As the body’s largest organ and one of its most essential systems, the skin’s functions go far beyond the aesthetic. The skin regulates internal temperature; perceives external stimuli and transmits the information to the brain; protects from potentially harmful substances; and stores essential nutrients.
Skin is comprised of three main layers – epidermis, dermis and hypodermis – all of which vary significantly in their anatomy and function.
Epidermis
The epidermis is the outer-most layer of the skin and continually renews and regenerates. Ultimately, all the cells in the epidermis originate from a single layer of basal cells in its basement membrane – these are called keratinocytes, which are stacked on top of each other to form several strata, melanocytes and dendritic cells.
Keratinocytes develop in the basal layer and rise, losing their central nucleus and producing skin proteins called keratins and fats called lipids, before being shed from the surface of the skin as dead cells. As they move towards the surface of the skin, their form changes and they create distinct layers known – from the bottom up – as the basal layer, the spiny layer, granular layer, stratum and stratum corneum.
In the spiny layer, the keratinocytes make lipid fat cells which discharge as the cells move up through the granular layer, forming a moisture-carrying mortar around the basal cells, which are now called corneocytes. These effectively dead and flattened cells form the protective outer layer of skin, which is worn away in a process known as desquamation. The entire process normally takes around 30 days – although sunburn, injury or cosmetically stripping the outer layer speeds the process.
The stratum corneum retains moisture in the lower skin layers, comprising up to 15 percent water. Moisture loss is regulated by the lipids, while dead cells are cleared away by skin enzymes.
Men tend to have a thicker stratum corneum than women; they tend to have more collagen and produce more sebum, making the lipid layer of their skin thicker and the cell renewal process slower.
Melanocytes produce melanin, which contributes to skin colour and provides UV protection. Dendritic (or Langerhans) cells are involved in the immune system of the skin. They consume foreign materials that invade the epidermis and transfer out of the skin to stimulate an immune response.
Dermis
Between the epidermis and the subcutaneous fat layer lies the dermis, which contains connective tissue and houses the arteries, hair follicles, lymph vessels, sensation receptors, sweat glands and veins. This thicker dermal layer is composed primarily of collagen, which is responsible for the strength and elasticity of the skin. It is held together by a protein called elastin that is produced by fibroblast cells. The dermis also contains moisture-storing glycoproteins and hyaluronic acid, which have the ability to attract and bind hundreds of times its weight in water. Here, collagen and elastin production decline with ageing.
Hypodermis
The hypodermis, also referred to as the subcutis, subcutaneous layer or superficial fascia, is the bottommost layer. As the deepest component of skin, the hypodermis is made up of a loose network of fat and collagen cells. It acts as a protective cushion and helps insulate the body by monitoring heat gain and heat loss. The thickness of this layer can vary significantly in individuals and in different parts of the body. [A]A
