetus. (E and F) Hair follicle bud and peg at 87-day and 101-day EGA development respectively. (G) Scanning electron micrograph of the surface periderm at 112-day EGA. Note the microvilli and blebs. ×2500.Courtesy: (Part A, B, E and F) Dr Karen Holbrook.
By 15 weeks gestation, the interfollicular epidermis is stratified with one or two additional layers of intermediate cells, but until the end of the second trimester there is no evidence of keratinization (Fig. 1). The adult pattern of keratins is maintained in the basal cells (K5 and K14), and the suprabasal keratinocytes of all intermediate layers express K1 and K10. Keratin filaments have continued to increase in quantity in the intermediate layers. Glycogen remains a significant component. The surface of the skin is still covered by a complete layer of periderm. The nondividing, periderm cells have attained a peak stage of structural modification in which one or more simple or complex blebs project from the surface; all portions of the periderm cells facing the amniotic cavity are covered with microvilli.55 The internal morphology of the cells, however, is variable. Some of the cells contain the organelles associated with active metabolic processes (e.g., mitochondria, rough endoplasmic reticulum, Golgi); others have few organelles and are filled with filaments. The latter cells have a thickened cell envelope, which is morphologically and biochemically equivalent to the cornified cell envelope. Involucrin and loricrin have been identified at the boundary of the periderm cell56 and an active epidermal transglutaminase is present that appears able to cross-link the envelope proteins.2,56 At later stages of this period (~18 weeks) the periderm cells flatten, the blebs regress to small, button-like protrusions,2 and the subcellular morphology is indicative of a nonfunctional, perhaps nonviable, apoptotic remnant of this tissue.9
Between 22 and 24 weeks EGA, the interfollicular epidermis begins to keratinize; the exact timing depends on the region of the body.8 Early evidence for keratinization is the appearance of lamellar granules and small keratohyalin granules in the uppermost intermediate cell layer (Fig. 5). This is the first age at which filaggrin is expressed in the fetal interfollicular epidermis;4 thus the onset of filaggrin synthesis appears to correlate precisely with the morphologic appearance of keratohyalin granules. The first few layers of cells that keratinize are a combination of “regressed” periderm cells that contain primarily filaments and have a cornified cell envelope, and incompletely keratinized keratinocytes; the latter cells are characterized by a condensed nucleus, dense organelles, cornified cell envelope, and flattened shape of the stratum corneum cell. The numbers of layers of keratinized cells and the size of keratohyalin granules gradually increase so that the epidermis of the third trimester fetus appears similar to that of the neonate and adult with the exception that glycogen remains in the cytoplasm of the keratinocytes. Coincident with keratinization, the periderm is sloughed from most of the skin surface revealing the underlying keratinized epidermis (Figs. 2 and 5).
FIGS. 5A TO C: Light, transmission, and scanning electron micrographs of late fetal human skin. Histology of normal human skin at the late fetal stage (A) 143-day estimated gestational age (EGA), and (B) 189-day EGA. Note the stratification and the keratinization of the epidermis as well as the complex structural organization of the dermis. (C) Scanning electron micrograph of the periderm of human skin 160-day EGA, note the periderm sheet sloughed from the skin surface.
The surface of the skin is coated with the lipid-rich material of the vernix caseosa, a combination of sebum secreted from the 10sebaceous gland of the follicle, epidermal lipids, desquamated cells, hair, and other tissue debris.57,58 Like sebum, the vernix contains a significant level of triglycerides and wax esters, and like cornified cells it has a high proportion of sterols. Other lipids that are not characteristic of either of these sources contribute to the unique content of this material.57 Vernix caseosa is often apparent on the skin at birth.
The second trimester dermis contains an extensive amount of fibrous connective tissue, including elastic fibers. The elastic microfibrils are synthesized early in development but the elastin gene does not appear to be expressed until approximately 15 weeks EGA.59 Elastic fiber networks are seen in the skin by histochemical and immunohistochemical methods around 20 weeks EGA.51,60 These fibers are immature in structure, more similar to adult elaunin fibers, and thus it is unrealistic to consider using the structure of the elastic fibers to identify a fetus at risk of a genetic disorder of elastic connective tissue such as cutis laxa. Even at birth, the amount of elastin associated with elastic fibers is minimal.
