Figure 1420

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Photomicrograph showing a late stage in the epidermal repair of a skin wound. The initial injury was caused by an incision through the full thickness of the skin and partially into the hypodermis, which contains adipose ceils (A). The epidermis has re-formed beneath the scab. The asterisk marks an artifact where epithelium separated during specimen preparation. The scab, which contains numerous dead neutrophils in its inferior aspect, is close to the point of release. The dermis at this stage shows little change during the repair process but will ultimately reestablish itself to form a continuous layer. xllO.

mate. It consists of densely packed keratin filaments embedded in a matrix of amorphous keratin with a high sulfur content, which is responsible for the hardness of the nail. The process of hard keratin formation, as with the hair cortex, does not involve keratohyalin granules. In addition, a cornified cell envelope contains proteins similar to those found in the epidermis.

The constant addition of new cells at the root and their keratinization account for nail growth. As the nail plate grows, it moves over the nail bed. On the microscopic level, the nail plate contains closely packed interdigitating corneocytes lacking nuclei and organelles.

The crescent-shaped white area near the root of the nail, the lunula, derives its color from the thick, opaque layer of partially keratinized matrix cells in this region. When the nail plate becomes fully keratinized, it is more transparent and takes on the coloring of the underlying vascular bed. The edge of the skin fold covering the root of the nail is the eponycbium, or cuticle. The cuticle is also composed of hard keratin, and for this reason it does not desquamate. Because of its thinness it tends to break off or, as with many individuals, it is trimmed and pushed back. A thickened epidermal layer, the byponychium, secures the free edge of the nail plate at the fingertip.

The skin, or integument, consists of two main layers: the epidermis, composed of stratified squamous epithelium that is keratinized, and the dermis, composed of connective tissue. Under the dermis is a layer of loose connective tissue called the hypodermis, which is also generally referred to as the subcutaneous tissue or, by gross anatomists, as the superficial fascia. Typically, the hypodermis contains large amounts of adipose tissue, particularly in an adequately nourished individual.

The epidermis gives rise to nails, hairs, sebaceous glands, and sweat glands. On the palms of the hands and soles of the feet, the epidermis has an outer keratinized layer that is substantially thicker than that over the other parts of the body. Accordingly, the skin over the palms and soles is referred to as thick skin, in contrast to the skin over other parts of the body, which is referred to as thin skin.

There are no hairs in thick skin. In addition, the interface between the epidermis and the dermis is more complex in thick skin than in thin skin. The finger-like projections of the dermis into the base of the epidermis, the dermal papillae, are much longer and more closely spaced in thick skin. This provides greater resistance to frictional forces acting on this skin.

In this sample of thick skin, the epidermis (Ep) is at the top; the remainder of the field consists of dermis, in which a large number of sweat glands (SW) can be observed. Although the layers of the epidermis are examined more advantageously at higher magnification (e.g., Fig. 3), it is easy to see, even at this relatively low magnification, that about half of the thickness of the epidermis consists of a distinctive surface layer that stains more lightly than the remainder of the epidermis. This is the keratinized layer. The dome-

shaped surface contours represent a cross section through the minute ridges on the surface of thick skin that produce the characteristic fingerprints of an individual.

In addition to sweat glands, the dermis displays blood vessels (BV) and adipose tissue (AT). The ducts of the sweat glands (D) extend from the glands to the epidermis. One of the ducts is shown as it enters the epidermis at the bottom of an epithelial ridge. It will pass through the epidermis in a spiral course to open onto the skin surface.

A sample of thin skin is shown here to compare with the thick skin in Figure l. In addition to sweat glands, thin skin contains hair follicles (HF) and their associated sebaceous glands (SGI). Each sebaceous gland opens into a hair follicle. Often, as in this tissue sample, the hair follicles and the glands, both sebaceous and sweat, extend beyond the dermis (De) and into the hypodermis. Note the blood vessels (BV) and adipose tissue (AT) in the hypodermis.

Figure 3, skin, human, H&E X320; inset x640.

The layers of the epidermis of thin skin are shown here at higher magnification. The cell layer that occupies the deepest location is the stratum basale (SB). This is one cell deep. Just above this is a layer several cells in thickness, the stratum spinosum (SS). It consists of cells that have spinous processes on their surface. These processes meet with spinous processes of neighboring cells and, together, appear as intercellular bridges (arrows, inset). The next layer is the stratum granulosum (SGr), whose cells contain kerato-

hyalin granules (arrowhead, inset). On the surface is the stratum corneum (SC). This consists of keratinized cells, i.e., cells that no longer possess nuclei. The keratinized cells are flat and generally adhere to other cells above and below without evidence of cell boundaries. In thick skin, a fifth layer, the stratum lucidum, is seen between the stratum granulosum and the stratum corneum. The pigment in the cells of the stratum basale is melanin; some of this pigment (P) is also present in connective tissue cells of the dermis.

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