This structure is clearly visible in mice, but it can be only roughly recognized in human skin in the regions where skin is thin and flat [3]. There are many types of hairs, ranging from the coarse hairs of the eyelashes and eyebrows to the barely visible hairs on the abdomen and back. 17-49). The simple mammary duct system that was laid down in the embryo remains in an infantile condition until it is exposed to the changing hormonal environment at the onset of puberty (Fig. Interfollicular epidermis, which is by far the largest part of the skin surface, has a relatively simple organization depending directly upon the dermo-epidermal junction. 9.9) and have shown a correlation between whorls and the direction of hair growth and the tension on the epidermis at the time of formation of the hair follicles.
A prominent feature of the skin, particularly the thick skin of the palms and soles, is the presence of epidermal ridges and creases. Hairs are specialized epidermal derivatives that arise as the result of inductive stimuli from the dermis. Postnatally, keratinocytes commonly spend about 4 weeks in their passage from the basal layer of the epidermis to ultimate desquamation, but in some skin diseases, such as psoriasis, epidermal cell proliferation is poorly controlled, and keratinocytes may be shed within 1 week after their generation. The stem cell “niche” in the interfollicular epidermis is thus apparently dependent only on the environment provided by the surrounding keratinocytes, and it is established as a consequence of autonomous autoregulatory feedback signals [4]. Many of the peridermal cells undergo programmed cell death (apoptosis) and are sloughed into the amniotic fluid. Keratinocytes can than be transferred multiple times directly onto plastic dishes, or on covered feeder dishes for extended periods of time (Redvers and Kaur, 2005). Several types of anomalies affect the integumentary system. The proportions of stem cells (10 to 30% of basal cells) and transit amplifying cells (10 to 40%) in cultured human IFE determined according to these criteria20 are in remarkably good agreement with the proportions estimated to exist in mouse epidermis; the remaining nonstem, nontransit cells are postmitotic and committed to terminal differentiation.11 Much more work is needed to relate the in vitro clonality studies to the in vivo behavior of the cells, but one possibility is that holoclones, meroclones, and paraclones represent a hierarchy of “stemness” within the stem cell compartment. Known as, The pattern of epidermal appendages such as hairs has been shown experimentally to relate to patterns generated in the dermis. In a rapid response to the suckling stimulus, the ejection of milk is triggered by the release of oxytocin by the posterior portion of the pituitary. A prominent feature of the skin, particularly the thick skin of the palms and soles, is the presence of epidermal ridges and creases. If ectoderm from one part of the body is combined with dermis from another area, the ectoderm differentiates into a regional pattern characteristic of underlying dermis, rather than a pattern appropriate for the site of origin of the ectoderm (Fig. In case tunica is multilayered, which is likely to happen. Erupted hairs are first seen on the eyebrows shortly after 16 weeks. The number of pigment cells in the skin does not differ greatly among the various races, but the melanocytes of dark-skinned individuals contain more pigment granules per cell. In humans, particularly in the regions of thick papillary dermis, the EPUs are larger, containing up to four dozen cells. 9.16 Precartilaginous primordia in the 9-mm long human embryo. The first morphological evidence of mammary gland development is the appearance of two bands of ectodermal thickenings called. (The development of teeth is discussed in Chapter 14.). At specific periods during embryogenesis, the cartilage is replaced by true bone through the process of endochondral ossification. Jun 16, 2016 | Posted by admin in EMBRYOLOGY | Comments Off on Integumentary, Skeletal, and Muscular Systems. These range from complete hair loss (atrichia), caused by mutations in the hairless gene (HR), to hypertrichosis, a condition characterized by excessive of ectopic hair growth. The future dermis is initially represented by loosely aggregated mesenchymal cells that are highly interconnected by focal tight junctions on their cellular processes. 9.4 Scanning electron micrographs of the ventral surfaces of the hand (A) and the foot (B) of a human embryo at the end of the second month. Because the number of clones is independent of the quantity and quality of the plated cells able to replicate, the underlying hypothesis is that all the proliferating basal stem cells in epidermis are able to organize a new EPU when appropriately stimulated, returning thus from the transitory amplification cell pool to the central permanent stem cell pool of long life and capacity to maintain epidermal homeostasis. Dermal-Epidermal Interactions B, Early hair peg (15 to 16 weeks). In line with this idea, overexpression of Eda leads to increased size and irregular shape of the early hair placodes. Abnormalities associated with the integumentary system are presented later in Clinical Correlation 9.1. A number of families have been identified with germline mutations in the P-cadherin gene. E-cadherin, which is present throughout the basal layer, is downregulated in placodes, while P-cadherin is upregulated (Jamora et al., 2003; Muller-Rover et al., 1999). As a result of inductive influences by the dermis, the epidermis produces a wide variety of appendages, such as hair, nails, sweat and sebaceous glands, mammary glands, and the enamel component of teeth. Although the mesoderm controls the branching pattern of the ductal epithelium, the functional properties of the mammary ducts are intrinsic to the epithelial component. 