Coarse fiber bundles of the deep dermis clearly distinguish it from the fine connective tissue of the hypodermis. This pattern is reversed from the situation in younger skin where the density of matrix in the subcutaneous tissue is greater than that of the dermis. The organization of the hypodermis is readily apparent at 15 weeks EGA, and by 18 weeks EGA there is a small accumulation of subcutaneous fat.
By late fetal stage, the hair follicle morphogenesis has proceeded and the hair canals can be seen as elongated ridges visible at the epidermal surface. The integrity of the periderm is disrupted along the tops of the ridges where the hair canal will rupture and open to the surface. The length of the canals, the time of hair release, and the density of the hair are dependent on the region of the body.61 The position of eyebrow and scalp hairs can be recognized as early as 15 weeks EGA by the presence of short, closely positioned, periderm-covered hair canals. At 21 weeks, the hair canals on the trunk begin to rupture, interrupting the continuity of the periderm over the body surface and releasing a fully elongated hair.61 The eccrine sweat glands develop on the body late in the second trimester, >2 months after their development initiated on the palms and soles.
By birth, the skin has the final structure that it will maintain through life (Fig. 6). The detailed characterization of the skin structure, composition, and function, as well as the epidermal appendages will be the focus of the next section of this chapter.
SKIN DEVELOPMENT IN IMMATURE/PREMATURE SKIN
The skin of the premature infant is that of the third trimester fetus. During this period, the skin increases in bulk, primarily by the addition of connective tissue to the dermis, the epidermis is keratinized, all of the epidermal appendages are formed, and the dermis contains all of the matrix proteins characteristic of the newborn and adult. Nonetheless, the skin is still immature in both structural and physiologic properties.
FIGS. 6A AND B: Architecture of normal human skin at birth. (A) Schematic representation of normal human skin at birth. Magnification delineates the distinct layers of the epidermis, cellular components, and hair morphology. (B) Light micrograph of a developing hair follicle at 15-week estimated gestational age (EGA). The regions of the follicle, sebaceous gland, and bulge are indicated. Note the keratinization of the developing hair and of the lining of the hair canal in the region of the infundibulum and within the epidermis (×400).(B: stratum basalis; Bu: bulge; C: stratum corneum; DP: dermal papilla; EGA: estimated gestational age; G: stratum granulosum; HS: hair shaft; LC: Langerhans cell; M: melanocyte; S: stratum spinosum; SG: sebaceous gland)
The epidermis has all of the layers of the adult epidermis, but it is thinner. The cells retain a substantial amount of glycogen, and the stratum corneum forms a less formidable barrier. The relatively poor barrier properties of the preterm infant are of great importance when considering topical application of various pharmacologic and cleansing compounds.62,63 The uptake of pharmacologic compounds applied topically to the skin is much more rapid in the 28- to 34-week premature newborn than in older newborns and the loss of water through the skin and evaporative water loss from the surface decreases exponentially with increasing gestational age.64,65 The amount of transepidermal water loss through the skin of a 25- to 30-week EGA infant can be so substantial that death can result from dehydration. In the premature infant, it is suggested that the vernix caseosa functions to augment the poorly established 11barrier properties of the epidermis or serve as a natural emollient. The skin of the premature infant is more resistant to desquamation than that of the term newborn.
The structure of the DEJ of the premature infant correlates with gestational age. As the age increases, the contours (rete ridges and dermal papillae) become more prominent. In the youngest (26–34 weeks EGA) premature infants, the DEJ is relatively flat, although the structural components of the DEJ that anneal the epidermis and dermis are well established. The papillary dermis immediately underlying the DEJ is edematous and the bundles of collagen fibrils are smaller and more widely spaced than those of the term newborn or adult, and thus the epidermal–dermal integrity might be expected to be more easily compromised than in the term newborn.66
The dermis of the premature infant is ~3 quarters of the thickness of adult dermis and comparable in connective tissue organization. Fine collagen fibrils and the small-sized collagen fiber bundles give the dermis a highly cellular and delicate appearance. The water content of the dermis remains high in premature babies, but this, like many properties of the premature and newborn skin depends on the nutritional status of the fetus in addition to gestational age.
