Definition of Skin .
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| structure of skin |
Skin is the general covering of the entire external surface of the body, including the external auditory meatus and the outer surface of tympanic membrane. It is continuous with the mucous membrane at the orifices of the body. Because of a large number of its functions, the skin is regarded as an important organ of the body .
Surface Area of Skin .
1. In an adult the surface area of the skin is 1.5-2 (average 1.7) sq. metres. In order to assess the area involved in burns, one can follow the rule of nine: head and neck 9%; each upper limb 9%; the front of the trunk 18%; the back of the trunk (including buttocks) 18%; each lower limb 18%; and perineum 1 % .
2. The surface area of an individual can be calculated by Du Bois formula. Thus, A = W x H x 71.84, where A = surface area in sq. cm, W= weight in kg, and H= height in cm.
Pigmentation of Skin .
The colour of the skin is determined by at least five pigments present at different levels and places of the skin. These are:
1. Melanin, brown in colour, present in the germinative zone of the epidermis.
2. Melanoid, resembles melanin, present diffusely throughout the epidermis.
3. Carotene, yellow to orange in colour, present in stratum corneum and the fat cells of dermis and superficial fascia.
4. Haemoglobin (purple).
5. Oxyhaemoglobin (red), present in the cutaneous vessels.
The amounts of first three pigments vary with the race, age, and sart of the body. In white races, the colour of the skin depends chiefly an the vascularity of the dermis and thickness (translucency) of the keratin. The colour is red where keratin is thin (lips), and it is white where keratin is thick (palms and soles).
Thickness of the Skin .
The thickness of the skin varies from about 0.5 to 3 mm.
Structure of Skin .
The skin is composed of two distinct layers epidermis and dermis.
A. Epidermis layer of Skin .
[1]. It is the superficial, avascular layer of stratified squamous epithelium. It is ectodermal in origin and gives rise to the appendages of the skin, namely hair, nails, sweat glands and sebaceous glands.
[2]. Structurally, the epidermis is made up of a superficial cornified zone and a deep germinative zone. The cells of the deepest layer proliferate and pass towards the surface to replace the cornified cells lost due to wear and tear.
[3]. As the cells migrate superficially, they become more and more flattened, and lose their nuclei to form the flattened dead cells of the stratum corneum.
[4]. In the germinative zone, there are also ‘dopa’ positive melanocytes (melanoblasts, dendritic cells, or clear cells) of neural crest origin, which synthesize melanin.
B. Dermis or corium layer of Skin .
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| Flexure lines (Langer’s lines) |
[1]. Dermis or corium is the deep, vascular layer of the skin, derived from mesoderm . It is made up of connective tissue (with variable elastic fibres) mixed with blood vessels, lymphatics and nerves. The connective tissue is arranged into a superficial papillary layer and a deep reticular layer.
[2]. The papillary layer forms conical, blunt projections (dermal papillae) which fit into reciprocal depressions on the undersurface of the epidermis. The reticular layer is composed chiefly of the white fibrous tissue arranged mostly in parallel bundles.
[3]. The direction of the bundles, constituting flexor or cleavage lines (Langer’s lines), is longitudinal in the limbs and horizontal in the trunk and neck . In old age the elastic fibres atrophy and the skin becomes wrinkled.
[4]. Overstretching of the skin may lead to rupture of the fibres, followed by scar formation. These scars appear as white streaks on the skin (e.g. lineae gravidarum). At the flexure lines of the joints, the skin is firmly adherent to the underlying deep fascia.
[5]. Dermis is the real skin, because, when dried it makes green hide, and when tanned it makes leather. Its deep surface is continuous with the superficial fascia.
Surface Irregularities of the Skin .
The skin is marked by three types of surface irregularities, the tension lines, flexure lines and papillary ridges (Montagna and Lobitz, 1964).
1. Tension lines.
It form a network of linear furrows which divide the surface into polygonal or lozenge-shaped areas. These lines to some extent correspond to variations in the pattern of fibres in the dermis.
