Ultrastructure of the fibrous substance in the keratinocytes of the epidermis in healthy individuals.

The present findings in the epidermis of healthy individuals suggest that the fibrous substance in the keratinocytes is subjected to an individual differentiation as the cells move from stratum basale through strata spinosum and intermedium to stratum coreum. In the basal cells the fibrous substance consists of opaque tonofilaments, mostly aggregated in dense tonofibrils, but also occurring randomly scattered and in loose tonofibrils (fibrous substance of basal-cell type). In most cells the fibrous substance undergoes differentiation: a) in stratum spinosum into compact tonofibrils with no individual tonofilaments visible (fibrous substance of spinous-cell type), b) in stratum inter-medium into compact tonofibrils with intensely stained regions or keratohyalin (fibrous substance of intermediate-cell type), and c) in stratum corneum into tonofibrils with a keratin pattern (fibrous substance of horny-cell type). There are also keratinocytes in which the fibrous substance undergoes only an incomplete differentiation, with the appearance of horny cells with a fibrous substance of spinous-cell type, intermediate-cell type or horny-cell type with keratohyalin. Finally, there are keratinocytes in which the fibrous substance does not undergo any differentiation at all, but which display a fibrous substance of basal-cell type throughout the entire epidermis.