All of the adult epidermal appendages are established in premature skin. The fully formed, hair-synthesizing lanugo follicles of fetal skin are equivalent to those of the term newborn or adult; however, the eccrine sweat glands have formed only a few coils of the glandular segment and the light and dark cells are not easily distinguishable.66 Sweat ducts are partially occluded until the end of the seventh month of gestation. The sweating response is limited or absent in premature infants67 and appears to have a strong correlation with gestational age; there is a tendency toward total anhidrosis in the premature neonate,68 although this dysfunction rapidly resolves after a few days of extrauterine life.
A summary of all the major morphological events that occur through the embryogenesis of the human skin is presented in Table 1.
REFERENCES
- Kalinin A, Marekov LN, Steinert PM Assembly of the epidermal cornifies cell envelope. J Cell Sci. 2001;114:3069-70.
- Holbrook KA. Structure and function of the developing human skin. In: Goldsmith LE (Ed). Physiology, Biochemistry and Molecular Biology of the Skin, 2nd edition. New York: Oxford University Press; 1991. p. 63.
- Candi E, Schmidt R, Melino G. The cornified envelope: a model of cell death in the skin. Nat Rev Mol Cell Biol. 2005;6:328-40.
- Dale BA, Holbrook KA, Kimball JR, Hoff M, Sun TT. Expression of the epidermal keratins and filaggrin during fetal human development. J Cell Biol. 1985;101:1257-69.
- Moll R, Moll I, Wiest W. Changes in the pattern of cytokeratin polypeptides in epidermis and hair follicles during skin development in human fetuses. Differentiation. 1982;23:170-8.
- Akiyama M, Smith LT, Yoneda K, Holbrook KA, Hohl D, Shimizu H. Periderm cells form cornified cell envelope in their regression process during human epidermal development. J Invest Dermatol. 1999;112:903-9.
- M'Boneko V, Merker H-J. Development and morphology of the periderm of mouse embryos (days 9–12 of gestation). Acta Anat (Basel). 1988;133:325-36.
- Richardson RJ, Hammond, NL, Coulombe PA, Saloranta C, Nousiainen HO, Salonen R, et al. Periderm prevents pathological epithelial adhesions during embrogenesis. J Clin Invest. 2014;124:3891-900.
- Holbrook KA, Odland GF. The fine structure of developing human epidermis: light, scanning and transmission electron microscopy of the periderm. J Invest Dermatol. 1975;65:16-38.
- Polakowska RR, Piacentini M, Bartlett R, Goldsmith LA, Haake AR. Apoptosis in human skin development: morphogenesis, periderm and stem cells. Dev Dyn. 1994;199:176-88.
- Lane AT, Negi M, Goldsmith LA. Human periderm: a monoclonal antibody marker. Curr Probl Dermatol. 1987;16:83-93.
- Schofield OMV, McDonald JN, Fredj-Reygrobellet D, Hsi BL, Yeh CJ, Ortonne JP, et al. Common antigen expression between human periderm and other tissues identified by GB1-monoclonal antibody. Arch Dermatol Res. 1990;282:143-8.
- Mears GJ, Van Petten GR. Fetal absorption of drugs from the amniotic fluid. Proc West Pharmacol Soc. 1977;20:109-14.
- Parmely TH, Seeds AE. Fetal skin permeability to isotopic water (THO) in early pregnancy. Am J Obstet Gynecol. 1970;108:128-31.
- Riddle CV. Intramembranous response to cAMP in fetal epidermis. Cell Tissue Res. 1985;241:687-9.
- Lind T, Kendal A, Hytten FE. The role of the fetus in the formation of amniotic fluid. J Obstet Gynaecol Br Commonw. 1972;79:289-98.
- Pispa J, Thesleff I. Mechanisms of ectodermal organogenesis. Dev Biol. 2003;262:195-205.
- Priolo M, Lagana C. Ectodermal dysplasias: a new clinical-genetic classification. J Med Genet. 2001;38:579-85.
- Priolo M. Ectodermal dysplasias: An overview and update of clinical and molecular-functional mechanisms. Am J Med Genet A. 2009;149A:2003-13.
- Itin PH. Etiology and pathogenesis of ectodermal dysplasias Am J Med Genet A. 2014;164A:2472.
- Foster CA, Holbrook KA. Ontogeny of Langerhans cells in human embryonic and fetal skin: expression of HLA-DR and OKT6 determinants. Am J Anat. 1989;184:157-64.
