Ultrastructure of the contrasting types of keratinization seen in the tail epidermis of the laboratory mouse Mus musculus.

The mouse tail epidermis undergoes contrasting forms of keratinization. Around the hair follicle there is a granular layer containing keratohyalin granules, and nuclei are absent from the horny layer. In the scale regions keratohyalin is not formed and nuclear remnants are retained in the horny cells as in parakeratosis generally. These findings from light microscopy were confirmed by transmission electron microscopy. The complete breakdown of organelles in the follicular regions contrastred with the retention of effete nuclei in the scales. Some of these nuclear remnants were pyknotic as in abnormal human parakeratosis, but most were further degraded with loss of nuclear membranes. In the boundary zone between the follicular and scale regions the epidermal cells had a few small keratohyalin granules and also showed incomplete degradation of nuclear remnants in the horny cells. The change from living epidermal cells to dead keratinized cells was abrupt in both the follicular and scale regions. In both sites the plasma membranes of the horny cells were thickened and there was a cytoplasmic meshwork of microfibrils in the cells.

Some ultrastructural observations on human palmar stratum corneum.

Ultrastructure of human palmar stratum corneum was examined for cell junctions and cytoplasmic material. Interdigitated microvilli are shorter and less uniform in man. Most of these processes are either conical or truncated in shape, as distinct from the relatively smooth cell surfaces except for sigma-shaped junctions in hairy sites. It is suggested that this provides strength against shearing force in the palm and sole. There is less cytoplasmic breakdown in palmar horny cells than in hairy cells.

Bio-assay of corticosteroids for topical application.

Five commercially available corticosteroid creams were compared with respect to their thinning action on mouse tail epidermis. The greatest effects were produced by Synalar and Sintisone; hydrocortisone had the least action. In addition it was shown that the epidermal thickening induced by vitamin A could be significantly reduced by Ledercort cream. The thinning action of the fluorinated steroids on the epidermis is an important property of these agents when used for the treatment of dermatoses having a hyperactive epidermis. The mouse tail test is considered to be a useful screening test for new topical applications of these substances.

Ultrastructural demonstration of cystine in guinea pig back stratum corneum.

A method for the ultrastructural demonstration of cystine in keratinized epidermal cells of guinea pig back hairy skin is described. This involves selective oxidation by peracetic acid and formation of an electron dense reaction product with uranyl acetate. Unkeratinized cells were unstained except for nuclei and keratohyalin. The thickened plasma membranes of transitional and keratinized cells were darkly stained. In transitional keratinized cells darkly stained membranes occurred initially in relation to desmosomes and then spread to intervening membrane areas. Dense material, probably matrix keratin containing cystine was found in the interiors of basal horny cells but not in the intermediate or superificial cells of the stratum corneum. It is suggested that the unstable matrix keratin is probably broken down by enzymatic hydrolysis above the basal zone of the stratum corneum.

Plantar epidermis of the guinea pig and characteristics of the stratum corneum.

The guinea pig plantar epidermis was examined by light-microscopical histochemical methods and by transmission electron microscopy. Autolysis of cell structure was much less complete in guinea pig plantar horny layer than in the back, and stainable cytoplasm was retained in keratinized cells but organelles were lost except for some degraded ultrastructural remnants. By light microscopy the whole thickness of the horny layer showed bound phospholipid and bound cysteine, and there was a weak cystine reaction at the peripheries of the keratinized cells. In ultrastructure the keratohyalin contained slightly larger subparticles than in the back skin. The horny layer was not divisible into basal, intermediate and superficial regions as in hairy skin. The stratum lucidum of light microscopy was not defined in electron micrographs. Osmium-stained cytoplasmic material was retained in horny cells about to be desquamated, in contrast to the empty appearance of these cells in hairy skin. Epidermal cells in plantar skin have ultrastructural cytoplasmic processes which are longer than they are broad. In the horny layer these interdigitate with those of neighbouring cells and are held together by lateral demonsomal junctions. Probably this gives mechanical strength against shearing forces experienced by the plantar horny layer.

Vitamin A-induced synthesis of alkaline phosphatase.

Incorporation of radioactive leucine into electrophoretically separated proteins from mouse tail epidermis indicates that synthesis of alkaline phosphatase is stimulated by vitamin A. It is suggested that some of the diverse effects of vitamin A may be the result of alkaline phosphatase induction.