The skin is the largest organ of the body in both mass and surface area. The skin is necessary for survival. If 60% of the skin is destroyed, such as by being burned, death is likely.
The function of the skin is to protect the internal organs of the body from the corrosive world outside. The skin is also a barrier which prevents bacterial invasion. If the skin is broken, so as to expose blood vessels, an antiseptic is needed to prevent bacterial infection.
There are three layers of skin: the epidermis--outer layer, the dermis--the middle layer, and the hypodermis--the inner layer. Of these layers the dermis is the largest layer. The epidermis sheds its cells constantly so that we get new skin each day.
The epidermis is composed of the stratum corneum and the stratum germinativum. The stratus corneum is the outer most layer which is exposed to air. The stratum germinativum is composed of cuboidal cells which divide rapidly. As they divide they stack on top of each other. The cells on top of this stack have endoplasmic reticulum which begins to synthesize the protein keratin. Keratin is highly insoluble to water and helps make our skin nearly water-proof. As these cells become filled with keratin they are said to be cornified. When cornified cells move toward the skin surface to become the stratum corneum they are flattened like pancakes.
The skin is metabolically active. At the base of the stratus corneum are found cells called melanocytes. These melanocytes produce a pigment called melanin. As basal cells divide melanocytes inject melanin into these new cells. It is melanin that gives pigment (color) to the skin. The color of the skin is not related to the number of melanocytes, but to how much melanin they produce. Albinos do not produce melanin and have very white colored skin.
The rapidly dividing cells of the stratum germinativum are very sensitive to the ultra violet radiation of the sun. Excessive exposure to ultra violet radiation can alter the genes of these cells to produce skin cancer. Ultra violet radiation stimulates the melanocytes to inject more melanin into the higher layers of the epidermis, resulting in a sun tan.
Iron pigments in melanocytes cause people to be fair skinned. When exposed to the sun this iron pigment causes freckles.
The skin has embedded in it specialized neurons which are the receptors for the
sensitivity of the skin. Some of these receptors are:
1. Pacinian corpuscles--encapsulated dendrites located deep in the skin. They respond to heavy pressure.
2. Meissner's corpuscles--encapsulated dendrites which are located near the surface of the skin. They respond to light pressure.
3. Krause end bulbs--detect a change in temperature in the direction of coldness, as well as touch and pressure.
4. Ruffian end organs--detect a change in temperature in the direction of warmth, as well as touch and pressure.
5. Free nerve endings--are found on the top of the dermis and bring in information about pain.
The skin contains many sweat glands which help to regulate the temperature of the body. Sweat glands are usually coiled structures which release a liquid called sweat. When sweat evaporates it cools the skin. Sweat is also excretory, meaning that it gets rid of things that the body does not need.
There are two types of sweat glands. Sweat glands that produce a 99% water sweat used to cool the body. These sweat gland are found in their highest concentration on the back and abdomen. The other type of sweat glands produce a thicker sweat known as apocrine sweat. These sweat glands are located where we have hair patches, such as the arm pits and the pubic area. We also have a few of these sweat glands on the soles of our feet. Apocrine sweat contain butyric acid, a fatty acid. These sweat glands respond to emotion. Apocrine sweat is odorous and is used by animals, along with urine, to mark their territories. Cats have apocrine sweat glands in their neck. They use these glands to rub against people they like to mark these people as their property. In people, apocrine sweat is eaten by bacteria producing an odorous by-product. Anti-perspirants contain chemicals which blocks the production of sweat by apocrine sweat glands. Deodorants contain anti-bacterial agents which prevent the invasion of bacteria.
The skin contains hair follicles which extend from the epidermis to the dermis in a canal called a skin pore. Each hair follicle has a nerve attached to it. Therefore, hair follicles allow us to feel very light pressure. This is also why it hurts to pull on hair. At the base of a hair follicle are found sebaceous glands which produce a skin oil called sebum. Ordinarily the skin pores are straight and open to the surface of the epidermis. However, during puberty when there is rapid growth, there is an increase in the production of sebum, and the pores of the skin have a tendency to become narrow beneath the epidermis. This traps sebum which bacteria feed upon causing infection. This infection is called acne. Medication such as benzoyl peroxide kills these invading bacteria. Medication such as Retin-A and Accutane help straighten and open the skin pore, so that excess sebum is more easily removed when we wash the skin.
Hair is made of flattened cells that grow in a circle. This gives hair its round-cylindrical appearance. Hair cells are the same cells found in the stratum germinativum, and they cornify rapidly. Melanin is injected into these cells to give hair color. The more melanin, the darker the hair. When the melanocytes change to produce less melanin, the hair becomes gray. Albinos have hair containing no melanin, causing their hair to be white. As hair turns gray it changes to cause holes in the hair follicle. This makes hair become more course. This will sometimes cause naturally curly hair to become straight.
Each hair is connected to a little muscle called an erector muscle. When erector muscles contract, they cause the hair follicle to rise resulting in goose bumps. These erector muscles tend to contract when it is cold. The erector muscles of birds cause their feathers to fluff on a cold day. This results in more dead air spaces which serves to insulate their bodies from the cold. For people, dogs and cats these erector muscles respond to anger and fear causing hair to rise.
Hair is a sensory organ in that it allows us to feel things on our skin. If an
ant is crawling on a hairless part of our body we are less likely to feel its
presence. Hair also protects us from heat loss. When we exert ourselves, a large
percent of heat loss occurs through our head. Since there is not much fat tissue
on the head, hair serves as insulation to prevent heat loss. So head hair serves
as a thermal insulator. Finger nails and toe nails are modified hair which
protect the fingers and toes, and which we use as tools. By the time we can
actually see hair, it contains no living cells. So hair, like finger and toe
nails, is dead.