Since all humans have approximately the same number of melanocytes, racial differences in skin coloring are due to differences in melanocyte activity. As the keratinocytes are pushed upward toward the epidermal surface, they move further from the dermis, where the blood vessels are. This means that they move further from their source of oxygen and nutrients. The nuclei of the cells degenerate and the keratinocytes can no longer divide or carry out metabolic activities. The stratum corneum is the outermost layer of the epidermis, consisting of 25 to 30 layers of dead, keratin-filled squamous cells.
Keratin is a tough, water-repellent protein.
Millions of dead cells from the stratum corneum are worn off by abrasion every day, and we have a totally new epidermis every 2-4 weeks. In healthy epidermis, the production of new cells balances the cell loss at the skin surface. In body areas regularly subjected to friction, such as hands and feet, both cell production and keratin formation are accelerated. They are easily damaged by UV radiation. The dermis is thick in the skin of the palms of the hands and the soles of the feet, and thinner in the skin of the eyelids, penis and scrotum.