Several kinds of cells make up our skin. However, today, I’m just going to talk about the main cell type or the keratinocyte.
Keratinocytes start from the basal layer of our skin’s epidermis. Here, they divide several times to produce more keratinocytes. Several of them are later pushed up slowly to the surface of our skin. As they move up, they lose their ability to divide but become more specialized by accumulating keratin filaments in their cytoplasm. As more keratin accumulate in their cytoplasm, some are also secreted out into the surroundings of the cell and create a barrier. Thus, nutrients can no longer move into the cells, and they die. The topmost cells of our skin are therefore dead keratinocytes. They are later removed from our skin surface and will be replaced by new cells coming from the basal layer of our skin. Keratin plus other molecules joined with them make our skin waterproof.
The whole process of skin renewal takes about 20 – 30 days. That means every 20 -30 days we have new cells on the surface of our skin. If one has the skin disease, psoriasis however, new skin is produced in less than 20 days. Thus, people with psoriasis have portions of “bumpy” skin.
By the way, formation of new keratinocytes by mitosis usually takes place at night while we sleep. This might explain why our skin suffers if have too many late nights.