Development of horns in merino sheep

Abstract

Development of horns has been studied in 20 foetuses ranging in age from 55 to 144 days of gestation, and in 16 lambs, ranging in age from birth to 42 days. Samples from one adult have also been examined. An increase in the thickness of the epidermis appears to be the first indication of horn development, recognizable in a 75-day-old male foetus and in an 84-day-old female foetus. Primary hair follicles develop in the horn region and produce emerging hairs, but these follicles later degenerate and disappear. The sweat glands and sebaceous glands formed in association with these follicles also degenerate, usually after hair growth has ceased. Fusion of primary follicles is a common feature in the horn growing skin, particularly before hair formation; a common connective tissue sheath surrounds the lower parts of these fused follicles. No secondary follicles are formed in the horn region but some branching of the primary follicles has been observed. By 118 days of gestation the epidermis in the horn region becomes greatly thickened, with very long dermal papillae which are well vascularized and innervated. Medulla-like columns of cells are formed above each dermal papilla and these cells keratinize later than the cells between the columns. These medullary cells remain in contact with each other longer than do the other cells of the horny sheath. Numerous arteriovenous anastomoses, which develop at two levels in the dermis of the horn region, are in continuity with capillaries which enter the dermal papillae of the epidermis. An outgrowth of bone from the frontal region of the skull, which later becomes the bony core of the horn, is first seen in a male foetus at 118 days. Acetylcholinesterase-positive branched cells (Langerhans cells), present in the lower part of the thick epidermis of developing horns, are not discernible in fully formed horns.