It is now right to ask ourselves what the epidermis is, what new ideas about it we have acquired and why the old schemes must be basically modified.
By epidermis, we mean the section embodied between the basal membrane (which separates it from the dermis) and the external surface, i.e. the stratum corneum. This membrane, which is at the base of all epithelia, is synthesized in its upper half by the overlying epitheliocytes, while its lower half is produced by elements in the underlying connective tissue, i.e. the dermis.
Immediately above the basal membrane we find the germinative cells, characterized by intense mitotic activity, since the elements afterwards detected in the overlying layers are reproduced by endogenous division of the mother cell which remains attached to the basal membrane and maintains its germinative capacity.
The basal cell by mitosis forms a new epithelial element which begins to migrate towards the surface by means of a process of maturation characterized by active and varied protein syntheses. The most important, and now made quite clear , is the synthesis of "light keratin", a cytoplasmatic protein filament sole and typical of all epithelial or epithelium-derived cells.
Keratin is the IF (Intermediate Filaments) type which identifies all ephitelia, whether keratinized or not; keratin and filaggrin synthesis can indeed take place before the changes, which lead to the keratinization of an epithelium.
An adequate availability of HRP (Histionine Rich Protein) forms the biochemical basis for complete aggregation of the fibrous keratin filaments in the amorphous keratin matrix. All things considered, it would lead to the formation of a highly efficient corneous barrier.
At the same time, profilaggrin is dephosphorylized, thus providing for uniform aggregation of the keratinic matrix and fibres, then to degrade, leaving the cell to partake in the formation of epicutaneous NMF. In the next cornification phase of the epithelial cell, the membrane proteins are deposited (involucrin, keratolinin, vinculin, etc.) causing inspissation (of non keratinic nature, but much more resisting than the keratin around which it is wrapped). Obviously, each cellular layer of the epidermis has a well defined, reciprocally dependent biosynthesis.
Now we may explain why we are dealing with light keratin (previously named cytokeratin). Usually this protein (MW from 46,000 to 58,000 Daltons) is found only in the basal elements, most likely as a cytoskeleton, and is closely linked to mitotic phenomena. During the maturation from the epidermal basal layers to the surface ones, the keratinic proteins polymerize and become heavy keratins (in the most external layers their MW ranges from 63,000 to 67,000 Daltons). Nevertheless the light form remains within the cell as a product of the previous synthesis.