Skin

the flexible tissue (integument) enclosing the body of vertebrate animals. In humans and other mammals, the skin operates a complex organ of numerous structures (sometimes called the integumentary system) serving vital protective and metabolic functions. It contains two main layers of cells: a thin outer layer, the epidermis, and a thicker inner layer, the dermis. Along the internal surface of the epidermis, young cells continuously multiply, pushing the older cells outward. At the outer surface the older cells flatten and overlap to form a tough membrane and gradually shed as calluses or collections of dead skin. Horns, hoofs, hair (fur), feathers, and scales are evolutionary adaptations of the epidermis. Although the epidermis has no blood vessels, its deeper strata contain melanin, the pigment that gives color to the skin. The underlying dermis consists of connective tissue in which are embedded blood vessels, lymph channels, nerve endings, sweat glands, sebaceous glands, fat cells, hair follicles, and muscles. The nerve endings, called receptors, perform an important sensory function. They respond to various stimuli, including contact, heat, and cold. Response to cold activates the erector muscles, causing hair or fur to stand erect; fright also causes this reaction. From the outer surface of the dermis extend numerous projections (papillae) that fit into pits on the inner surface of the epidermis so that the two layers are firmly locked together. In humans, whorls on the fingers show where the epidermis falls between rows of papillae, making the patterns used in fingerprinting. The skin provides a barrier against invasion by outside organisms and protects underlying tissues and organs from abrasion and other injury, and its pigments shield the body from the dangerous ultraviolet rays in sunlight. It also waterproofs the body, preventing excessive loss or gain of bodily moisture. Human skin performs several functions that help maintain normal body temperature: its numerous sweat glands excrete waste products along with salt-laden moisture, the evaporation of which may account, in certain circumstances, for as much as 90% of the cooling of the body; its fat cells act as insulation against cold; and when the body overheats, the skin's extensive small blood vessels carry warm blood near the surface where it is cooled. The skin is lubricated by its own oil glands, which keep both the outside layer of the epidermis and the hair from drying to brittleness. Human skin has remarkable self-healing properties, particularly when only the epidermis is damaged. Even when the injury damages the dermis, healing may still be complete if the wounded area occurs in a part of the body with a rich blood supply. Deeper wounds, penetrating to the underlying tissue, heal by scar formation. Scar tissue lacks the infection-resisting and metabolic functions of healthy skin; hence, sufficiently extensive skin loss by widespread burns or wounds may cause death.




Structure of Skin

The skin is made up of three successive levels from the surface to in-depth: the epidermis, the dermis, and the hypodermis.


It is covered by a mixture of sweat and sebum, the hydro-lipidic film, the first protective barrier against exterior aggressions, which limits the development of bacteria through its acidity. This fine emulsion also maintains cutaneous moisturisation and gives the skin its velvety look.



The epidermis

The outermost covering, the epidermis, is the first protective barrier the organism has. It is formed by many layers of cells which are perfectly stratified and contain no blood vessels. The superficial layer, the corneal layer, is made of keratinised cells, which are constantly eliminated through exfoliation. These cells have gone through a specific growth process and have lost their nuclei and become flat to form the superimposed slats which scale. The corneal layer varies in thickness according to the area of the body, the thickest part covering the palms of the hands and soles ofthe feet act against friction and constraints. However, the skin covering the mucous membrane does not contain keratin and is therefore without a corneal layer. The deepest layer of the epidermis, made up of germinal cells, ensures the continuous renewal of the corneal layer after cellular growth. The epidermis takes four to six weeks to be completely renewed. In the deeper part of the epidermis, we can also find another type of specialized cells called melanocytes that govern skin colour through the production of melanin. This pigment is found in greater quantities in dark-skinned people than in fair-skinned people.




The dermis

The dermis is the skin's supporting tissue. Its specialized cells, called fibroblasts, are in charge of the production of collagen and elastin fibres. Collagen fibres ensure maintenance and resistance of the tissues by forming a densely organized weft-like network. Finer elastin gives the skin its suppleness and elasticity. They decrease in number during the ageing process and disappear completely after the age of 45. These fibres are swimming in a gel, rich in hyaluronic acid, which plays a part in skin moisturisation, locking water into molecules. The dermis also contains blood vessels, which carry out the role of feeding the above epidermis and also participate in thermal regulation. The dermis is also particularly rich in nerve endings, which are specifically sensitive to touch, to pain, and to temperature, making the skin a sensorial organ.




The hypodermis

The hypodermis is the organism's adipose mattress giving the figure a more or less harmonious outline. It is the most important reservoir the body has, working through the stocking and liberation of fatty acids. These fatty cells, the adipose, are the voluminous cells whose nuclei have been flattened and pushed to the side by a drop of lipids. Lipocytes are distributed differently depending on sex: for a woman they predominate in the buttocks and thigh area, whereas, for a man, they are to be found in the abdominal region. In the hypodermis, we find sweat glands as well as body hair follicles, to which the sebaceous glands are annexed.





