Skin Cell types
Keratinocytes are cells that form the majority of the epidermis. They are cells that release very little extracellular matrix, so the skin cell pro membranes of adjacent keratinocytes are usually very close. This is further favored by the numerous desmosomes that allow the cohesion and integrity of the epidermis to be maintained. The main family of proteins that keratinocytes synthesize are keratins, a type of intermediate filament in the cytoskeleton. Keratinocytes have a characteristic life cycle that begins at the basal part of the epithelium and ends at the most superficial part of the epithelium.
1. Morphology and differentiation
The morphology of keratinocytes is not constant throughout its life, which in humans is approximately one month. This change occurs progressively from the basal part of the epidermis, where they are born, to the most superficial part, where they die and eventually come off. The morphological differences that occur in this process are manifested in the form of layers or strata. The strata, from the innermost part to the most superficial, are: basal, prickly, granular and horny. Except the basal, which is relatively constant, the thickness of the strata is variable depending on the body region (Figure 1). The set of strata, that is, the epidermis, can vary from about 50 µm thick in the body areas with little friction to more than 1 mm in areas such as the soles of the feet or hands, where the friction is intense.
2. Basal or germ layer
Figure 2. Strata in the thick skin of a rat's palm. Observe how the morphology of keratinocytes changes from the germinative or basal layer to the corneal or keratinized one.
The basal or germinative layer (Figures 2 and 4) is a layer of one cell thick, essentially formed by keratinocytes, which is located in the innermost part of the epidermis. The cells of this layer are attached to the basal lamina through hemidesmosomes, allowing the stability of the epithelium and the control of the division and differentiation of the keratinocytes.