The integumentary system consists of integument, called skin or cutis, and the structures developed from the skin are termed the skin derivatives. The study of the structure and functions of the skin and its derivatives is called dermatology. SKIN Meaning of Skin. The skin is defined as a surface covering easily separable from the underlying muscle layer of the body wall. Physical Features of Skin. Skin is the largest organ of the body. A 1.8 m. tall man of average weight has a skin area of about 9.7 square metres. The skin accounts for about 16% of total body weight. Thickness of the skin varies in different parts of the body. It is thickest on the palms and soles, where it may be about 1.5 mm. thick. On the palms and fingers, the skin (epidermis) has numerous ridges, forming arches, whorls and loops in highly characteristic patterns. These patterns, called finger marks, are determined genetically and serve as a mark of personal identification. No two persons have identical patterns. The finger marks develop during the third and the fourth months of foetal life, and never change afterward. Finger marks were discovered by Henry Fauld, a Scottish medical missionary, in 1880. By greasy finger marks on a bottle, he identified the person who had been drinking the rectified spirit from the dispensary. Impressions of the inked bulbs of the distal phalanges of the fingers are called finger prints. The skin is elastic and can cover a large surface when swelling occurs. Histology of Skin. Human skin consists of two distinct regions of different origins : epidermis and dermis or corium. The two regions are fitted together by ridges and depressions. Epidermis. Epidermis is the outer thin, nonvascular region of the skin. It is nourished by the tissue fluid from the inner region of the skin. It develops from the ectoderm of the embryo. It is a keratinized, stratified squamous epithelium. The nature of cells varies in different layers. The innermost layer consists of columnar cells arranged perpendicular to the dermis. It is known as Malpighian layer, or stratum germinativum, or stratum basale. It lies on a basement membrane of epidermal origin. The cells of this layer have abundant desmosomes in their lateral and upper surfaces for binding to adjacent cells, and hemidesmosomes in their inner surfaces for binding to basement membrane. The cells of the germinative layer are active and produce new cells by mitotic divisons, hence the name germinative layer. The newly formed cells pass outward and become progressively flattened. The region of epidermis where flattening of cells takes place is called transitional region. In this region the cytoplasm of the cells is slowly replaced by a hard, insoluble, fibrous, sulphur-containing protein, the keratin, or horn. The outer layers of epidermis are, thus, composed of very thin, keratinized cells which lose nuclei and die. These cells together constitute the horny layer, or stratum corneum. This layer is water-proof and germproof. It provides epidermis its characteristics toughness. Its thickness increases over parts of the body which are subjected to considerable friction such as ball of the foot and bases of the finger. The horny layer is cast off as small pieces at intervals. During our lifetime, we shed about 23 kg. of skin in this manner. The rate at which the horny cells are cast off is almost the same at which cells are formed from the Malpighian layer. This keeps the thickness of the epidermis constant. The human epidermis is renewed about every 15 to 30 days. The outer surface epidermis bears numerous minute pores of sweat glands and hair follicles.
Finger prints. A and B – Loops, C –Arches, D–Whorls, F–Circles, F–Scar-line The rate of cell division in the stratum germinativum is highest during sleep and lowest during muscular exercise and stress. - 63 -
Thick Skin. Over the parts of the body with thick skin, e.g., palms and soles, the transitional region of the epidermis further shows three region : the inner stratum spinosum, the middle stratum granulosum, and the outer stratum lucidum. The cells of the stratum spinosum are polygonal, slightly falttened and with prickly surface. The cells of the stratum granulosum contain keratohyalin granules. The cells of the stratum lucidum are rather transparent and contain a substance called eleidin. This substance is thought to be an intermediate product in the change of keratohyalin granules to keratin of the outer stratum corneum. Melanocytes. The lower layer of the epidermis has pigment cells, the melanocytes1. The melanocytes send long proceses between or under the epidermal cells. They produce the dark pigment melanin as granules called melanosomes. The latter pass into the epidermal cells. Melanin in the epidermal cells gives protection by absorbing ultraviolet rays of the sun. Tanning result from an increase in melanin production in response to prolonged exposure to sunlight. Amount of melanin varies greatly different races. In general, where the sun’s rays are more intense, have a heavier deposit of mealanin than the persons of colder climates. This is racial adaptation. A few people do not develop melanin and they cannot remain in bright sunlight for more than a few minutes at a time without developing serious burns. Such persons are called albinos. The lack of melanin is termed albinism. Dermis. Dermis is the inner thick, vascular region of the skin. It develops from the mesoderm. It is about 2 – 3 times as thick as the epidermis.
(a) Structure. The dermis is composed of dense connective (areolar) tissue containing bundles of wavy, unbranched white, or collagen, fibers; straight, branching yellow, or elastin, fibers and various types of cells, fibroblasts, histiocytes and mast cells. The white fibers attached the skin firmly to the underlying muscles. Their toughness limits stretching of the skin. Gradual starching of collagen fibers causes wrinkles in a later life. The yellow fibers are elastic. They bring about recovery of the skin after its stretching. The fibroblasts secrete the yellow and white fibers as well as the matrix in which fibers and cells lie. The histiocytes eat up the injured cells and foreign germs entering the skin. The mast cells secrete heparin and histamine in man as already mentioned. The dermis also contains blood vessels, nerve fibers and lymph vessels. The blood vessels send capillaries to meet the epidermis. Motor nerve fibers innervate the muscles and glands in the dermis, while sensory nerves fibers carry nerve impulses from sensory receptors present in the dermis. These receptor detect heat, cold, touch, pain, pressure, etc. Some smooth muscles fibers are also present in the dermis. A few macrophages are also present. Adipocytes may also occur.
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(b) Arteriovenous Anastomoses. In certain areas of the dermis, blood can pass directly from arteries to veins through the arteriovenous anastomoses or shunts. The latter play an important role in temperature and blood pressure regulation, since the skin can hold about 4.5 % of the blood volume. (c) Regions. The dermis has 2 regions : (i) outer thin papillary layer that consists of loose connective tissue and sends projections into the epidermis, forming, dermoepidermal junctions and (ii) inner thick reticular layer which consists of dense connective tissue and has some reticular fibers in addition to collagen and elastin fibers. The reticular layer has more fibers and fewer cells than the papillary layer. Subcutaneous Tissue. Beneath the dermis is a layer of loose connective tissue. This layer is called the subcutaneous tissue. It loosely attaches the skin to the muscles inside so that the skin can slide over the muscles. The access of the body fat is deposited many regions in the subcutaneous tissue. The subcutaneous fat insulates the body against cold and heat, and acts as a shock absorber. In palms and soles the fibers of the subcutaneous tissue are tightly interwoven with those of the dermis. Therefore, the skin is more firmly attached in these regions. DERIVATIVES OF SKIN Hair, glands and nails are formed from the skin in man. (1) Hair. Hair is a characteristic