Antlers do not grow directly from deer skulls. Instead, they emanate from the top of permanent bumps on the head called pedicles, which begin development in the foetus. The pedicle develops from the periosteum of the frontal bone and has the same structure as that bone. The pedicle increases its diameter throughout the stag's life, by annual deposition of a ring of bone. If the pedicle is removed, antlers can no longer develop. Pedicle development is chemically controlled by steroids.
The pedicles start to grow out of the head when the deer approaches puberty (usually around 5-7 months of age) although growth can be retarded by sub-optimal health and nutrition. Precise timing of initiation is highly correlated with body weight which itself is related to nutrition. When pedicles reach a specific height for the species in question (5-6cm in red deer), a shiny skin develops on the top. This marks the transformation of pedicle to first antler. The tissue at this stage is no longer scalp-like but is in velvet-like form.
After transformation from pedicle to antler, the antler enters a rapid growth phase. The first antler usually forms a long, single tapering beam, called a spike, hence a yearling stag is typically referred to as a 'spiker'. However often multi-spikes are observed. The typical pattern of growth of the pedicle and first antler is shown in the figure below.
The spike becomes fully calcified bone as the rutting season and full puberty approaches. Calcification, or 'mineralisation' completely shuts down the supply of nutrients to the antlers.
On account of the lack of mineral supply to the calcified antler, velvet is shedded from the antler, to expose bare, calcified antler bones.
The first hard antlers are cast in the following spring. The clean, hard antler is essentially a dead bone attached to living bone (the pedicle) and so its casting from the stag is not surprising. The main thing that affects the timing of antler casting is changes in day length but nutrition has some effect. After casting of first antlers, development of subsequent antlers enters a well-defined cycle.