A magnification of the four distinct strata of human skin. At the top is the stratum corneum consisting of several layers of flat, dead, waterproof keratinocytes -- the outer layer of skin cells. Beneath the stratum corneum are the strata granulosum, spinosum and basale. It is in the stratum basale that resident stem cells differentiate to provide new cells and renew the skin. Below the stratum basale is the dermis, a collagen rich tissue that cushions the body.