A new insight into hormone control over bone growth, wound healing and tissue regeneration has come from observations of the annual regrowth of antlers of red deer in Scotland.
The results were presented at a recent meeting of the Royal Society in London as a unique example of the link between the regrowth of tissue in mammals and its control by sex hormones.
The investigation had other unusual aspects: it was a joint inquiry between the Medical Research Council's reproductive biology unit in Edinburgh and an animal research group at the Institute of Zoology, at Regent's Park.
The complex nature of the cycle, in which the animal sheds its antlers at the end of the breeding season and redevelops them each year, became apparent in research started 15 years ago by Dr Gerald Lincoln, of the reproductive biology unit. He began by examining free-living red deer on the Isle of Rhum, off the west coast of Scotland, and the measurements have continued using a small group of stags kept on a deer farm near Auchtermuchty, Fife.
The rate at which the new antlers grow is a remarkable physiological phenomenon. They develop at up to one inch a day and a complete new set, weighing as much as 25 kg in the moose, may be produced in three months.
The medical and animal research team are fascinated by the biology which underlies both the seasonal deciduous process and the genetics that determine the shape of the antlers.
The center of a growing antler is formed of cartilage, which becomes progressively harder with deposits of calcium. The growing antler is provided with an abundant supply of nutrients from a prolific network of blood vessels in the thick covering of skin, which is also rich in nerve fibers and the hair follicles that produce the velvet fine covering of hair.
When the antlers reach maturity, a rapid change occurs. A final period of calcification takes place. The blood supply is suddenly restricted and the layer of skin is shed. When exposed the hard underlying bone dies and then remains intact for six months.
Each stage of the cycle, from the shedding of the velvet and casting of the dead antler to regrowth, is associated with changes in the concentrations of the male hormone testosterone.
The pattern of the male hormone levels is synchronized to the time of the year and day length, involving the secretion of melatonin from the pineal gland. That in turn influences the release of the stimulant from the brain, LHRH, that triggers the secretion of the gonadotrophic hormone by what is sometimes regarded as the master control of the body's endocrine system, the pituitary gland.
Stags given an implant of long-acting doses of hormones retained their antlers for longer periods. Similarly, regrowth was interrupted if there was a deliberate change in the normal growth of skin over the pesicle from which the antler had been shed.