The function of skin is to protect the body and to serve as an organ of heat regulation, excretion and sensation. It consists of an epithelial part – epidermis and a connective tissue part – the dermis and other structures, such as hair follicles, sebaceous glands, sweats gland etc. The hair, the horny portion of the hoof, nails or claws, on the digits, the horns of cattle, etc., are also derived from the skin, by special modification and development.
The skin or cutis is divided into the epithelial epidermis and the connective tissue corium of dermis. The underlying loose subcutis attaches the skin to the subjacent organs. The thickness of the skin varies considerably in different parts of the body. The relative proportions of epidermis and dermis also vary. Hair follicles are generally present but there are some non-hairy regions and the epidermis show considerable thickening in these regions.
Epidermis
- This is a stratified squamous epithelium covered by a stratum corneum and rests on the papillae of dermis.
- The free surface may be smooth or show elevation caused by underlying papillae.
- Epidermis consists of two main layers – a deep layer stratum germinativum or stratium Malphigi and a superficial horny layer.
- The deep layer is subdivided into two and superficial into three. So starting from the corium one can distinguish the following layers:
- Stratum Cylindricum: consists of a layer of columnar cells resting on a thin basement membrane.
- Stratum spinosum: composed of polygonal cells arranged in a number of layers. The cells show spherical nuclei and basophilic cytoplasm. They are arranged more transversely, towards the surface. In routine preparation examined under light microscope fine processes may be seen extending between the cells. These were referred to as “intercellular bridges” and since the cell showed spines or processes projecting form their surfaces, they were called “Prickle Cells”. EM studies have shown that these intercellular bridges are artifacts. The membranes of the cells normally in close apposition but tend to pull apart except at the region of desmosomes due to the shrinkage caused by technical procedures.
- Numerous mitotic figures may be seen in this layer and both stratum cylindricum and spinosum form the stratum germinativum.
- Stratum granulosum: consists of a few layers of flattened rhomboid cells. These cells show first signs of cornification in the form of basophilic granules of Keratohyalin which increase in size and number toward surface and lend a granular appearance to this layer. With increase of granules, nucleus disintegrates and becomes pale.
- Stratum lucidum: is a shiny acidophilic layer of homogeneous appearnce. Here the Keratohyalin granules have liquefied to form eleidin, which is uniformly distributed throughout the many layers of cell. The cells show neither nucleus nor cell boundaries.
- Stratum Corneum: Consists of many layers of flat elongated cornified cells. The material forming the cells is keratin or true horny substance. Nucleus is absent. The superficial cells are dried horny plates, which are constantly shed off. New ones from the deeper layers replace the cells thus lost. The superficial layer, which is constantly desquamated, is known as stratum disjunctum.
Corium or Dermis
- The dermis varies in thickness and is composed of dense irregularly arranged connective tissue. It contains the connective tissue fibres, fibroblasts and histiocytes. It can be divided into a superficial or subepithelial papillary payer and a deeper reticular layer, though the two layers blend without any distinct line of demarcation.
- Papillary layer: This is just below the epidermis and there is a dense interweaving of fine collagenous bundles. This layer bears on its surface contain papillary bodies, over which the epithelium is moulded. These papillae may contain capillary loops or sensory nerve endings of tactile sense. (Tactile or Meissner’s corpuscles). Between the bases of papillae, three may be down growths as inter-papillary pegs of epithelium.
- Stratum reticulare: The reticular layer is deeper and thicker, not sharply marked off from papillary layer. These are characterized by a network of coarse collagenous and elastic fibres. The fibres form an extensive feltwork with rhomboid meshes, the direction of fibres being parallel to the surface of the skin. The elastic fibres form basket-like capsular condensations around hair-bulbs, sweat and sebaceous glands.
Subcutis or Hypodermis
- Consists of loose collagenous tissue which contain many elastic fibres cross with each other to form a meshwork. Smaller bundles subdivide the meshes.
- A homogeneous adhesive ground substance converts the fibre nets into thin membranes. The degree to which skin can be displaced or folded depends on the development of subcutis i,e., the thickness, length, extensibility of fibre bundles.
- The spaces of subcutis are filled with adipose tissue forms a flat cushion (panniculus adiposus) which in well conditioned animals extends to the deeply placed cutaneous muscles. There is no clear demarcation between corium and subcutis.
Regional difference
- The epidermis is considerably thicker in the palm, sole and volar surfaces of digits and all layers are distinct.
- The stratum corneum is very thick and forms as major part. In these regions the hair follicles are absent.
- Tactile corpuscles are numerous in the dermal papillae in the skin, at the tip of fingers and palm.
- In other region, the epidermis is thinner. All the layers are reduced and stratum corneum and stratum germinativum are constantly present, the stratum granulosum and lucidum being indistinct or totally absent.
- In the dermis the papillae are not prominent and are less numerous. Most of the area of the skin over the body contains hair follicles.
- The colour of the skin is primarily dependent upon the presence of melanin pigment in the epidermal cells. Certain patches of skin are especially rich in pigment (nipples, axilla, circumanal region, scrotum, labia majora), while practically no pigment is present in some regions (palm, sole).
- Melanin is formed in specialized cells known as melanocytes. These are found in the basal layers of stratum germinativum. The cell bodies of melanocytes appear clear, free of granules located among the basophilic epidermal cells in routine preparations. The processes of melanocytes are demonstrable only by special techniques, extend between the epidermal cells.
- The melanin granules as they are formed are pushed to the periphery and hence the processes of melanocytes will contain pigment but the cell body appears relatively clear. From the processes of melanocytes, the pigment is distributed to the epidermal cells. In white races the pigment granules occur only in the deepest cell layers and in coloured races, the pigment is distributed in more layers, throughout the stratum germinativum.
- Melanocytes stain black on treatment with Dioxy phenylalanine (Dopa reaction) and this reaction is attributed to the presence of the enzyme tyrosinase in the cytoplasm of melanocytes. The formation of melanin is stimulated by ultraviolet radiation.
- In the coloured races, pigmented connective tissue cells containing melanin, called dermal chromatophores may also occur in the dermis, close to the epithelium.
