When testosterone levels are extremely low, the new antler re-generates out of scar tissue from the previous antler casting, unlike the first antlers which generate from normal skin tissue. Mesodermal cells of the scar tissue over the pedicle differentiate to produce the growing antler tissue. The antler tissue is laid down, and the points of growth duly advance so as to always be present at the tips of the antler. Skin covers the growing antler, from which hairs protrude, giving a velvet-like appearance. Antler during this stage is thus referred to as 'velvet'. Between the skin and the growing cells of the antler is a later of connective tissue. This layer has a blood supply.
It is well known that there is a general relationship between antler size and body size in that larger deer have larger antlers. The entire growth phase is 120 days commencing from new antler re-generation. Some growth occurs during the mineralisation phase as, prior to the antler becoming fully calcified, some nutrients continue to be supplied. Growth rates of up to 3cm per day have been recorded at the peak of the rapid growth phase. While the rate of antler growth is affected by nutrition, the actual causative mechanism is not known. However, it seems likely that the influence of nutrition is mediated by hormones.
Whereas skeletal bones of red deer grow at 34g/day during the first 18 months, their antlers may grow at 100g/day. Minerals obtained from the diet such as calcium and phosphorus are likely to be the key source of antler material, and both feral and farmed deer have been observed chewing shed antler and bones during antler development.
Calcification occurs during and after the rapid growth phase. However, it does not usually commence until 55-60 days after growth has commenced. Since the deer antler velvet will not grow any larger without compromise of its full nutrient supply, velvetting for oriental medicinal purposes which seek optimum nutrient levels typically takes place 55-65 days after casting. Calcification is complete by February to March in time for the rut.
The process of calcification involves osteoblasts being converted into osteocytes. Bony 'trabeculae' are formed by the deposition of calcium salts onto a matrix laid down by the osteocytes. These trabeculae become surrounded by more osteocytes and increase in size. The antler structure has a dense trabecular bony centre containing small amounts of non-haemopoietic marrow surrounded by very strong peripheral compact bone.
Upon completion of calcification, the velvet begins to die and assumes a shrivelled appearance. It is triggered by rising testosterone levels.
The mature antler is fixed to the pedicle strongly, to allow carriage of the antlers by the pedicle and to withstand the impacts of fighting. The junction of the antler to the pedicle consists of compact bone without a spongy core. There are few blood vessels in this area.
Just before casting, the strong junction between the pedicle and calcified antler is eroded on account of increasing osteoclastic activity and vascularisation. Physical assistance by the stag rubbing against objects, combined with erosion of the junction brings about casting. Blood loss is observed and scar tissue forms at the site.