Keratinocytes account for 95% of the cells of the epidermis. These cells are mainly responsible for the production of the family of fibrillar proteins known as keratins, but other proteins and sterols are, to a lesser degree, also synthesized. Keratinocytes begin their life as germinative, undifferentiated cells at the dermal-epidermal junction. These cells are termed basal cells. Stem cells in the basal layer (and, in certain circumstances, the layer immediately above) are continuously dividing in such a way that one-half of the cells remain in place to provide cells for replenishment, while the other half enter a progressive path to the skin surface where they will be exfoliated. These basal cells (which already show some degree of perinuclear keratin formation) are attached to one another and to the underlying basement membrane in a series of junction points known as desmosomes. These desmosomal connections are continuously reabsorbed and rebuilt as cells move by one another during their outward journey. Through these desmosomes and other connecting points known as gap junctions, a rather constant 200-A distance is kept between adjacent cells. This intercellular space allows for the diffusion of nutrients to the cells as they move outward.
As cells migrate outward from the basal layer, the desmosomes, as a result of histologic fixation artifact, become visible as prickles or spines. Thus the middle layer of the epidermis has become known as the spinous or prickle cell layer. Cell activity in this layer is dominated by the production of keratin, the fibrils of which are now seen to extend to the cytoplasmic wall where they connect as tonofilaments to the cell membrane at the site of desmosome formation.
In the outermost portion of the spinous layer, the keratinocytes begin to change rather dramatically. Two types of granules appear in the cytoplasm, and the cells become increasingly flattened. The large keratohyalin granules contain the protein filaggrin, which assists in forming the aggregates of keratin fiblils. This process may be visualized as the formation of steel rod-reinforced concrete. The smaller lamellar granules (also called membrane-coating granules or Odland bodies) attach to the cytoplasmic membrane where their lipid contents are released into the intercellular space as f1attened liposomes. These lipids (often likened to the “mortar” between the keratinocyte “bricks”) are responsible paradoxically for both water retention in the outer epidermis and for the formation of the water-impervious barrier zone.
At the outer portion of the granular layer the flattened keratinocytes lose visible evidence of the keratohyalin granules and, as they die, lose their nuclei. These dead keratinocytes form the stratum corneum layer and are eventually exfoliated at a rate that, in the absence of disease, allows for a constant thickness of the epidermis.
Results of studies of cell kinetics suggest that in normal epidermis, mitotically active basal cells have a cell cycle time of 200-400 hours. The cells released from the basal layer traverse the remainder of the epidermis to the level of the stratum corneum in approximately 2 weeks. The rate of cell division and transit time to the surface is controlled by a number of growth factors and other molecules. Disturbances in epidermal cell kinetics occur in many skin diseases and have been especially well studied in psoriasis.
Tags:basal cells, cytoplasm, Human Anatomy and Physiology, keratin, proteins stem cells
Leave a Reply