To say that the epidermis is made of dead cells is to diminish its importance and to promote the (false) idea that it is OK to tamper with it with expensive devices and chemicals. In any case, it is a bad idea to use pejorative terms when talking about our bodies, and that includes our skin.
The skin is very a very complex organ, the result of evolutionary processes that occurred through millions of years (you can google Saccorhytus). The skin makes it possible for humans to survive outside the water. In Arizona, that can mean 29% relative humidity like today, meaning that the air contains just 29% of what would be necessary to saturate the air. To deal with this dryness, plants have stomata, and we, humans, have our skin.
You can read in my blog about the intricate sequence of events that go from the time you cut your finger while peeling a potato until healing is complete and it looks like it nothing ever happened to it. Then you will not talk so dismissively about the skin!
Let’s have a look at how the epidermis is remade all the time, allowing and fixing for damage done by the sun, pollution and accidents.
The epidermis is not made of “dead cells” but is a dynamic tissue in which cells are constantly in motion; cell populations pass one another and also melanocytes and Langerhans cells as they move toward the surface of the skin.
The epidermis is made of four layers classified by cell morphology and position: the basal cell layer (stratum germinativum), the squamous cell layer (stratum spinosum), the granular cell layer (stratum granulosum), and the cornified or horny cell layer (stratum corneum). The lower three layers constitute the living, nucleated cells. The epidermis is a continually renewing layer and gives rise to other structures, like the pilosebaceous structure, nails, and sweat glands. The basal cells of the epidermis undergo proliferation cycles that provide for the renewal of the outer epidermis.
The keratinocytes, which make up most of the cells of the epidermis, have intercellular bridges and large amounts of cytoplasm. The epidermis also contains other cell populations, such as melanocytes (pigment forming), Langerhans cells (involved in the immune response), and Merkel cells (involved in feeling touch and pressure). The differentiation process that occurs as the cells migrate from the basal layer to the surface of the skin results in keratinization, a process in which the keratinocyte first passes through a synthetic and then a degradative phase. In the synthetic phase, the cell builds up a cytoplasmic supply of keratin, a fibrous protein that serves as part of the cell’s cytoskeleton. Bundles of these keratin filaments converge on and terminate at the plasma membrane forming the intercellular attachment plates known as desmosomes. Lipids, like ceramides, are also synthesized and are fundamental to the role of the skin as a barrier to water loss. During the degradative phase of keratinization, cellular organelles are lost, the contents of the cell are consolidated into a mixture of filaments and amorphous cell envelopes, and the cell finally is known as a horny cell or corneocyte. So, yes, in the end the skin cells are dead, but only as a complex process that involves specialized metabolism and produces a skin barrier that will prevent water loss, infection and penetration of dangerous chemicals present in the environment. The skin is also involved in the regulation of body temperature and in alerting us about dangers through transmitting sensations.
The basal layer of the epidermis contains column-shaped keratinocytes attached to the basement membrane, with their long axis perpendicular to the dermis. These basal cells form a single layer and adhere to one another as well as to the squamous cells through desmosomal junctions. Other distinguishing features of the basal cells is the presence of melanin pigment transferred from adjoining melanocytes.
The basal layer is the primary location of the stem cells, the mitotically (for mitosis, cell division) active cells of the epidermis that will form the cells of the outer epidermal layers. Skin stem cells (like adult stem cells in other organs) are limited in that they can only produce skin cells (unlike embryonic stem cells). Not all basal cells have the potential to divide. Epidermal stem cells in the basal layer are clonogenic cells with a long lifespan that progress through the cell cycle very slowly under normal conditions. Wounding can increase the number of cycling cells in the epidermis by stimulating division of stem cells. Migration of a basal cell from the basal layer to the cornified layer in humans takes at least 14 days, and the transit through the cornified layer to the outermost epidermis requires another 14 days.
The dermal epidermal junction (basement membrane)
The basement membrane allows for the exchange of fluids and cells between dermis and epidermis, while holding them together. It also regulates adhesion, movement, and growth of keratinocytes and fibroblasts, and provides structural support for the epidermis. There are some fancy and very specialized proteins here, like collagen XVII, integrin α6β4, laminin 111, laminin 332 and more. These proteins can be damaged by ROS* coming from the environment or formed by UV radiation reaching the skin. Some of these proteins are rich in amino acids that are easily damaged by ROS* and whose oxidation will make the protein unable to function.
It may interest you that some disorders of pigmentation may be cause or aggravated by damage to the basement membrane. Apparently, melanocytes associated with a damaged dermal-epidermal junction changed shape and malfunction, causing so called “sun spots”.
How can this information help you?
If your epidermis is alive and important, you should remember not to submit to procedures and “treatments” that can damage it, unless a good dermatologist advises you to.
Knowing that the epidermis is home to your valuable skin stem cells is a reminder to protect it from the sun and pollution, because stem cells with damaged DNA will produce mutated skin cells.
Proteins are key to the normal functioning of the epidermis and its anchors to the dermis, so protecting skin proteins from reactive oxygen species (ROS*) and repairing those damaged is key to protecting your skin so that it can do the job of protecting your body!
We ate Skin Actives take all of this information into account when we choose our ingredients and formulate our products.