Embryologic, Histologic, and Anatomic Aspects
The skin of infants is traversed by a subtle maze of ridges that develops during the fourth and fifth months of fetal life. The ridges become increasingly prominent during childhood. Those surface markings remain stable throughout life and are nearly inextinguishable. Swirled patterns characterize the palms and soles. Small, roughly diamond-shaped outlines crisscross the rest of the body surface and are seen particularly well on the volar aspect of the wrists, in the antecubital and popliteal fossae, and between the knuckles (Fig. 1.8). Just as the surface of the epidermis is marked by diverse configurations on different anatomic sites, so, too, the undersurface of the epidermis is stamped by varied contours on different regions of the body (Fig. 1.9).
Gross aspects anatomically of normal skin. A. Lateral ankle. B. Knuckle. C. Index finger. D. Knee. (Courtesy of Gary Wagner.)
Moldings of various sizes and shapes on the undersurface of epidermis at the junction of nipple and areola of a woman. (Courtesy of William Montagna, Ph.D.)
Etchings that cover the entire surface of palms and soles, excluding flexion creases, are termed, collectively, “dermatoglyphic patterns” (Fig. 1.10). Ridges and furrows in parallel form loops, whorls, and arches on the fingertips in patterns so highly individualistic that fingerprinting is used for identification of persons, even for distinguishing between “identical,” i.e., monozygotic, twins. Study of dermatoglyphics has contributed to early detection of genetic abnormalities, e.g., Down’s syndrome, and of defects caused by infections in utero, e.g., rubella. Palmar and plantar skin is typified histologically by a thick cornified and granular layer, a prominent undulate pattern of epidermal rete ridges and dermal papillae, numerous eccrine units and nerve endings, and absence of infundibuloapocrine-sebaceous-follicular units. The corrugated palmar skin, like the treads of tires, is well suited for gripping and grasping objects. The exquisite epicritical sensitivity of the fingertips to tactile stimuli made it possible for Braille to develop his “reading” system for the blind.
Dermatoglyphic pattern on the thumb of a dermatopathologist who was 35 years old in 1971 when this photograph was taken for inclusion in the first edition of this work.
In humans, hair is largely ornamental, whereas in other mammals it serves principally as a furry insulating cover. Eyelashes, like fly-swatters, protect eyes from objects that might blow into them and thereby be injurious, and eyebrows, like awnings, protect the eyes from sunlight and sweat that might be hazardous. Although on casual inspection a human is largely a naked animal (except for the scalp, axillae, and pubes in both sexes and the region of the beard and chest in men), in actuality, the entire body surface, except for palms, soles, glans penis, labia minora, dorsal aspects of terminal phalanges, and mucocutaneous junctions, is punctuated by fine hairs. The tiny ostia from which those vellus hairs emerge are discernible readily on a forearm, for one example. Vellus hairs on the face of women, particularly on the upper lip, usually are inconspicuous until menopause when hormonal changes may cause them to become the thicker and darker terminal hairs of hirsutism. The dense pelage that blankets the scalp is seen by conventional microscopy to be a consequence of numerous large terminal follicles rooted deep in the subcutaneous fat, whereas the wisps of hair that characterize most anatomic sites are manufactured by tiny vellus follicles situated high in the dermis (Fig. 1.11).
Vellus and terminal hair follicles. Both consist of an upper and lower segment, the former permanent and composed of a single part, an isthmus, and the latter transient and made up of a stem and bulb.
The formidable skin of the back, made up mostly of a thick dermis that consists of broad bundles of collagen arranged orthogonically, is well constructed to withstand the stress of the upright posture of humans. In contrast, the distensible skin of the eyelids, with its thin dermis, is designed for accommodating rapid blinking movements necessary to guard the eyes (Fig. 1.12).
Variation in structure of skin and subcutaneous fat on different anatomic sites.
Other regions of the skin have distinguishing features. The middle of the face, particularly in adolescents, is greasy from secretion produced by numerous large sebaceous glands affiliated with infundibula that have prominent ostia (Fig. 1.13B). The helix of ears is covered by numerous vellus hairs and, through a microscope, is seen to possess closely set, minute follicles (Fig. 1.14B). Pigmented zones, such as those of the areolae, contain an increased amount of melanin in the epidermis. The hairy, moist axillae harbor conglomerations of follicles and sebaceous glands in conjunction with countless apocrine and eccrine glands (Fig. 1.12). Erectile tissues, such as clitoris and penis, are endowed with highly vascularized smooth muscles.
Ostia of infundibula on the skin of the nose, a region in which those openings in adults may be prominent and patulous. A. View clinically. B. View histologically. Note numerous sebaceous glands whose duct enters the base of an infundibulum at the junction of that part of the epidermis and the isthmus of the follicle. (x22)
Vellus hairs from vellus follicles on the pinna of an ear. A. View clinically. B. View histologically. (x27)
At the body openings, skin is continuous with mucous membranes from which it differs, principally, by possessing a cornified layer of cells that lack a nucleus, a granular zone, and distinctive epithelial and nonepithelial adnexal structures (Fig. 1.15).
Mucous membrane. Unlike epidermis, nuclei are present within cells at the surface of an epithelium that cornifies only slightly. (x187)
An inevitable consequence of the upright posture of humans is elevated hydrostatic pressure, which, in time, causes the wall of superficial blood vessels of the legs, especially those far below the knees, to become thickened and its lining to display plump endothelial cells (Fig. 1.16). The cutaneous vasculature reflects eloquently intense emotions, among them fear (pallor from constriction), shame (blush from dilation), and anger (lividity from dilation). Telltale signs of anxiety are cold hands and sweaty palms, transmitted incontrovertibly by a “cold, clammy handshake.”
Skin of the ankle. A. View clinically. B. View histologically. Superficial dermal blood vessels in the region of an ankle have a thick wall and plump endothelial cells, those findings being consequent to the effects of stasis. (x250)
The skin is remarkably diversified regionally, grossly and by examination microscopically, and those variations correlate with the multiform functions of it.