2. Flexure lines (skin creases or skin joints).
There are certain permanent lines along which the skin folds during habitual movements (chiefly flexion) of the joints. The skin along these lines is thin and firmly bound to the deep fascia. The lines are prominent opposite the flexure of the joints, particularly on the palms, soles and digits .
3. Papillary ridges (friction ridges) .
There are confined to palms and soles and their digits. They form narrow ridges separated by fine parallel grooves, arranged in curved arrays. They correspond to patterns of dermal papillae. Their study constitutes a branch of science, called dermatoglyphics (Cummins and Midlo, 1961). Three major patterns in the human fingerprints include loops, whorls and arches. These patterns and many other minor features are determined genetically by multifactorial inheritance .
APPENDAGES OF SKIN .
1. Nails .
Nails are hardened keratin plates (cornified zone) on the dorsal surface of the tips of fingers and toes, acting as a rigid support for the digital pads of terminal phalanges.
Each nail has the following parts.
(a) Root is the proximal hidden part which is burried into the nail groove and is overlapped by the nail fold of the skin.
(b) Free border is the distal part free from the skin.
(c) Body is the exposed part of the nail which is adherent to the underlying skin. The proximal part of the body presents a white opaque crescent called lunule .
Each lateral border of the nail body is overlapped by a fold of a skin, termed the nail wall. The skin (germinative zone + corium) beneath the root and body of the nail is called nail bed.
The germinative zone of the nail bed beneath the root and lunule is thick and proliferative (germinal matrix), and is responsible for the growth of the nail.
The rest of the nail bed is thin (sterile matrix) over which the growing nail glides. Under the translucent body (except lunule) of the nail, the corium is very vascular. This accounts for their pink colour.
2. Hair .
[1]. Hair are keratinous filaments derived from invaginations of the germinative layer of epidermis into the dermis. These are peculiar to mammals (like feathers to the birds), and help in conservation of their body heat. However, in man the heat loss is prevented by the cutaneous sensation of touch.
[2]. Hair are distributed all over the body, except for the palms, soles, dorsal surface of distal phalanges, umbilicus, glans penis, inner surface of prepuce, the labia minora, and inner surface of labia majora.
[3]. The length, thickness and colour of the hair vary in different part of the body and in different individuals. Each hair has an implanted part called the root, and a projecting part called the shaft.
[4]. The root is surrounded by a hair follicle (a sheath of epidermis and dermis), and is expanded at its proximal end to form the hair bulb. Each hair bulb is invaginated at its end by the hair papilla (vascular connective tissue) which forms the neurovascular hilum of the hair and its sheath.
[5]. Hair grows at the hair bulb, by proliferation of its cells capping the papilla. The hair follicles, enclosing hair roots, lie obliquely to the surface of the skin, which is responsible for the characteristic hair streams in different parts of the body.
[6]. The arrectores pilorum muscles (smooth muscles supplied by sympathetic nerves) connect the undersurface of the follicles to the superficial part of the dermis. Contraction of these muscles leads to erection of hair, squeezes out the sebum, and produces ‘goose skin’ .
[7]. The foetal skin is covered by fine hair called lanugo (primary hair). These are mostly shed by birth, and are replaced during infancy by another set of fine hair called vellus (secondary hair).
[8]. These are retained in most parts of the body, but are replaced by the thick and dark terminal hair of the scalp and eyebrows, and other hairy areas of the adult skin.
[9]. The hair grow at the rate of about 1.5-2.2 mm per week; their growth is controlled by hormones. The life span of the hair varies from 4 months (eyelashes, axillary hair) to 4 years (scalp hair).
3. Sweat Glands .
[1]. Sudoriferous or sweat glands are distributed all over the skin, except for the lips, glans penis, and nail bed. These glands are of two types, eccrine and apocrine (Zelickson, 1971). The eccrine glands are much more abundant and distributed in almost every part of the skin.
