1. Stratum basale (stratum germiuativum). This single layer of columnar basophilic keratinocytes rests on the basal lamina that separates epidermis from dermis. These cells divide continuously and give rise to the keratinocytes in all other layers. They attach to their neighbors by desmosomes and to the basal lamina by hemidesmosomes. Cytokeratin intermediate filaments (tonofilaments) are important components of both junctions. The cytokeratin content increases as these cells approach the stratum corneum, where it constitutes about 50% of their total protein. The basophilia of the basal layer is caused by ribosomes.
2. Stratum spinosum. This comprises several layers of large keratinocytes overlying the stratum basale. The cells are cuboidal or polygonal in the deeper layers and slightly flattened in the upper layer. Tonofibrils (tonofilament bundles) fill the cytoplasm, extend into the numerous cell processes that give these cells their spiny appearance, and insert into the desmosomes that attach the tips of these processes to those of adjacent cells. The mitotic rate here is lower than in the stratum basale. Mitosis occurs only in the malphighian layer, which includes the stratum basale and stratum spinosum.
3. Stratum granulosum. This lies above the stratum spinosum and, in thick skin, consists of 3-5 layers of flattened polygonal (often diamond-shaped) cells that contain numerous mem braneless keratohyalin granules. The intense basophilia of these granules is caused by their content of a phosphorylated histidine-rich precursor of the protein filaggrin. Cells in this layer also contain small ovoid or rodlike lamellar granules. These fuse with the plasma membrane and release their contents (glycosaminoglycans and phospholipids) into the inter cellular spaces. This material may be important in sealing the deeper layers of the skin from the external environment and in protection from dehydration.
4. Stratum lucidum, This layer overlies the stratum granulosum and is apparent only in thick skin. It is a narrow, acidophilic, translucent band of flattened keratinocytes whose nuclei, organelles, and intercellular borders are not visible. The cytoplasm contains dense cytokeratin aggregates embedded in an amorphous electron-dense matrix derived from the keratohyalin granules. This intracellular mixture of intermediate filaments and matrix con stitutes the immature keratin, sometimes called eleidin,
5. Stratum corneum, The outermost layer, this consists of many layers of dead, platelike enucleate keratinocytes with thickened plasma membranes. These cells represent the final stage of keratinization and are filled with mature keratin, a birefringent scleroprotein consisting of at least 6 polypeptides. The molecular weights of the polypeptides of mature keratin in the stratum corneum is higher than those of immature keratin in deeper, less differentiated cells. Keratin's substructure includes tonofilament subunits formed by 3 coiled and intertwined polypeptide chains. Nine of these subunits coil together to form each IO-nm thick intermediate filament. As they aggregate end to end, the tonofilament increases in length. Tonofilaments are embedded in and bound together by the amorphous matrix first found in keratohyalin granules. Dead cells are continuously sloughed (exfoliated) from the surface and replaced, through successive waves of mitosis and differentiation, by cells from the deeper waves.