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This skin is the largest single organ in the body. It weights nine pounds, is 0.5mm to 5mm thick and has a surface area of some eighteen square feet (area of single bed) in a medium sized adult male. The skin covers the entire surface of the body and is turned inwards at the nostrils, mouth, anus and genital organs, where it is continuous with the wet epithelium termed the mucous membrane. It varies considerably in thickness over different parts of the body beings thickest over the soles of the feet and palms of the hands. In general, the skin covering the flexor and medial surfaces of the extremities is thinner than that on the extensor and lateral surfaces, while that on the ventral trunk, and neck is thinner than on the dorsal surface. Pressure of friction tends to increase the thickness of the skin.
The skin is composed of an external stratified epithelial portion called the epidermis (derived from ectoderm), and an internal fibrous portion containing blood vessels and nerves called the dermis or corium which develops from the mesoderm. Anatomy of the Skin
The skin consists of two interacting tissue sheets, the epidermis and the dermis. It rests on the superficial or subcutaneous fascia, an adipose connective tissue membrane that is an important part of the integumentary system.
Epidermis: The epidermis is a superficial sheet of keratinized stratified squamous epithelium that varies in thickness depending on its location. It is thickest on friction areas of the skin, such as the palms of the hands and the soles of the feet, and thinnest on the lips and eyelids.
In humans the epidermis has a wavy surface of ridges and grooves. This is because the underlying dermis is thrown into a series of projections called dermal papillae. The epidermis lying on top of these papillae is thrown into ridges. The patterns of epidermal ridges on the palms and soles, which appear as loops, arches and whorls are largely inherited and unique to each individual. In the form of fingerprints and footprints, they serve as a means of identification.
Like other epithelial sheets the epidermis is constantly being worn away from the surface and renewed by mitotic division of basal cells. Newly formed cells mature, become keratinized, and die as they are pushed towards the surface. From base to surface the epidermis is subdivided into four layers or strata as described below:
Stratum Germinativum The stratum germinativum consists of several rows of cells ranging from low columnar in the bottom row to polyhedral (many sided) in the top row. The cells in this layer still retain the capacity to divided; hence the name germinativum. Stratum germinativum cells actively synthesize precursors of the waterproofing protein known as keratin. Keratin is a hard fibrous protein with high sulfur content.
Stratum Granulosum: The stratum granulosum consists of one or two rows of fusiform cells that produce keratohyalin, another precursor of keratin. The presence of keratohyalin granules in these cells gives the name granulosum to this layer. Mitotic division does not occur in the stratum granulosum. Also, as their cytoplasm fill with keratohyalin granules, the nucleus and cytoplasmic organelles of the granulosum cells begin to degenerate.
Stratum Lucidum: The stratum lucidum consists of several rows of flattened, closely compacted cells that are filled with keratin and lack nuclei. In light microscopic preparations it appears as a clear wavy band that separates the stratum granulosum from the stratum corneum. The stratum lucidum is more prominent in thick skin than in thin skin and may even be absent.
Stratum Corneum: The stratum corneum consists of several to many rows of scale like keratinized cells lacking nuclei and cytoplasmic organelles. These cells are cemented together with intercellular substance to form a tough waterproof barrier that protects the living cells below from dehydration. No living cell could stand the constant exposure to air, water, and temperature changes that these dead cells face. Ultimately, the dead cells are sloughed off from the surface of the epidermis as dry flaky scales.
Dermis: The dermis, also called the corium or cutis, is a sheet of connective tissue that supports and maintains the overlying epidermis. It consists of two poorly demarcated layers that are described here.
Subcutaneous Fascia: The superficial fascia (tela subcutanea or hypodermis) is a sheet of adipose connective tissue lying between the dermis and the deep fascia covering the muscles. Its collagen and elastic fibres are continuous with those of the dermis and form fibre bundles that mainly run parallel to the long axis of the skin. These fibre bundles are thickest and most numerous on the palms and soles, where the superficial fascia is firmly attached to underlying structures. The number of fat cells in the superficial fascia varies with sex, age, nutritional state of the individual, and region of the body. When properly distributed, the fat content of the superficial fascia gives pleasing contours to the body. The superficial fascia is pierced by many large blood vessels, which, with the high content of fat cells, permit the superficial fascia to act as a reservoir, an insulator, and a temperature-regulating device. Skin Colour
Normal colour: Normal skin colour is produced by the interaction of several pigment presents in the skin. These pigments include melanin, carotene and hemoglobin.
