Skin, Hair, and Nails - The organ called skin

It is always a little surprising to hear, for the first time, the skin referred to as a single organ. This is not to say it is a simple organ; on the contrary, it is an exceedingly complex and varied one. However, despite variations in appearance from part to part of the body, our entire outer wrapper (the more technical word is integument ) is similarly constructed.

One might object: “But my nails and hair certainly look different from the skin on my nose!” This is perfectly true, but nails and hair are extensions of the skin, and wherever they occur are composed of similar tissues: the nail on a little toe is made of the same material as the hair.

Functions of the Skin

The skin has three main functions, and its different outward appearance on different parts of the body reflects to some extent which of these functions a certain area of skin primarily serves. The three main functions are protection (from germs or blows), temperature control (e.g., through perspiration, to aid in keeping the body's internal organs near our normal internal temperature of 98.6° F.), and perception. Nerve endings in the skin give us our sensations of touch, pain, heat, and cold. Associated with the skin's important role in temperature regulation is its function as an organ of excretion—the elimination, via perspiration, and subsequent evaporation of water and other substances. Skin is also the site of the body's natural production of vitamin D, stimulated by exposure to sunlight.

Anatomy of the Skin

Skin has three more or less distinct layers. The outermost is called the epidermis; the middle, the dermis; and the innermost, subcutaneous (under-skin) tissue. The epidermis may also be called cuticle; and the dermis either corium or true skin .

The Subcutaneous Layer

The subcutaneous layer is really a rather vague border zone between muscle and bone tissues on one side, and the dermis on the other, a kind of springy, fatty padding that gives bounce and a look of firmness to the skin above it. With age, the fatty cells of the subcutaneous layer are not continually replaced as they die, and this layer tends to thin out. The result is wrinkles, which form where the outer layers of skin lose their subcutaneous support, much like the slipcover of a cushion that has lost some of its stuffing.

The Dermis

The dermis is serviced by the multitude of tiny blood vessels and nerve fibers that reach it through the subcutaneous tissue. In addition, many special structures and tissues that enable the skin to perform its various functions are found in the dermis: sebaceous (skin oil) glands and sweat glands; minuscule muscles; and the roots of hairs encased in narrow pits called follicles .

The topmost layer of the dermis, interconnecting with the epidermis above it, resembles, under a microscope, nothing so much as a rugged, ridge-crossed landscape carved with valleys, caves, and tunnels. The basic forms of this microscopic terrain are cone-shaped hills called papillae; between 100 and 200 million of them are found in the dermis of an adult human being.

Because the dermal papillae serve as the bedrock for the surface layer of skin, the epidermis, we can understand why there is really no such thing as smooth skin; even the smoothest patch of a baby's skin appears ridged and cratered under a magnifying glass.

The distribution of papillae in the skin falls into certain distinctive patterns that are particularly conspicuous on the soles of babies’ feet and on the fingertips, and give each of us our unique finger, toe, and footprints. The mathematical possibility of one person having the same fingerprints as another is thought to be about one in 25 billion. The papillae ridges on the fingertips also make it easier for us to pick up and handle such things as needles or pencils or buttons.

Finally, because there are relatively more papillae concentrated at the fingertips than on most other areas of the body, and because papillae are often associated with dense concentrations of nerve endings, the fingertips tend to be more responsive to touch sensations than other parts of the body.

The Epidermis

The bottom layer of the epidermis, its papillae fitted into the pockets of the layers of the dermis beneath it, is occupied by new young cells. These cells gradually mature and move upward. As they near the surface of the skin, they die, becoming tough, horny, lifeless tissue. This is the outmost layer of the epidermis, called the stratum corneum (horny layer), which we are continually shedding, usually unnoticed, as when we towel off after a bath, but sometimes very noticeably, as when we peel after a sunburn.

A suntan, incidentally, is caused by the presence of tiny grains of pigment, called melanin , in the bottom layers of epidermis. Sunlight stimulates the production of melanin, giving the skin a darker color. A suntan fades as the melanin granules move to the surface and are shed with dead skin cells.