Upward of 300

Source: From Refs. 2 and 3.

corneum covering the greater part of the human body has less capacity to imbibe water. Nevertheless, all horny tissue becomes hydrated to some degree when the natural evaporation of water from the skin's surface, so-called insensible perspiration, is held in check upon applying an occlusive dressing. The horny tissue becomes more pliable. Consequently, molecules diffuse through it with greater facility. And it is likely that some substances may exhibit greater solubility within the hydrated horny mass, adding further to their ease of permeation. Conversely, the stratum corneum becomes brittle when it dries out. Ultradry, inelastic horny tissue splits and fissures when stretched, giving rise to the common conditions we know as chapped lips, windburn, and dishpan hands.

The stratum corneum is thus a dense, polyphasic epidermal sheathing made from dehydrated and internally filamented former cells held together by desmosomes, tonofibrils (intercellular anchors), and interstitial lipid. It has been estimated that it contains 10 times the fibrous material of the living epidermis in roughly one-tenth the space (7). The stratum corneum is in contact with the living epidermal mass at its undersurface. Its external surface interfaces the environment. Cells at the basement of the stratum corneum contain water at high thermodynamic activity of the physiological milieu, whereas air at the surface of the skin tends to have a far lower water activity. As a result, under ordinary circumstances, water diffuses out through the skin (down the implied activity gradient) and into the environment. This process is known as insensible perspiration. About 5 mL of water is lost this way per square meter of intact body surface per hour (or 0.5 mg/cm2/hr) (8).

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