The skin is the ultimate vessel for the human body; it receives and transports, accepts and expels according to the body’s needs. It is container, defender, regulator, breather, feeler, and adaptor. But success in these roles is not accomplished automatically. As sturdy as it is, the skin requires attention and maintenance to function properly. Without such care, the complex organization of the skin breaks down, making it and the body it protects susceptible to injury and disease. Thus, the body’s coverall organ is as fragile as it is seemingly unyielding. Skin is made up of 3 primary layers that differ in function, thickness, and strength. From outside to inside, they are the epidermis and its sublayers, and the dermis, after which is found subcutaneous tissue, or the hypodermis. The 2 layers are further differentiated by their respective amounts of hair follicle, pigmentation, cell formation, gland makeup, and blood supply. Moreover, these layers are present in the two general types of skin, thin and hairy, and thick and hairless. The former is more prevalent on the body, while the latter is found on parts of the body that are used heavily and experience extreme friction, like the palm and the heel.
There are three types of specialized cells in the epidermis. The melanocyte produces pigment (melanin), the Langerhans' cell is the frontline defense of the immune system in the skin, and the Merkel's cell's function is not clearly known.
The second, larger layer of skin is called the dermis. Its main roles are to regulate temperature and to supply the epidermis with nutrient-saturated blood. The dermis is made up of fibroblasts, which produce collagen connective tissues and which lend elasticity and support to the skin. It is the seat of hair follicles, nerve endings, and pressure receptors. Furthermore, the dermis defends the body against infectious invaders that can pass through the thin epidermis, the first defense against disease.
The subcutaneous tissue is a layer of fat and connective tissue that houses larger blood vessels and nerves. This layer is important is the regulation of temperature of the skin itself and the body. The size of this layer varies throughout the body and from person to person.
The skin is a complicated structure with many functions. If any of the structures in the skin are not working properly, a rash or abnormal sensation is the result. The whole specialty of dermatology is devoted to understanding the skin, what can go wrong, and what to do if something does go wrong.
The dermis is also subdivided into two divisions, the papillary dermis, and the reticular layer. The papillary dermis is the main agent in dermis function. It is from here that the dermis (1) supplies nutrients to select layers of the epidermis and (2) regulates temperature. Both of these functions are accomplished with a thin but extensive vascular system that operates like vascular systems throughout the body. Constriction and expansion control the amount of blood that flows through the skin and dictate whether body heat is dispelled carefully in times of heat or conserved for the cold. The reticular layer is much denser than the papillary dermis; it strengthens the skin, providing structure and elasticity. As a foundation, it supports other components of the skin, such as hair follicles, sweat glands, and sebaceous glands.
The epidermis is the outer layer of skin. The thickness of the epidermis varies in different types of skin. It is the thinnest on the eyelids at .05 mm and the thickest on the palms and soles at 1.5 mm.
The epidermis contains 5 layers. From bottom to top the layers are named stratum basale, stratum spinosum, stratum granulosum, stratum licidum, and stratum corneum. The bottom layer, the stratum basale, has cells that are shaped like columns. In this layer the cells divide and push already formed cells into higher layers. As the cells move into the higher layers, they flatten and eventually die. The top layer of the epidermis, the stratum corneum, is made of dead, flat skin cells that shed about every 2 weeks.