Melanin: Melanin is a brown-black pigment formed in melanocytes, the pigment cells. Melanocytes are located at the dermo-epidermal junction and transfer melanin-producing granules into young epidermal cells. Melanin helps protect epidermal cells from the damaging effect of ultraviolet rays in sunlight. For example, people develop a tan when exposed to strong sunlight, and human populations living close to the equator, where the sunlight is intense, have darker pigmentation than those who live nearer to the poles. The number of melanocytes in the skin is approximately the same in all races; but the individual’s colour (black, brown, red, tan or white) depends upon type of melanin, the amount of melanin produced, and the degree of granule aggregation.
Carotene: Carotene is a yellow pigment, normally found in epidermal cells, that gives the skin its yellowish tones. This same pigment is responsible for the colour of carrots and other yellow vegetables. Excessive ingestion of carrots causes a person’s skin to develop a pronounced yellow colour.
Haemoglobin: Haemoglobin is an oxygen-binding pigment found in red blood cells. When combined with oxygen, it colours the blood bright red. Oxygenated blood flowing through superficial dermal vessels gives the skin its pinkish tones. Skin Appendages
These are three classes of skin appendages in humans: glands, hair, and nails. All are derivatives of the epidermis. Glands: Integumentary glands are exocrine glands whose secretory units are located in the dermis or subcutaneous fascia. Excretory ducts connect them either to the skin surface or to a hair follicle.
- Sweat Glands: Sweat, or sudoriferous, glands occur in two types: eccrine sweat gland and apocrine sweat glands.
Eccrine Sweat Glands: Approximately two million eccrine sweat glands are present in the skin of the average adult. These glands are widely distributed over all the hairless areas of the body, except for the eardrum and the glans penis, and are most numerous on the palms and soles. Eccrine sweat glands are simple tubular glands whose coiled secretory units lie in the dermis; their excretory ducts spiral through the epidermis to terminate in minute pores on the free surface. This type of gland produces a colourless, odorless, watery secretion called sweat. Sweat plays an important role in temperature regulation; its evaporation from the surface of the skin rapidly lowers body temperature.
Apocrine Sweat Glands: Apocrine sweat glands are highly coiled simple tubular glands located primarily in the armpits and around the anus. These glands release their secretions into hair follicles. Apocrine sweat glands are basically scent glands that produce a thick milky secretion in response to stress or sexual stimulation. This secretion is odorless until degraded by bacteria on the skin.
- Ceruminous Glands:Ceruminous glands are modified sweat glands located in the external auditory canal of the ear. They secrete cerumen, or earwax, a substance that protects the eardrum. However, excessive accumulation of earwax in the external ear canal can cause a partial hearing loss.
- Mammary Glands: Mammary glands (breasts) are specialized integumentary organs containing modified sweat glands to secrete milk. The breasts of the human female, reach their greatest development during her childbearing years under the stimulus of pituitary and ovarian hormones.
- Sebaceous Glands: Sebaceous glands are simple branched acinar glands derived from developing hair follicles. Most sebaceous glands remain connected to hair follicles into which they release their secretion. Only on the lips, eyelids, and external genital organs are sebaceous glands found independent from hair follicles. The oily secretion of the sebaceous gland, called sebum, lubricates the skin and provide waterproofing. Acne vulgaris, an inflammation of the sebaceous glands commonly seen in adolescents, is due to sebaceous gland hyperactivity, which is probably stimulated by rising levels of sex hormones. Some forms of acne leave permanent pits and scars in the skin. Comedones, or black heads, are plugs of keratin and dirty dried-up sebum that block the opening of hair follicles.
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