- Schuster C, Vaculik C, Fiala C, Meindl S, Brandt O, Imhof M, et al. HLA-DR+ leukocytes acquire CD1 antigens in embryonic and fetal human skin and contain functional antigen-presenting cells. J Exp Med. 2009;206:169-81.
- Schuster C, Mildner M, Mairhofer M, Bauer W, Fiala C, Prior M, et al. Human embryonic epidermis contains a diverse Langerhans cell precursor pool. Development. 2014;141:807-15.
- Holbrook KA, Underwood RA, Vogel AM, Gown AM, Kimball H. The appearance, density and distribution of melanocytes in human embryonic and fetal skin revealed by the anti-melanoma monoclonal antibody, HMB-45. Anat Embryol. 1989;180:443-55.
- Barla-Szabo L. Ejection of melanocytes and melanin from fetuses and newborn mammalian animals. Acta Morphol Acad Sci Hung. 1970;18:213-25.
- Hamada H. Age changes in melanocyte distribution of the normal, human epidermis. Jpn J Dermatol. 1970;82:223-32.
- Moll I, Moll R, Franke WW. Formation of epidermal and dermal Merkel cells during human fetal skin development. J Invest Dermatol. 1986;87:779-87.
- Moll R, Moll I, Franke W. Identification of Merkel cells in human skin by specific cytokeratin antibodies: changes in cell density and distribution in fetal and adult plantar epidermis. Differentiation. 1984;28:136-54.
- Moll I, Lane A, Franke W, Moll R. Intraepidermal formation of Merkel cells in xenografts of human fetal skin. J Invest Dermatol. 1990;94:359-64.
- Moll I, Moll R. Early development of human Merkel cells. Exp Dermatol. 1992;1:180-4.
- Kim D-G, Holbrook KA. The appearance, density and distribution of Merkel cells in human embryonic and fetal skin: their relation to sweat gland and hair follicle development. J Invest Dermatol. 1995;104:411-6.
- Narisawa Y, Hashimoto K. Immunohistochemical distribution of nerve–Merkel cell complex in fetal human skin. J Dermatol Sci. 1991;2:361-70.
- Narisawa Y, Hashimoto K, Nakamura Y, Kohda H. A high concentration of Merkel cells in the bulge prior to the attachment of the arrector pili muscle and the formation of the perifollicular nerve plexus in human fetal skin. Arch Dermatol Res. 1993;285:261-8.
- Sharp F. A quantitative study of the glycogen content of human fetal skin in the first trimester. J Obstet Gynaecol Br Commonw. 1971;78:981-6.
- Moll R, Franke WW, Schiller DL. The catalog of human cytokeratins: patterns of expression in normal epithelia, tumors and cultured cells. Cell. 1982;31:11-24.
- Tseng SCG, Jarvinen M, Nelson WG, Huang JW, Woodcock-Mitchell J, Sun TT. Correlation of specific keratins with different types of epithelial differentiation: monoclonal antibody studies. Cell. 1982;30:361-72.
- Yin T, Green KJ. Green, Regulation of desmosome assembly and adhesion. Semin Cell Dev Biol. 2004;15:665-77.
- McMillan JR, Eady RAJ. Hemidesmosome ontogeny in human fetal skin. Arch Dermatol Res. 1996;288:91-7.
- Smith LT, Holbrook KA, Madri JM. Collagen types I, III and V in human embryonic and fetal skin. Am J Anat. 1986;175:501-21.
- Smith LT. Patterns of type VI collagen compared to types I, III and V collagen in human embryonic and fetal skin and in fetal-skin derived cell cultures. Matrix Biol. 1994;14:159-70.
- Breen M, Weinstein HG, Johnson RL, Veis A, Marshall RT. Acid glycosaminoglycans in human skin during fetal development and in adult life. Biochim Biophys Acta. 1970;201:54-60.
- Johnson CL, Holbrook KA. Development of human embryonic and fetal dermal vasculature. J Invest Dermatol. 1989;93:10S-17S.
- Bickenbach JR, Holbrook KA. Proliferation of human embryonic and fetal epidermis in organ culture. Am J Anat. 1986;177:97-106.
- Bickenbach JR, Holbrook KA. Label-retaining cells in human embryonic and fetal epidermis. J Invest Dermatol. 1987;88:42-6.
