Anatomy and Physiology of the Skin: Keratinocytes,Basal Layer & Squamous Cell Layer
Keratinocytes
At least 80% of cells in the epidermis are the ectodermally derived keratinocytes. The differentiation process that occurs as the cells migrate from the basal layer to the surface of the skin results in keratinization, a process in which the keratinocyte first passes through a synthetic and then a degradative phase (Chu, 2008). In the synthetic phase, the cell builds up a cytoplasmic supply of keratin, a fibrous intermediate filament arranged in an alpha-helical coil pattern that serves as part of the cell’s cytoskeleton. Bundles of these keratin filaments converge on and terminate at the plasma membrane forming the intercellular attachment plates known as desmosomes. During the degradative phase of keratinization, cellular organelles are lost, the contents of the cell are consolidated into a mixture of filaments and amorphous cell envelopes, and the cell finally is known as a horny cell or corneocyte. The process of maturation resulting in cell death is known as terminal differentiation (James et al., 2006).
Basal Layer
The basal layer, also known as the stratum germinativum, contains column-shaped keratinocytes that attach to the basement membrane zone with their long axis perpendicular to the dermis. These basal cells form a single layer and adhere to one another as well as to more superficial squamous cells through desmosomal junctions (Murphy, 1997). Other distinguishing features of the basal cells are their dark-staining oval or elongated nuclei and the presence of melanin pigment transferred from adjoining melanocytes (Murphy). The basal layer is the primary location of mitotically active cells in the epidermis that give rise to cells of the outer epidermal layers. However, not all basal cells have the potential to divide (Jones, 1996; Lavker & Sun, 1982). Epidermal stem cells in the basal layer are clonogenic cells with a long lifespan that progress through the cell cycle very slowly under normal conditions. Hyperplasiogenic conditions, such as wounding, can increase the number of cycling cells in the epidermis by stimulating division of stem cells. DNA damage caused by carcinogenic agents may mutate cell proliferation machinery and can also affect the rate of cellular division. Migration of a basal cell from the basal layer to the cornified layer in humans takes at least 14 days, and the transit through the cornified layer to the outermost epidermis requires another 14 days (Chu, 2008).
The basal layer, also known as the stratum germinativum, contains column-shaped keratinocytes that attach to the basement membrane zone with their long axis perpendicular to the dermis. These basal cells form a single layer and adhere to one another as well as to more superficial squamous cells through desmosomal junctions (Murphy, 1997). Other distinguishing features of the basal cells are their dark-staining oval or elongated nuclei and the presence of melanin pigment transferred from adjoining melanocytes (Murphy). The basal layer is the primary location of mitotically active cells in the epidermis that give rise to cells of the outer epidermal layers. However, not all basal cells have the potential to divide (Jones, 1996; Lavker & Sun, 1982). Epidermal stem cells in the basal layer are clonogenic cells with a long lifespan that progress through the cell cycle very slowly under normal conditions. Hyperplasiogenic conditions, such as wounding, can increase the number of cycling cells in the epidermis by stimulating division of stem cells. DNA damage caused by carcinogenic agents may mutate cell proliferation machinery and can also affect the rate of cellular division. Migration of a basal cell from the basal layer to the cornified layer in humans takes at least 14 days, and the transit through the cornified layer to the outermost epidermis requires another 14 days (Chu, 2008).
Squamous Cell Layer
Overlying the basal cell layer is a layer of the epidermis that is 5–10 cells thick and known as the squamous cell layer or stratum spinosum (Murphy, 1997). The squamous layer is composed of a variety of cells that differ in shape, structure, and subcellular properties depending on their location. Suprabasal spinous cells, for example, are polyhedral in shape and have a rounded nucleus, whereas cells of the upper spinous layers are generally larger in size, become flatter as they are pushed toward the surface of the skin, and contain lamellar granules (Chu, 2008). Lamellar granules are membrane-bound organelles containing glycoproteins, glycolipids, phospholipids, free sterols, and a number of acid hydrolases, including lipases, proteases, acid phosphatases, and glycosidases. The abundance of hydrolytic enzymes indicates that the lamellar granules are a type of lysosome. Although the lamellar granules primarily are active in cells at the interface between the granular and cornified layers, they also function in cells of the upper spinous layer to deliver precursors of stratum corneum lipids into the intercellular space (Haake & Hollbrook, 1999). Intercellular spaces between spinous cells are bridged by abundant desmosomes that promote mechanical coupling between cells of the epidermis and provide resistance to physical stresses. Organized concentrically around the nucleus, keratin filaments in the cytoplasm are bound to desmosomal plaques at one end and remain free at the end closer to the nucleus (Murphy, 1997). The desmosomal plaques are composed of six polypeptides found on the cytoplasmic side of the cell membrane that are important in the regulation of the calcium required for desmosomal assembly and maintenance (Fairley, Scott, Jensen, Goldsmith, & Diaz, 1991; Hennings & Holbrook, 1983; Lin, Mascaro, Liu, Espana, & Diaz, 1997). The spine-like appearance of the numerous desmosomes along cell margins is where the stratum spinosum derives its name (Chu, 2008). Gap junctions are another type of connection between epidermal cells. Essentially forming an intercellular pore, these junctions allow for physiologic communication via chemical signals that is vital in the regulation of cell metabolism, growth, and differentiation (Caputo & Peluchetti, 1977).
No comments