Stratum Corneum

The outermost layer of the skin appearing in the exploded epidermal sketch of Figure 1 represents the stratum corneum (the horny layer). Stratum corneum, the principal barrier element of the skin, is an essentially metabolically inactive tissue that comprises acutely flattened, stacked, hexagonal cell-building blocks formed from once-living cells. These cellular building blocks are layered 15 to 25 cells deep over most of the body (2). At some sites, the cells appear to be stacked in neat columns. The stratum corneum exhibits regional differences in thickness, being as thick as several hundred micrometers on the friction surfaces of the body (palms and soles). However, over most of the body, the tissue measures only about 10 |im thick, dimensionally less than a fifth of the thickness of an ordinary piece of paper (2,3). It is a dense tissue, about 1.4 g/cm3 in the dry state, a fact that has led it to also be referred to as the stratum compactum.

The stratum corneum is continuously under formation. Microscopic flakes (squama) dislodged from the surface through wear and tear are replaced with new cells from beneath, with complete turnover of the horny layer occurring roughly every two weeks in normal individuals (4). In humans the cells that give rise to the stratum corneum originate exclusively in the basal layer of the epidermis. Cell division in the basal layer begins an extraordinary process in which daughter cells are pushed outward, first to form a layer of so-called spinous or prickle cells and then, serially, the granular, lucid, and horny layers. As suggested in Figure 1, during their transit through the epidermal mass, the cells flatten acutely. The protein and lipid components that eventually characterize the fully differentiated horny layer are synthesized in transit, with the initial strands of the structural protein that will eventually fill the cell interior being formed in the basal layer. During their inexorable migration to the stratum corneum, the protein content of the cells expands to the point that massed proteins of several kinds are distinguishable as they merge into the granular layer. In culmination, the intracellular space of the fully differentiated horny cell is literally packed with structural protein, namely, semicrystal-line a-keratin intermixed with more amorphous P-keratin. Nothing but keratin is visible inside the fully differentiated horny cell by electron microscope. The intracellular space is dense, offering little freedom of movement to organic molecules that may become dissolved within it. Moreover, because of its remarkable ionic character, the intracellular keratin mass borders on being thermodynamically impenetrable.

Lipid synthesized during a keratinocyte's epidermal transit is collected in small vesicles that become visible within the granular layer. These were designated membrane-coating granules long before their content and function were known. As granular cells undergo transformation, the "coating granules" gravitate to the outermost cell membrane and are passed exocytotically into the intercellular space. A lipid "mortar" is formed that seals the horny structure. Virtually all the lipid of the stratum corneum is in the interstitial space, much of it being present in liquid crystalline, bilayer assemblage (5). The densely packed keratin platelets caulked with intercellular lipid make the stratum corneum, pound for pound, an incredibly resistive moisture barrier. An exoskeleton (infrastructure) of residual cell membranes bound together by desmosomes and tonofibrils acts to separate the keratin and lipid domains. The lipid content of the horny layer represents about 20% of the stratum corneum's dry weight, while the endoskeleton contributes roughly 5% to the weight (Table 1) (2,4,6).

In its normal state at ordinary relative humidity, the stratum corneum takes up moisture to the extent of 15% to 20% of its dry weight (2). The ionic character of keratin is certainly a factor of consequence here. Should the skin become waterlogged, the water content of horny tissue covering the friction surfaces (callused surfaces) can rise to several multiples of the tissue's dry weight. The water content of stratum corneum appears to be less affected under the same circumstance. It appears that the stratum

Table 1 Composition of the Stratum Corneum

Percentage of

Table 1 Composition of the Stratum Corneum

Percentage of

Tissue component

Gross composition

dry weight

Cell membrane

Lipid, protein

«5

Intercellular space

Mostly lipid, some protein, and

«20

0 0

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