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.

Skin as a Toxin Storage Organ in the Endemic New Guinean GenusPitohui

“…Our findings are consistent with past work on avian integument that has led to the view that the entire avian epidermis should be considered a holocrine unit (Lucas 1970, Menon et al 1980, Menon 1984, Menon and Menon 2000. Because of the skin's adaptations as a holocrine unit and as a lipid-rich water barrier, the skin may be pre-adapted or exapted (Brower et al 1988) for sequestering and storing certain types of compounds, including defensive toxins.…”

Fine structural basis of the cutaneous water barrier in nestling Zebra Finches Poephila guttata

“…An inverse relationship between feathering and epidermal lipid secretion has been recognized recently in avian skin. In many bird species, such as certain ibises and storks, epidermal lipogenesis increases in body areas that lose feathers permanently and develop into neoapteria during post-juvenile development (Menon 1984). T h e present observations show that epidermal lipogenesis is high prior to plumage development as well.…”

Cosmetic Coloration in Birds: Occurrence, Function, and Evolution

“…Besides forming a dynamic barrier regulating water loss through the skin, epidermal lipids could also have antimicrobial properties, offer protection against UV light, and act as solvents for carotenoids and other pigments (see review in Menon 1984).…”