<P> The Malpighian layer (stratum malpighi) is both the stratum basale and stratum spinosum . </P> <P> The epidermis is separated from the dermis, its underlying tissue, by a basement membrane . </P> <P> 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, 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 . The entire epidermis is replaced by new cell growth over a period of about 48 days . </P> <P> Keratinocyte differentiation throughout the epidermis is in part mediated by a calcium gradient, increasing from the stratum basale until the outer stratum granulosum, where it reaches its maximum, and decreasing in the stratum corneum . Calcium concentration in the stratum corneum is very low in part because those relatively dry cells are not able to dissolve the ions . This calcium gradient parallels keratinocyte differentiation and as such is considered a key regulator in the formation of the epidermal layers . </P>

Which layer of the epidermis is responsible for cell division and replacement