Volume XXI - Nr.1 - March 2006
Keratin and keratinocytes
What is epidermis?
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.
Keratin is the IF (Intermediate Filaments) type
which identifies all ephitelia, whether keratinized
or not; its synthesis and that of filaggrin
can indeed take place before the changes, which
lead to the keratinization of an epithelium,
occur.
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.
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.
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.
Keratinocytic functions
Light keratin
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.
In this context, the keratinocytic functions of the Malpighian layer can be synthetized as follows:
- Basal and prickle cells: these provide for synthesis and assembly of the various, relatively low sulphur fibrous keratins;
- Granular cell: producing the amorphous keratin matrix with a high sulphur content in which profilaggrin provides for aggregation of the fibrous keratin filaments into orderly macrofibrils. The granular cell also produces the proteins for induration and thickening of the corneous cell membrane;
- Corneous cell; in the initial corneification process, the granular cell takes on the appearance of a transition cell, losing its nucleus as well as its cytoplastic organules and lipid Odland corpuscles. This material is discharged from the cell and will partly form the intercellular cementing lipid mass, while partly degrading so as to form the NMF .
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.