<P> Elevation of extracellular calcium concentrations induces an increase in intracellular free calcium concentrations . Part of that intracellular increase comes from calcium released from intracellular stores and another part comes from transmembrane calcium influx, through both calcium - sensitive chloride channels and voltage - independent cation channels permeable to calcium . Moreover, it has been suggested that an extracellular calcium - sensing receptor (CaSR) also contributes to the rise in intracellular calcium concentration . </P> <P> Epidermal organogenesis, the formation of the epidermis, begins in the cells covering the embryo after neurulation, the formation of the central nervous system . In most vertebrates, this original one - layered structure quickly transforms into a two - layered tissue; a temporary outer layer, the periderm, which is disposed once the inner basal layer or stratum germinativum has formed . </P> <P> This inner layer is a germinal epithelium that give rise to all epidermal cells . It divides to form the outer spinous layer (stratum spinosum). The cells of these two layers, together called the Malpighian layer (s) after Marcello Malpighi, divide to form the superficial granular layer (Stratum granulosum) of the epidermis . </P> <P> The cells in the stratum granulosum do not divide, but instead form skin cells called keratinocytes from the granules of keratin . These skin cells finally become the cornified layer (stratum corneum), the outermost epidermal layer, where the cells become flattened sacks with their nuclei located at one end of the cell . After birth these outermost cells are replaced by new cells from the stratum granulosum and throughout life they are shed at a rate of 0.001 - 0.003 ounces of skin flakes every hour, or 0.024 - 0.072 ounces per day . </P>

The main two layers of the skin are the stratum granulosum and stratum spinosum