Morphological and histological studies on the skin of the pig (sus domesticus)

Abstract

Studies on the skin of Large White Yorkshire pigs were conducted using 12 animals of six to ten months of age. The project was undertaken to study the morphology, morphometry, histology and the distribution of hair and to compare the sex differences if any, in the skin of pig. Skin samples were collected from eight areas of the body viz., the snout, dorsal nasal, dorsal neck, ventral neck, dorsal abdomen, lateral abdomen, ventral abdomen and carpal regions. After recording gross parameters, material was fixed in 10 per cent neutral buffered formalin and standard procedures were adopted for histoarchitectural and histochemical studies. In general, skin of male animals was slightly thicker than that of the females. Maximum thickness for the skin, epidermis and the dermis was noticed in the snout and minimum in the ventral abdomen. Skin was thicker on the dorsal surface of the body than on the ventral surface. Contribution of the epidermis to the total skin thickness was maximum in the snout region. Subcutaneous fat layer was slightly thicker in females. A highly significant positive correlation was noticed between the skin thickness and the thickness of the epidermis in the snout, dorsal nasal and carpal regions in both male and female animals. Among the five layers of the epidermis, stratum basalis, spinosum and corneum were always present and formed continuous layers throughout the body surface. The stratum granulosum was not continuous in ventral neck and lateral and ventral abdominal regions. A definite stratum lucidum was seen only in the snout, dorsal nasal and ventral abdominal areas. The rete pegs and the dermal papillae were most abundant in the snout region and minimum in the lateral abdominal region. Stratum basalis was made up of a single layer of columnar to cuboidal cells. Clear cells could be located in the stratum basalis and stratum spinosum. Stratum spinosum was the thickest layer of the epidermis consisting of large, irregular and polyhedral cells with distinct boundaries. Prekeratin granules were detected in the upper layers of stratum spinosum. Thickness of this layer was maximum in the snout. Stratum granulosum consisted of two to four rows of flattened, diamond-shaped cells. Cytoplasm showed keratohyalin granules. Stratum lucidum appeared as a clear, bright, homogenous, strongly eosinophilic layer. Stratum corneum consisted of keratinized, scale-like polygonal, clear cells. There was a significant positive correlation between the thickness of the skin and that of dermis in all regions under study in both sexes. Papillary layer of the dermis was made up of collagen fibres predominantly, which were finer and more closely arranged. Reticular layer consisted of large, coarse and loosely interwoven bundles of collagen fibres. Glomi were most numerous in the snout. Hair arrangement in swine was simple, but grouping of hairs was evident. Maximum hair density was noticed in the dorsal nasal area. Density of hair distribution was more in the male animals. Hair shaft was composed of a cuticle, thicker cortex and slender medulla. Hair follicle was composed of four parts, viz., hair papilla, hair matrix, inner root sheath and outer root sheath. Largest arrector muscles were noticed in the abdomen dorsal region. Interfollicular muscle connected adjacent hair follicles of its characteristic hair group. Sweat glands were of apocrine type in all the regions under study except in the snout and dorsal nasal regions where it was of eccrine type. In the latter, both clear and dark cells were identified. Sebaceous glands appeared as large, lobulated, sac-like structures associated with the hair follicles. The secretory units consisted of a solid mass of epidermal cells. Maximum subcutaneous fat thickness was noticed at the neck dorsal region. The subcutaneous tissue was composed of a loose meshwork of connective tissue fibres, cells, blood vessels and nerve fibres. PAS - alcian blue positive areas were detected in middle region of the epidermis and ground substance of the dermis. Cells of stratum corneum, stratum spinosum, sebaceous glands, their ducts and clear cells of eccrine sweat glands showed a positive reaction to Oil Red O. Most of the layers of the epidermis and the dermal papillae, blood vessels surrounding the hair follicles and the sweat glands showed a positive reaction to alkaline phosphatase. Epidermis and sebaceous glands showed a positive reaction for acid phosphatase.