Functions

Skin performs the following functions:

1. Protection: an anatomical barrier between the internal and external environment in bodily defense; Langerhans cells in the skin are part of the adaptive immune system

2. Sensation: contains a variety of nerve endings that react to heat and cold, touch, pressure, vibration, and tissue injury; see somatosensory system and haptics.

3. Heat regulation: the skin contains a blood supply far greater than its requirements which allows precise control of energy loss by radiation, convection and conduction to maintain blood flow and conserve heat. Erector pili muscles are significant in animals.

4. Control of evaporation: the skin provides on. Dilated blood vessels increase perfusion and heat loss while constricted vessels greatly reduce cutaneous a relatively dry and impermeable barrier to fluid loss. Loss of this function contributes to the massive fluid loss in burns.

5. Aesthetics and communication: others see our skin and can assess our mood, physical state and attractiveness.

6. Storage and synthesis: acts as a storage centre for lipids and water, as well as a means of synthesis of vitamin D by action of UV on certain parts of the skin.

7. Excretion: The concentration of urea is 1/130th that of urine. Excretion by sweating is at most a secondary function to temperature regulation.

8. Absorption: Oxygen, nitrogen and carbon dioxide can diffuse into the epidermis in small amounts, some animals using their skin for their sole respiration organ. In addition, medicine can be administered through the skin, by ointments or by means of adhesive patch, such as the nicotine patch or iontophoresis. The skin is an important site of transport in many other organisms.





Diseases

In medicine, the branch concerned with the skin is called dermatology. The skin is subject to constant attack from without, and so can be afflicted by numerous ailments, such as these:

Tumors:

1. Benign tumors of the skin such as Squamous cell papilloma
2. Skin cancer



Others:

1. Rashes
2. Blisters
3. Acne
4. Keratosis pilaris
5. Fungal infections such as athlete's foot
6. Microbial infections.
7. Calcinosis cutis
8. Ring worm
9. Sunburn
10. Keloid
11. Scabies


There are several other skin diseases as well

slender threadlike outgrowth from the skin of mammals. In some animals hair grows in dense profusion and is called fur or wool. Although all mammals show some indication of hair formation, dense hair is more common among species located in colder climates and has the obvious function of insulation against the cold. Other functions include camouflage and protection against dust and sand. The long, sensitive hairs, called tactile hairs, that are located around the mouth area of most mammals are extremely sensitive to touch. Each hair filament originates in a deep pouchlike depression of the epidermis, called a hair follicle, which penetrates into the dermis. The root of the hair extends down into the hair follicle and widens into an indented bulb at its base. Extending into the indentation is the papilla, the center of hair growth, which contains the capillaries and nerves that supply the hair. Newly dividing cells at the base of the hair multiply, forcing the cells above them upward. As the cells move upward, they gradually die and harden into the hair shaft. The hair shaft has two layers, the cuticle and the cortex. The cuticle (outer layer) consists of flat, colorless overlapping cells; below the cuticle is the cortex, containing pigment and a tough protein called keratin; it forms the bulk of the hair shaft. Coarse hair, such as that of the scalp, contains an additional inner core called the medulla. Hair is lubricated by sebaceous glands that are located in the hair follicle. Illness or stress may lessen the secretion of pigment, which normally gives color to hair, and cause the hair shaft to whiten. However, the normal process of whitening that comes with age is determined by heredity. In humans, scalp hairs are generally shed every two to four years, while body hairs are shed more frequently. Straight-textured hair, round in cross section, is common among Native Americans, Eskimos, and Mongoloid peoples. Kinky or woolly hair, flat in cross section, prevails among the dark peoples of Africa, Australia, and elsewhere. Wavy or curly hair, common among Caucasians, is oval in cross section. The color of hair is determined by the amount of pigment and air spaces in the cortex and medulla. Hair color and texture are inherited characteristics.











the horny outgrowth shielding the tip of the finger and the toe in humans and most other primates. The nail consists of dead cells pushed outward by dividing cells in the root, a fold of epidermis at the base of the nail. The hard material in nail cells is the tough protein material, keratin. If the root is destroyed, the nail ceases to grow. Otherwise, growth from root to tip is achieved in about four months. The small-celled and relatively bloodless tissue near the base of the nail forms a white, crescent-shaped spot called the lunula, or moon. No pigment occurs in nail cells, but since they are translucent, their appearance is pink because of blood vessels beneath. A painful inflammation (paronychium) of the fingertip may result from infection starting in a hangnail. Pressure from improperly fitting shoes may cause the large toenail to cut into the skin along its edges (the so-called ingrown toenail). Horny derivatives of the integument, homologous to the primate nail, have evolved into various structures in other animals, e.g., the hooves of horses and cattle and the claws of birds and reptiles.

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