Overview of Skin Anatomy
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.
The skin is the largest organ of the body and is as indispensable as the body's other major organs. But the skin is perhaps the organ that brings us closest to an understanding of organ function, because its anatomy and function can be partially observed by the naked eye. In addition to its ability to communicate internal physiological information, like the presence of fever, the skin also reacts to external stimuli, like sun exposure, toxins, and even psychological stimuli. From poison indication and warning, to the "cold sweats" and "goosebumps," the skin is a constant and dynamic interface between the body and its environment.
Skin is made up of two 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 two 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.
The epidermis, the outermost layer of skin, is thin but complex. Melanin, which is responsible for skin pigmentation, is found throughout the epidermis. The epidermis also keratinizes (described below) to produce nails, hair, sweat, and to regenerate. It is the foremost initiator of cell death and regeneration, the final boundary between body and environment. It is useful, however, to itemize the epidermis layer and its progressive keratinization from its innermost sublayer to its outermost, because cell growth naturally follows this pattern.
Keratinization, the maturation and migration of skin cells, begins in the innermost layer of the epidermis, the stratum germinativum. These cells, called keratinocytes, accumulate and move outward toward the next epidermis layer, the stratum spinosm, where they become dense. As they move into the stratum granulosm, skin cells pick up granules that contain lipids. Lipids assist in the formation of water barriers among the cells of the skin, which, in turn, help to ensure body moisturization.
At this point, the cell also becomes flattened, or horny, and the nucleus disappears; what remains is keratin. In the next layer, the stratum lucidum, the cell is prepared to move into its final sublayer with the addition of melanin granules. Then, sudden changes in enzyme function cause cell death. The products of this ongoing process form the stratum corneum, which is the outermost epidural layer consisting of neatly packed dead horny cells. Directly beneath this layer is a layer of live cells that is being pushed from underneath toward death and a place in the corneum. When these new cells reach the surface, their upward and outward force causes the dead cells to break apart and slough away, a process known as desquamation.
This stratum corneum may be as thin as a few cells, or as thick as 50 or more, again depending on its location on the body. The corneum of the scalp, for instance, may be very thin, perhaps five cells thick, while that of the elbow is more likely to be upwards of 50 cells thick. So the body provides for high-contact areas by maintaining a thicker and, therefore, more durable layer of protection.
It can take anywhere from six to 10 weeks for a cell to mature and journey through the layers of epidermis to its death and expulsion.
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 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.