Tumor-like growth of antlers in castrated fallow deer: an electron microscopic study.

Male deer regenerate new sets of antlers each year. When fully grown, rising levels of testosterone promote antler ossification, cutting off the blood flow and causing the velvet integument to be shed. After the mating season, the old antlers fall off to be replaced by new ones. When the adult fallow deer is castrated in autumn or winter, its bony antlers are shed and replaced by usually shorter regenerates that remain permanently viable and in velvet. If prevented from winter freezing, these antlers continue to grow thicker each year, eventually giving rise to amorphous outgrowths, or antleromas, from their sides. These growths mushroom out from the antler as clusters of nodules, developing in unpredictable locations, but commonly at the bases and ends of the antlers. Their integument contains numerous hair follicles. Internally, antleromas are composed of masses of collagen together with fibroblasts actively engaged in ribonucleic acid and protein synthesis. Thin basal laminae surround the blood vessels, and in the skin separate the overlying epidermis from the collagenous substance of the antleroma. Despite their superficial resemblance to hypertrophic scars, antleromas lack many of the characters by which they are diagnosed. They may be classified as benign tumors, at least in the generic sense. Antleromas would appear to represent a sustained expression of antler regeneration uncoupled from those morphogenetic influences responsible for the configurations into which deer antlers normally develop.