Ever wonder how the outer skin functions? The Epidermis.

The epidermis, the superficial layer of skin, is first made of cells called keratinocytes, which renew nonstop according to a 21-day cycle. That renewal of the epidermis is made thanks to the cell proliferation and the differentiation that keeps the balance of adult tissues. Therefore, keratinocytes divide at the level of the basal layer of the epidermis, which is mainly made of nondifferentiated cells, and migrate to the surface, changing their form: they lose their nuclei and load complex filaments of keratine. When they reach the cornified layer, they become corneocytes, dead cells that create a solid membrane (thanks to keratine) that is impermeable and protective: the protective natural barrier of the epidermis. Those built-up corneocytes will naturally break away and be shed. The alteration of that balance, essential to the good of tissues, called homeostasis, is responsible for physical changes linked to aging: skin wilting because of the decrease of cell proliferation, lack of healing in case of wounds, and loss of hair... Study of the proliferation and the differentiation of epidermis cells To show that the balance of tissues has been maintained, both the proliferation and differentiation of epidermis cells were studied. KI67 is an anti-gene that marks cell proliferation, and filaggrin is a protein that marks cell differentiation. Studies have been made on the reconstructed epidermis.