From BBC News:
The deer is unique among mammals in being able to regenerate a complete body part – in this case a set of bone antlers covered in velvety skin. Antlers are large structures made from bone that annually grow, die, are shed and then regenerate. They grow in three to four months, making them one of the fastest growing living tissues. After the antlers have reached their maximum size, the bone hardens and the velvety outer covering of skin peels off. Once the velvet is gone, only the bare bone remains – a formidable weapon for fighting.
At the end of the mating season, the deer sheds its antlers to conserve energy. Next spring, a new pair grows out of a bony protuberance of tissue at the front of the animal’s head. The research suggests that stem cells – the master cells of the body, with the ability to develop into many specialised cell types – underpin this process.