A: We distinguish the various layers of skin, because they have different functions and also can develop different sorts of medical problems.
In its broadest stroke, skin is divided into 3 layers.
The outermost layer, the ‘epidermis’, is a cell-rich structure that makes up much less than a 10th of the bulk of skin. The middle layer is the ‘dermis’. This layer is cell-poor and largely filled with supporting structures, such as collagen fibers and elastic fibers. It also contains blood and lymphatic vessels and nerve fibers. Other skin structures that originate from the epidermis, such as sweat glands and hair follicles, also are mostly found in the dermis, although they maintain connections to the overlying epidermis. Below the dermis is the 3rd layer of skin, the sub-cutaneous fat. The size of this layer, of course, varies greatly among individuals.
The Outermost Layer Of Skin: The Epidermis.
The gut brings stuff in; the skin keeps stuff from getting in or out.
Epidermis is the business end of skin. Because it faces the external world, its primary duty is to manage this interface – to hold the good stuff in and keep the bad stuff out. One way to think about skin is that it has the opposite charge to that of the gastrointestinal system. While the gut is primarily adapted to facilitate the absorption of food from our diet, skin evolved primarily to prevent our body water from escaping into the drier atmosphere. Of course, skin is also obliged to protect our tender interiors from all manners of injury.
Epidermis, too, is divided into a number of layers. The outermost layer is one that concerns our laboratory the most. This is the cornified cell layer, or the Stratum corneum, to put it into its official, Latin lingo. This is the site of the permeability barrier. It is composed of cells that have lost their nuclei and hence are considered to be dead cells – although that does not mean these cells aren’t doing anything.
For more about how the skin is structured and how the stratum corneum forms the permeability barrier, sign up to receive our free booklet, A Primer On The Skin Barrier.
The living or nucleated cells of the epidermis are also divided into several layers. The lower most, the basal cell layer or Stratum basale is purposed with replacing the stratum corneum cells that are shed from the skin surface, with new daughter cells.
The basal layer of the epidermis also has the important job of tethering the epidermis to the dermis.
This tethering prevents the thin and fragile epidermis from shearing off the dermis with even minor frictional injuries. We know how critical this function is, because of a family of rare inherited skin diseases, called epidermolysis bullosa, in which a genetic mutation results in one component of the tethering apparatus being weakened or absent. People with one of these disorders can develop large and painful blisters following only minor skin abrasions.
The epidermal layers between the basal and cornified cells layers – the inner or spinous cell layer (Stratum spinosum) and the outer or granular cell layer (Stratum granulosum)(not labelled on the image above) are concerned with elaborating the proteins and lipids that will be used to form the Stratum corneum.