The animate cells of the epidermis make a sharp, upper interface with the lifeless stratum corneum. They also have a well-demarcated, deep interface with the dermis (Figure 1). When physicochemically considered, the viable epidermis is nothing more than a wedge of tightly massed, live cells. Consequently, the whole of this live, cellular mass is regarded as a singular diffusional field (resistance) in percutaneous absorption models, although, when viewed under microscope, the tissue is clearly multilayered. The identifiable strata, bottom to top, are (i) the basal layer (stratum germinativum), a single layer of cubical or columnar cells that is unremarkable in appearance; (if) the multicellular spinous or prickle layer (stratum spinosum) in which the cells exhibit sharp surface protuberances; and (iii) the granular layer (stratum granulosum), a thin layer that stains to yield a mottled appearance. In some histological displays, a fourth, upper transitional and translucent layer is also distinguishable (stratum lucidum). These layers reflect the progressive differentiation of the cells that eventuates in their death and placement as "bricks" within the horny structure. As elsewhere in the body, water found in the live epidermis has an activity equivalent to that of a highly dilute, isotonic NaCl solution (0.9% NaCl). The density and consistency of the live epidermal composite are only a little greater than those found for water.
The interface the viable epidermis makes with the stratum corneum is flat. However, the interface with the dermis is papillose (mounded). Myriad tiny bulges of the epidermis fit with exacting reciprocity over dermal depressions and ridges. It is these ridges that give the friction surfaces of the body their distinctive patterns (e.g., fingerprints). Importantly, since hair follicles and eccrine glands have epidermal origins, cells capable of regenerating the epidermis actually extend well into and through the dermis by way of these tiny glands (Figure 1). When the skin is superficially injured, surviving cells at the base of these glands regenerate a scar-free surface. Discounting these deep rootages, the epidermis is on the order of 100 to 250 |im thick (9).
Table 2 Cells of the Skin
Cell type Principal function
Cells of the epidermis Keratinocytes Langerhans cells Melanocytes
Cells of the dermis Fibroblasts Mast cells Blood cells Endothelial cells Nerve cells and endings
Source: From Refs. 2 and 11.
Keratinocytes account for most of the cells of the epidermis. One also finds Langerhans cells of white blood cell progeny here. The latter well-dispersed cells function as antigen-presenting cells (APCs) in the skin's immunological responses. Yet another group of cells, melanocytes, are strategically placed in the epidermis just above the epidermal-dermal junction (Table 2). Acting under the influence of melanocyte-stimulating hormone (MSH), they synthesize and deposit the pigment granules into skin, which give the races of human beings their unique skin coloration. Melanocytes are also set into action by ultraviolet radiation. Their activities in this circumstance lead to sun tanning. Migrant macrophages and lymphocytes are occasionally found in skin sections. Such cells can be numerous in traumatized skin.
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