which layer of the epidermis is responsible for cell division and replacement
Epidermis The stratified squamous epithelium is maintained by cell division within the stratum basale. Differentiating cell delaminate from the basement membrane and are displaced outwards through the epidermal layers, undergoing multiple stages of differentiation until, in the stratum corneum, losing their nucleus and fusing to squamous sheets, which are eventually shed from the surface (desquamation). Differentiated keratinocytes secrete keratin proteins which contribute to the formation of an extracellular matrix and is an integral part of the skin barrier function. In normal skin, the rate of keratinocyte production equals the rate of loss,[4] taking about two weeks for a cell to journey from the stratum basale to the top of the stratum granulosum, and an additional four weeks to cross the stratum corneum.[2] The entire epidermis is replaced by new cell growth over a period of about 48 days.[11]
Melanocyte Melanocytes (/məˈlænəˌsaɪt, -noʊ-/ ( listen) or /ˈmɛlənəˌsaɪt, -noʊ-/[1][2]) are melanin-producing neural-crest derived[3] cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea),[4] the inner ear,[5] meninges,[6] bones,[7] and heart.[8] Melanin is the pigment primarily responsible for skin color. Once synthesised, melanin is contained in a special organelle called a melanosome and moved along arm-like structures called dendrites, so as to reach the keratinocytes.