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.