Our skin has three layers. The outermost layer, the epidermis, is the protector. The middle layer, the dermis, gives elasticity and structure. The innermost layer, the hypodermis, is the fat cushion.
Our outermost protector layer, the epidermis, is thin, as thin as a sheet of paper (0.1mm). It contains no blood vessels and is nourished by the blood vessels in the underlying dermis layer. The epidermis has about 50 cell layers in the thinnest areas such as eyelids and other areas around the eyes; it has about 100 cells layers in thicker areas. These cells constantly renew themselves. They are born on the bottom of the epidermis and journey upward toward the surface, changing shape and becoming less and less alive along the way, until they reach the very top and fall off. It takes about 28 days for a young person’s skin to completely renew itself. This skin renewal cycle takes longer and longer as we age, it takes twice as long at age 60.
The epidermis contains 4 to 5 sub layers and they work together to protect and rebuild the outmost layer of our skin, to keep water in and keep pathogens out. The epidermis also makes vitamin D and melanin when exposed to the sun.
The Basal Layer
On the bottom of the epidermis is one layer of elongated cells called basal cells. These are the factories that produce new cells for the epidermis. Basal cells sit directly on top of the middle layer, dermis, and get oxygen and nutrients from the blood capillaries in the dermis layer. They grow and divide off new cells that leave the basal layer and migrate upward to first become squamous layer cells. The melanin-producing cells, melanocytes, also live in the basal layer.
The Squamous Cell Layer
8 to 10 layers of cells pushing up from the basal layer form the squamous cell layer, also called stratum spinosum or pricle-cell layer. The squamous cells are called keratinocytes and they produce keratin protein. The cells move progressively upward through this layer, transforming in shape from elongated and standing-up like basal cells at the bottom to flatten out along the skin surface near the top. Langerhans cells live in the squamous layer and they bind antigens and prevent these unwanted substances from penetrating through the skin.
The Granular Layer
By time cells reach the top of the squamous layer, they are too far away from the dermis to receive nutrients. They become dried out, and the keratin protein remaining in and around them makes them tough and fibrous. 3-5 layers of these flattened and dried out, pretty much dead cells make up the granular layer, or stratum granulosum.
In finger tips, palms, and soles of the feet, there is a clear layer called stratum lucidum. It is 3-5 layers of very flat cells.
The Horny Layer
The top, outermost layer, 10 to 30 cells deep, is the horny layer, or stratum corneum. These dead skin cells are glued together by lipids to keep the moisture in and keep pollutants and microorganisms out. Cells at the top of the horny layer continuously shed and are replaced by new cells coming up from below.
The health of our epidermis determines whether we look fresh and radiant. It also determines whether our skin is dry or moist. As we age, our skin renewal process is slowed down substantially, and our skin becomes dry since the skin is thinner, and thus less able to hold in moisture. By removing the outermost layers of dead cells, gentle exfoliation (facial peel) prompts our skin to speed up the renewal process by which cells are born in the basal layer and migrate upward toward the surface. This makes the skin look fresher and helps it retain moisture better. For older people, speeding up skin renewal through exfoliation is like a time machine taking one back to the days of more youthful skin.