Layer 1: Stratum Basale/Germinativum
This is the deepest layer of the epidermis, composed of a single layer of cuboidal or columnar keratinocytes. Some stem cells are located in this layer undergoing cell division which continually produces new keratinocytes. Keratinocytes located in this layer have large nuclei, many ribosomes in the cytoplasm, a small GC, few mitochondria, and some rough ER. Throughout the cytoskeleton are scattered intermediate filaments called Tonofilaments . Tonofilaments are composed of a protein that will form Keratin in more superficial epidermal layers and attach to desmosomes which bind cells of adjacent layer. Tonofilaments also attach to hemidesmosomes, which bind the keratinocytes to the basement membrane positioned between the epidermis and the dermis. Merkel cells/discs and melenocytes are scattered throughout this layer.
Layer 2: Stratum Spinosum
This layer is superficial to the stratum basale arranged in 8-10 layers of keratinocytes fitting closely together. These keratinocytes have the same organelles as stratum basale. Bundles of tonofilaments are inserted into a desmosome, tightly joining the cells to one another. This provides both strength and flexibility to the skin. Langerhans cells and melanocytes are present in this layer.
Layer 3: Stratum Granulosum
Approximately the middle of the epidermis is this layer which is composed of 3-5 layers of keratinocytes undergoing apoptosis, or genetically programmed cell death. In this layer, a protein called Keratohyalin converts the tonofilaments into keratin. Also present in this layer and within the keratocytes are membrane enclosed Lamellar Granules which release a lipid-rich secretion. As mentioned above this secretion acts as a water-repellent sealant.
Layer 4: Stratum Lucidum
This layer is present only in the thicker areas of skin or hairless areas. It contains 3-5 layers of clear, flattened, dead keratinocytes with large amounts of keratin and thickened plasma membranes.
Layer 5: Stratum Corneum
This outer layer contains 25-30 layers of flattened, dead keratinocytes. Cells of this layer are continuously being shed and replaced by cells of the deeper strata. It slows water loss and acts as a barrier to substances from entering the skin. Each of these layers is of variable thickness depending on the location of the body. As an example, contrast the soft kin of the horse’s lower eyelid and the thick skin covering the dorsolateral aspects of the trunk which can be as much as 10 mm in thickness in stallions. Skin is thickest where the requirement for protection is greatest.
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