Skin consists of all 4 types of body tissue: nervous, muscle, connective, and epithelial. As a result, it is elastic, flexible, and responsive. Its thickness depends upon its function. For example, an extremely thin layer of skin covers the eardrums, which must be sensitive to sound waves. In contrast, thick skin covers the soles of the feet.
Layers of Skin
The skin consists of 2 layers. The skin outer layer is called the epidermis. The thick inner layer is called the dermis.
The epidermis itself has 2 layers. The top one is actually about 20 layers of dead, scale like, flattened cells. These cells die quickly because they are cut off from their food supply. They contain a protein called keratin, which makes them waterproof. The body loses several thousand of these cells each day, and new cells are produced by mitosis in the lower epidermal layer. As the surface cells disappear, those in the lower layer become the outer surface. It takes about 27 days for all of the outer skin cells to be replaced. In addition to these skin-generating cells, the lower layer has cells that contain melanin, the pigment that makes skin dark. Every person has approximately the same number of these cells. Therefore, skin color differences result from variations in the amount of pigment produced by these cells.
The dermis is composed mainly of connective tissue, which gives the skin its strength and elasticity. Blood vessels, nerves, hair roots, and oil and sweat glands are all located in the dermis.
A protective layer of loose fatty tissue and dense connective tissue called the subcutaneous layer attached the dermis to the bones and muscles. Although not technically part of the skin, the subcutaneous layer, like skin, helps protect the body against injury and heat loss.
Sebaceous glands, or oil glands, secrete sebum, an oil that reaches the skin’s surface through the places where hair emerges from the skin. The oil prevents hair and skin from drying out and helps waterproof the skin. A condition called acne commonly occurs during adolescence. Acne occurs when oil mixes with dead cells and plugs up pores in the skin, causing blackheads. In addition, inflammation of oil glands causes pimples. Acne may be related to hormonal changes that take place during adolescence.
More than 2,5 million sweat glands exist in the dermis. Most consist of a tiny duct that opens to the skin’s surface and rids the body of excess water and certain wastes. The evaporation of sweat also acts to cool the body when it becomes overheated.
Hair and nails
Hair is present on the skin over the entire body, except on the soles of the feet, the palms of the hands, and the lips. Hair is manufactured in hair follicles, which are small folds of epidermis that extend into the dermis. Tiny blood vessels at the base of the follicle nourish the hair root. A group of actively dividing cells near the base produces new hair. The hair shaft, which extends above the skin’s surface is composed of dead epidermis.
Nails are mainly dead cells composed of keratin that protect the tips of fingers and toes. At the base of the hard nail plate is a whitish, semicircular area called the lunula. Cell division takes place in the root of the nailbed.