Glandular Functions of Avian Integument

Abstract

This paper attempts a synthesis of available information on glandular functions of avian integument, a topic which has not received adequate attention. Morphologi cally distinct glands in the skin of birds are limited to the uropygial gland, ceruminous (wax) glands in external ear canal and integumentary glands of the vent (anal glands). Of these, only the uropygial gland has been studied in a considerable manner., whereas taxonomic and functional status of the latter two remain unclear at the present juncture. The holocrine ceruminous glands, though lipoid producing, differ greatly from the uropygial gland in structure and functions. The glands of the vent are the only mucoid producing glands found in avian skin. Thus the avian integument, on gross examination, appears to be poorly endowed with glands compared to that of other vertebrates. However, cells of avian body epidermis show a unique lipogenic potential, producing both keratin and sebum like lipoid material. Aptly termed as 'sebokeratocytes', the epidermal cells in glabrous portions of skin as well as specialized regions such as the rictus and toe web exhibit a remarkably high degree of sebogenic activity. Possible significance of this epidermal lipoid secretion in enhancing water repellancy of skin; preventing excessive cutaneous water loss; screening off harmful radiations, imparting colouration to the skin etcetera are discussed. In light of the widespread nature of epidermal lipoid secretion and the knowledge that many specialized types of feathers in various species produce and maintain a supply of material such as powder, grease or both, used in cosmetic purposes, the concept of secretory activity is reexamined. It is proposed that on a functional basis, the secretory epidermal regions and specialized feathers be considered as glandular units of skin, though they lack the conventionally accepted glandular profile.