The epidermis is avascular (no blood flow) and is nourished by diffusion from the dermis. Contains keratinocytes, melanocytes, Langerhans cells, Merkel cells andinflammatory cells. Keratinocytes are the most abundant cell type, constituting 95% of the cells that compose it. At some points the thickened epidermis invaginates into the interior of the dermis,forming Rete ridges, which in turn define the dermal papillae or dermal papilla.
The epidermis is composed of 4 or 5 layers, depending on the region of the skin. In order from the outermostto the innermost are named: stratum corneum (stratum corneum), translucent layer (stratum lucidum), granular layer (stratum granulosum), spinous (stratum spinosum) and basal or germinal layer(stratum basale / germinativum ). The term "Malpighi layer" or "Malpighian layer (stratum malpighian in honor of Marcello Malpighi) is commonly used to define the set of basal layer and cord.
Cell kineticsThe stratified squamous epithelium is maintained by continuous division of cells in the basal layer. The cells were differentiated on the basis of the basal lamina as they migrate into the outerlayers of the epidermis. When you reach the horny layer lose their nuclei and fuses to the flaky layers, which becomes detached from the skin surface by desquamation. In a normal healthy skin, thenumber of new cells being produced is equal to that of cells that break off, taking two weeks for a cell to travel from the basal layer to the top of the granular layer, and four additional weeks tocross the stratum corneum. The epidermis is completely renewed in a period of 48 days.
Epidermal organogenesis, ie the formation of the epidermis, begins in the cells lining the embryoafter neurulation, the formation of the central nervous system. In most vertebrates, this original monolayer structure is quickly transformed into a fabric composed of two layers: an outer layer...