temperature regulation sense perception mechanical protection water balance The two types of skin The skin can be divided into two classes:

temperature regulation sense perception mechanical protection water balance The two types of skin The skin can be divided into two classes: (i) the thin skin, and (2) the thick skin.The thick skin is found in the sole and palm of the hand; all other skin are thin skin. Characteristics, by which the thick skin differs from the thin skin are: the thick skin is - (i) more thick than the thin variety, the layer called stratum corneum (horny layer) being particularly thick,(ii) the thick skin contains no hair and no sebaceous gland in persons accustomed to walk barefooted, the skin of the sole may be as thick as 1 cm or so, whereas the skin on the upper eyelid may be as thin as 0. 5 mm only. Appendages of the skin The epidermis of the skin develops from the ectoderm of the fetus. This ectoderm, while developing into the skin ectoderm, may develop into nails, hairs, sebaceous glands and sweat glands. These structures are collectively called the 'appendages' of the skin. Microscopic anatomy of the skin From a physiological point of view, the term skin should include three layers, viz (i) epidermis, (ii) dermis, and (iii) the subcutaneous tissue. Some physiologists, indeed, have used the term skin in this way.* Classical histologists, however, include only the epidermis and the dermis in the skin (the subcutaneous tissue is thus excluded). In this book, the term skin will be used in the sense a classical histologist uses, because, as this author believes, this is the most popular usage. The skin, therefore, consists of two layers: (i) tha epidermis and (ii) the dermis. *Hellier, F. F : <>, in Bell, G. H., Emslie - Smith D, and Patterson, C. R. Text Book of Physiology. ELM. 10th Ed. Churchil, 1980 chap. 17 The epidermis (fig. 8.5.1) consists of 5 layers which are: stratum corneum stratum lucidum. stratum granulosum. stratum spinosum (or prickle cell layer) stratum geriminatirvum (the deepest layer of epi dermis) Fig. 8.5. 1. Microscopic features of skin The cells of the stratum germinativum (= the layer which generates, i. e. generates new cells) continuosly produce new cells (by mitotic division); these cells move continuously towards the surface side and on the way receive a special material, called , ultimately when the superficial most surface is reached, the cells become dead and highly keratinized and constitute the stratum corneum: Stratum germinativum cells are columnar and stand on the basement membrane. The basement membrane stands between the epidermis and the dermis. Stratum spinosum (prickle cell layer) is several (3 to S) cells thick, the individual cells have a prickly (thorny) appearance (and hence the name). The prickled appearance is due to the fact that cytoplasmic processes come out from the individual cells to meet their fellow cytoplasmic strands of the adjacent( cells. Stratum granulosum is about 3 cell layers thick and lies superficial to the prickle cell layer. Although the mechanism of synthesis of keratin is rather complicated, it may be (rather crudely) stated that the cells of the stratum granulosum (=the granular layer) are very important for keratin synthesis. The precursors of keratin take the shape of granules which are present within the cells of the stratum granulosum from which the name has been derived. Stratum lucidum, when present, is found in between the str. granulosum and sir. comeum (Fig. 8. 5.1). it is, when stained by hematoxylin and eosin, rather a clear (lucidum = clear) layer. Str. lucidum is found only in thick skin. When the granules of the str. granulosum are converted into eleidin, the cells become rather clear and form str. lucidum. Eleidin is precursor of keratin. Stratum corneum (comeus =