Stratum corneum morphology

Potts, R.O. Francoeur, M.L. The influence of stratum corneum morphology on water permeabihty. J. Invest. Dermatol. 1991, 96, 495-499. 

The innermost layer of the epidermis, the stratum basale, consists of a single layer of columnar-shaped, undifferentiated stem cells. Mitosis of these cells constantly renews the epidermis and this proliferation compensates for the loss of dead stratum corneum cells (corneocytes) from the skin surface. As the cells produced by the basal layer move upward, they alter morphologically as well as histochemically to form the outermost layer, the stratum corneum. Over a 4- to 5-week period the entire epidermis is renewed [5]. 

Bouwstra, J. A., et al. 2003. Water distribution and related morphology in human stratum corneum at different hydration levels. J Invest Dermatol 120 750. 

Sato, J., Yanai M., and Denda, M. (2000) Water content and thickness of stratum corneum contribute to skin surface morphology. Arch. Dermatol. Res. 292 412-417. 

In general, rat skin has been proven more permeable than human skin (ECETOC, 1993). This is likely to be caused by the different structure of the skin in both species (e.g. number of appendages, intercellular lipid composition of the stratum corneum and corneocyte surface area). It should be noted, however, that some cases have been reported where rat skin was found to be less permeable than human skin (Hotchkiss et al., 1992). Since the availability of human skin is limited, pig skin is often used. Because of its similar morphology and barrier function (Bronaugh et al., 1982), pig skin is often considered a good alternative to human skin. 

Stratum corneum, the outermost layer of mammalian epidermis, functions physiologically as the principal diflFusion barrier to molecules penetrating the skin and as a protective physical barrier to mechanical insults at the skin surface. Data suggest that these functions are critically dependent on the specific morphological and macromolecular organization of the membrane mosaic (16, 17, 18, 19, 20). Thus, alterations of biophysical properties arise from environmental factors acting directly on the membrane or upon the keratinization process, and they affect... 

Stratum corneum used in these studies was isolated from human, newborn rat, callus, and guinea pig foot pad. The methods of isolating the various tissues have been discussed elsewhere (10,11). Even though the various corneum tissues differ somewhat in morphology and chem-... 

Stratum corneum is a multicellular membrane of acutely-flattened, metabolically-inactive cells stacked in vertical columns (22). This stratified morphological organization is demonstrated in a freshly frozen transverse section of epidermis (Figure 2) where the swelling aids the visualization. The morphological architecture is such that the cells in one column interdigitate with those in adjacent columns to form a con-... 

Stratum corneum breaking strength decreases fourfold over the 0-100% RH range reaching a minimum at approximately 90% RH which is not lowered further by immersion in water. Of fundamental importance is the morphological location within the stratum corneum where failure occurs under a uniaxial load. Scanning electron microscopy (SEM) and conventional analysis of fractured samples indicate that the samples predominately fracture within the intercellular junctions rather than intracellularly (9). 

Thus we know, for example, that permeation of compounds across the skin is, in most cases, controlled by the stratum corneum, and that it is the chemical composition and morphology of this layer that usually determines the rate and extent of absorption (Elias 1981 Raykar et al. 1988). We also know how to modify this barrier, by chemical or physical means, to alter the rate of diffusion of many permeating molecules (Walters and Hadgraft 1993 Smith and Maibach 1995). 

Protein substantivity on the skin has been investigated by in vivo and in vitro studies. The adsorption of collagen peptides on human callus, morphologically and chemically similar to the stratum corneum of normal skin, has been demonstrated to occur rapidly from aqueous solutions and detergent formulations, even at very low protein content, and is quantitatively comparable to the adsorption on hair (79). 

The events which normally transform germinative epithelial cells of the human epidermis into differentiated squamous cells and then into more fully keratinized cells of the stratum corneum (normal keratinization) are poorly understood. Although the morphological consequences of these events are well described for both normal skin and the skin of various pathologic states, the critical biochemical and physiologic parameters which result in a smooth, supple, continuous skin surface or deviation from the more normal condition have not been identified. 

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