[2]. Each gland is a single tube, the deep part of which is coiled into a ball. The coiled part, called the body of the gland, lies in the deeper part of corium or in the subcutaneous tissue. The straight part, called the duct, traverses the dermis and epidermis and opens on the surface of the skin.
[3]. The glands are large in the axilla and groin, most numerous in the palms and soles, and least numerous in the neck and back. The eccrine glands are merocrine in nature, i.e. produce their thin watery secretion without any disintegration of the epithelial cells.
[4]. They are supplied and controlled by cholinergic sympathetic nerves. The glands help in regulation of the body temperature by evaporation of sweat, and also help in excreting the body salts. In dogs, sweat glands are confined to foot pads. Therefore, dogs do not sweat, they pant.
[5]. The apocrine glands are confined to axilla, eyelids (Moll’s glands), nipple and areola of the breast, perianal region, and the external genitalia. They are larger than eccrine glands and produce a thicker secretion having a characteristic odour. They develop in close association with hair and their ducts typically open into the distal ends of the hair follicles.
[6]. Ceruminous glands of the external auditory meatus are modified apocrine sweat glands. The apocrine glands also are merocrine in nature, but are regulated by a dual autonomic control. Some workers are not inclined to call them as sweat glands at all because they do not respond sufficiently to temperature changes.
[7]. In animals they produce chemical signals or pheromones, which are important in courtship and social behaviour. On an average one litre of sweat is secreted per day; another 400 ml of water is lost through the lungs, and 100 ml through the faeces. This makes a total of about 1500 ml, a rough estimate of the invisible loss of water per day.
[8]. However, in hot climates the secretion of sweat may amount to 3-10 litres per day, with a maximum of 1-2 litres per hour. So long the sweat glands are intact, the skin can regenerate. If the sweat glands are lost, skin grafting becomes necessary.
4. Sebaceous Glands .
[1]. Sebaceous glands, producing an oily secretion, are widely distributed all over the dermis of the skin , except for the palms and soles. They are especially abundant in the scalp and face, and are also very numerous around the apertures of the ear, nose, mouth, and anus.
[2]. Sebaceous glands are small and sacculated in appearance, made up of a cluster of about 2-5 piriform alveoli. Their ducts open into the hair follicles with the exception of lips, glans penis, inner surface of prepuce, labia minora, nipple and areola of the breast and tarsal glands of the eyelids, where the ducts open on the surface of the skin.
[3]. Sebaceous glands are holocrine in nature, i.e. they produce their secretion by complete fatty degeneration of the central cells of the alveolus, which are then replaced by the proliferating peripheral cells.
[4]. The secretion is under hormonal control, especially the androgens. The oily secretion of sebaceous glands is called sebum. It lubricates skin and protects it from moisture, desiccation, and the harmful sun rays.
[5]. Sebum also lubricates hair and prevents them from becoming brittle. In addition, sebum also has some bactericidal action. Sebum makes the skin water-proof. Water evaporates from the skin, but the fats and oils are absorbed by it.
Functions of Skin .
1. Protection :- Skin protects the body from mechanical injuries, bacterial infections, heat and cold, wet and drought, acid and alkali, and the actinic rays of the sun.
2. Sensory :- Skin is sensory to touch, pain and temperature.
3. Regulation of body temperature :- Heat is lost through evaporation of sweat; and heat is conserved by the fat and hair.
4. Absorption :- Oily substances are freely absorbed by the skin.
5. Secretion :- Skin secretes sweat and sebum.
6. Excretion :- The excess of water, salts and waste products are excreted through the sweat.
7. Regulation of pH :- A good amount of acid is excreted through the sweat.
8. Synthesis :- In the skin, vitamin D is synthesized from ergosterol by the action of ultraviolet rays of the sun.
9. Storage :- Skin stores chlorides.
10. Reparative :- The cuts and wounds of the skin are quickly healed .
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