- Gipson IK, Spurr-Michaud SJ, Tisdale AS. Hemidesmosomes and anchoring fibril collagen appear synchronously during development and wound healing. Dev Biol. 1988;126:253-62.
- Tidman MJ, Eady RAJ. Hemidesmosome heterogeneity in junctional epidermolysis bullosa revealed by morphometric analysis. J Invest Dermatol. 1986;86:51-6.
- Tidman MJ, Eady RAJ. Evaluation of anchoring fibrils and other components of the dermal-epidermal junction in dystrophic epidermolysis bullosa by a quantitative ultrastructural technique. J Invest Dermatol. 1985;84:374-7.
- Fine J-D, Horiguchi Y, Couchman JR. 19-DEJ-1, a hemidesmosomal-anchoring filament complex associated monoclonal antibody: definition of a new skin basement membrane antigenic defect in junctional and dystrophic epidermolysis bullosa. Arch Dermatol. 1989;125:520-3.
- Smith LT, Sakai LY, Burgeson RE, Holbrook KA. Ontogeny of structural components at the dermal-epidermal junction in human embryonic and fetal skin: the appearance of anchoring fibrils and type VII collagen. J Invest Dermatol. 1988;90:480-5.
- Smith LT, Holbrook KA, Byers PH. Structure of the dermal matrix during development and in the adult. J Invest Dermatol. 1982;79:93s-104s.
- Holbrook KA, Minami SA. Hair follicle morphogenesis in the human: characterization of events in vivo and in vitro. NY Acad Sci. 1991;642:167-96.
- Pinkus H. Embryology of Hair. In: Montagna W, Ellis RA (Eds) The Hair Growth. New York: Academic Press; 1958. p. 1.
- Akiyama M, Dale BA, Sun TT, Holbrook KA. Characterization of hair follicle bulge in human fetal skin: the human fetal bulge is a pool of undifferentiated keratinocytes. J Invest Dermatol. 1995;105:844-50.
- Nanbu Y, Fujii S, Konishi I, Nonogaki H, Mori T. CA 125 in the epithelium closely related to the embryonic ectoderm: the periderm and the amnion. Am J Obstet Gynecol. 1989;161:462-7.
- Holbrook KA, Underwood RA, Dale BA, Thacher SM, Wuepper KD, and Banks-Schlegel S. Cornified cell envelope (CCE) in human fetal skin: involucrin, keratolinin, loricrin and transglutaminase expression and activity. J Invest Dermatol. 1991;96:542.
- Kurkkainen J, Nikkari T, Ruponen S, Haahti E. Lipids of the vernix caseosa. J Invest Dermatol. 1965;44:333-8.
- Nazarro-Porro M, Possi S, Boniforti L, Belsito F. Effects of aging on fatty acids in skin surface lipids. J Invest Dermatol. 1979;73:112-7.
- Sephel GC, Buckley A, Davidson JM. Developmental initiation and elastin gene expression by human fetal skin fibroblasts. J Invest Dermatol. 1987;88:732-5.
- Deutsch TA, Esterly NB. Elastic fibers in fetal dermis. J Invest Dermatol. 1975;65:320-3.
- Holbrook KA, Odland GF. Structure of the hair canal and the initial eruption of hair in the human fetus. J Invest Dermatol. 1978;71:385-90.
- Nachman RL, Esterly NB. Increased skin permeability in preterm infants. J Pediatr. 1971;79:628-32.
- West DP, Worobec S, Solomon LM. Pharmacology and toxicology of infant skin. J Invest Dermatol. 1981;76:147-50.
- Hammarlund K, Sedin G. Transepidermal water loss in newborn infants. Acta Paediatr Scand. 1979;68:795-801.
- Faranoff AA, Wald M, Gruber HS, Klaus MH. Insensible water loss in low birth weight infants. Pediatrics. 1972;50:236-45.
- Holbrook KA. A histologic comparison of infant and adult skin. In: Maibach HI, Boisits EK (Eds). Neonatal Skin. New York: Marcel Dekker; 1981. p. 3.
- Sinclair JC. Thermal control in premature infants. Annu Rev Med. 1972;23:129-48.
- Green M. Comparison of adult and neonatal skin eccrine sweating. In: Maibach HI, Boisits EK (Eds). Neonatal Skin. New York: Marcel Dekker; 1981 p. 35.