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|The integumentary system is the protective cover for the human body. It has one organ, the skin,
a.k.a. the cutaneous membrane.
Functions of the skin include:
protection - protects against invasion of microorganisms, protects from water loss and dehydration.
defense - contains macrophages, lymph nodes and other structures which identify pathogens and provide first line of defense against them.
sensation - the skin contains sense organs for light touch, pressure, temperature, and pain.
secretion - the skin secretes the precursor to Vitamin D (this is then activated by processing in the liver and kidney), and melanin.
thermoregulation - by diverting blood into or away from the skin the body can release or conserve heat.
|The skin is made of two layers, the epidermis made stratified squamous epithelium, and the dermis made of areolar and dense irregular connective tissue. The epidermis is keratinized in the body's external skin to help protect from abrasion and water loss and non-keratinized in the internal skin (the linings of the mouth, esophagus, anus, and vagina). In all locations the epidermis is continually replaced by mitosis at its base and exfoliation from its surface. (See Figure 5.3, Modified).|
Above: Figure 5.2 Modified
|The epidermis is composed of several layers:
The stratum basale or stratum germinativum undergoes constant mitosis to replace the cells exfoliated from the skin's surface. The new cells push up into the intervening or transitional layers toward the surface. The stratum spinosum or "spiny layer" consists of cells attached to one another by fibrous desmosomes which enable the skin to be pulled and stretched without the cells pulling apart. As the cells push up through these layers they accumulate large amounts of keratin and keratohyalin (a glycoprotein) and this substance forms dense granules in the stratum granulosum. Some areas of the skin, notably the palms and soles have an additional layer, the stratum lucidum or "clear layer" which makes them thicker in order to resist pressure. The cells of this layer, like those in the outer layer, are dead cells impregnated with keratin filaments and keratohyalin. The outermost layer is called the cornified layer because its cells are stiff and horny. In fact the horns of animals such as the rhinoceros are made of exactly this material. This layer may be quite thick, 20 cells or so thick, and is constantly sloughing off. It is this process which produces the flaky skin seen in dandruff and other conditions. The cornified layer will increase in thickness when subjected to continuous pressure and abrasion. This is what produces the "corns" and callouses seen on the feet and hands.
Melanocytes associate with the cells of the stratum basale. All races have about the same number of melanocytes, in association with keratinocytes (epidermal cells) in ratios of from 1:4 to 1:10 depending on the skin location. They secrete melanin in granules which are taken into the keratinocytes by endocytosis. Some melanin also remains in the interstitial space. Melanin helps protect the cells from UV radiation. Melanin varies in color among the races from a brown to a reddish tint. In addition to melanin, skin color is influenced by the presence of blood vessels and carotene. In Caucasian skin the melanin is broken down rapidly by enzymes from the keratinocytes.
Cancer occurs when genetic changes result in turning on, or not turning off, genes which regulate normal cell division and tissue growth. In the case of skin cancer the stimulus for such changes usually lies in exposure of the skin to UV radiation. Three basic types exist:
1) squamous cell carcinoma: A moderately dangerous cancer that occurs in the keratinocytes of the intermediate or transitional layers of the epidermis. Normally these cells undergo apoptosis, [See Cell Suicide in Health and Disease in the Dec. 1996 Scientific American, available on reserve in the HSC Library] the suicide or self destruction which results in the protective layer of flattened, dead cells found in the outer cornified layer. And the acceleration of apoptosis is an important response of these cells to sun exposure. If apoptosis fails then these cells can become abnormal and produce cancer.
2) basal cell sarcoma. An abnormal growth of the basal cells. These cells are normally protected from UV exposure by the melanin secreted by nearby melanocytes. This form of cancer can usually be successfully treated surgically if caught in time.
3) melanoma. This is a cancer of the melanocytes which is the most dangerous because it often metastasizes to other tissues and organs.
|The diagram of a hair root shown at left (and in class, See
also Figure 5.6) illustrates the anatomy of
the hair and associated structures, as well as the blood supply to the skin which
allows it to act in thermoregulation. Vessels immediately under the epidermis
can be perfused with blood to allow heat to be radiated to the environment, or
they can be bypassed with shunts connecting arteries and veins to allow the
blood to return to the warmer core of the body.
The hair itself, together with the fingernails, is made of flattened keratinized cells thickly stacked together. Melanin granules secreted around these cells give hair its color which can vary as can skin by the amount and color of the melanin granules. Hairs can also be raised by pilo arrector muscles to produce "goose bumps", also part of thermoregulation. The hairs are bathed by an oily substance called sebum which helps the skin to remain moist. Lanolin, which has been used in many cosmetics, is sebum from sheep.
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