9.14B). In the head, preskeletal cells of the neural crest may receive information at levels ranging from the neural tube itself, to sites along their path of migration, to the region of their final destination. The dermis arises from several sources. At specific periods during embryogenesis, the cartilage is replaced by true bone through the process of endochondral ossification. Epidermal Appendages These cells produce molecules characteristic of bone (type I collagen, osteocalcin, and osteopontin) and form spicules of intramembranous bone. Later formation of a hair is structurally and biochemically an extremely complex process, which, among other things, involves the expression of a range of Hox genes in specific locations and at specific times along the length of each developing hair. Early in the second month, melanoblasts derived from the neural crest migrate into the embryonic dermis; slightly later, they migrate into the epidermis. Mammary ductal epithelial downgrowths (Fig. The continual turnover of the epidermis is mediated by epidermal proliferative units, which consist of a stem cell in the stratum basale and several transit amplifying cells. Epidermis is usually multilayered (e.g., Nerium and Ficus elastica) and possesses sunken stomata covered with numerous hair (e.g., Nerium). In many animals, this increases the insulation properties of the hair. Some cranial bones (e.g., the bones making up the roof and much of the base of the skull) are mesodermal in origin, but the facial bones and some of the bones covering the brain arise from mesenchyme derived from the ectodermal neural crest. The epidermis, a multilayered epithelium, surrounds and protects the vertebrate body. Fibroblast growth factor (FGF) and Wnt (mainly Wnt-11) signaling from the dermis, along with the inactivation of local bone morphogenetic proteins (BMPs), stimulates the activation of other Wnts in the ectoderm to form an epidermal placode. 9.12) are associated with two types of mesoderm: fibroblastic and fatty. 22.1E). Cross-species recombination experiments have shown that, even in distantly related animals, skin ectoderm and mesenchyme can respond to each other’s inductive signals. 9.7). As the cells move into the outer layer, the stratum corneum, they lose their nuclei and resemble flattened bags densely packed with keratin filaments. During the later stages of hair formation, the hair bulb becomes infiltrated with melanocytes, which provide color to the hair. The proliferation of basal epidermal cells is under the control of a variety of growth factors. However, this does not promote formation of extra hair follicles (Mustonen et al., 2004). Much of the nail is covered by a thin eponychium, most of which will eventually degenerate. The histogenesis of each of these tissues exemplifies important aspects of development. FGF-18, produced by the perichondrium, inhibits the maturation of the chondrocytes around the periphery of the mass of cartilage as those at the center are undergoing hypertrophy. These basal cells continuously produce the overlaying cells of the spinous and granular layers and finally transform into dead corneous squames of the stratum corneum that eventually slough off the skin surface. As is the case with testosterone effects, estrogen effects on the epithelium of the mammary ducts are mediated via paracrine influences from the mammary connective tissue stroma, which contains the estrogen receptors. The nonabortive clones would include holoclones and meroclones and even paraclones. Tissue recombination experiments on a variety of vertebrate species have shown that the underlying dermis determines the course of development of the epidermis and its derivatives, and that the ectoderm also influences the developmental course of the dermis. Cultivation of human epidermal keratinocytes at clonal density on a feeder layer of 3T3 cells. 9.17). The movement of epidermal cells away from the basal layer is preceded by a loss of adhesiveness to basal lamina components (e.g., fibronectin, laminin, and collagen types I and IV). Adjacent hairs are frequently in different phases of the hair cycle. The formation of a hair involves a series of inductive interactions mediated by signals that are only partly understood. 9.7). On the tips of the digits, the ridges form loops and whorls in fingerprint patterns that are unique to the individual. Integumentary System 23-2, F). name 3 characteristics of the lining of the esophagus. This artificial skin sustains itself on the patient for extended periods of time, and the extent of its survival is thought to reflect the ability to capture the highly proliferative putative stem cells from the tissue. The deep skeletal elements of the body typically first appear as cartilaginous models of the bones that will ultimately be formed (Fig. Skin on our scalp is an example … These patterns form the basis for the science of dermatoglyphics, in which the patterns constitute the foundation for genetic analysis or criminal investigation. First, they stimulate further downgrowth of the mammary epithelial bud. (From Turnpenny P, Ellard S: Emery’s elements of medical genetics, ed 14, Philadelphia, 2012, Churchill Livingstone.) The pattern of the epidermal ridges is correlated with the morphology of the volar pads when the ridges first form. … Although melanoblasts can be recognized early by staining with a monoclonal antibody (HMB-45, which reacts with a cytoplasmic antigen common to melanoblasts and melanomas [pigment cell tumors]), these cells do not begin to produce recognizable amounts of pigment until midpregnancy. D, Lactating adult. During wound healing, several different populations of the bulge region of the hair follicle can contribute to reestablishment of the epidermis (see Section 18.3.2.5.2 below for details) [74]. This multilayered structure has the appearance of an onion. Clinical Correlation 9.1 summarizes several types of anomalies that affect the integumentary system. How the epidermal placodes are spaced in such a geometrically regular fashion is still not well understood. As is the case with testosterone effects, estrogen effects on the epithelium of the mammary ducts are mediated via paracrine influences from the mammary connective tissue stroma, which contains the estrogen receptors. During the later stages of hair formation, the hair bulb becomes infiltrated with melanocytes, which provide color to the hair. Individual mammary placodes form from aggregation and proliferation of ectodermal cells of the milk line under the inductive influence of the signaling molecule neuregulin-3. These cellular properties can be explained by the loss of several integrins, which attach the basal cells to the underlying basal lamina. EPUs have a long life, and under normal conditions their distribution on the skin surface is stable. These cells produce molecules characteristic of bone (type I collagen, osteocalcin, and osteopontin) and form spicules of intramembranous bone. E, At 6 months. Because of their high content of keratin, epidermal cells are given the generic name keratinocytes. These findings, especially when focal, are not necessarily associated with a scarring outcome.19, Fiona M. Watt, in Handbook of Stem Cells, 2004. Late in the first trimester, the epidermis is invaded by, Progression from a single-layered ectoderm to a stratified epithelium requires the activation of the transcription factor, When the multilayered epidermis becomes established, a regular cellular organization and sequence of differentiation appear within it (. 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