Horn is keratin. Antler is bone. When in velvet, antlers have a good blood supply and are growing, so by that, antler is not "dead" bone. The blood veins are in the velvet. When the blood ceases to flow to the antlers, the velvet is shed. If you have ever seen a deer that is shedding the velvet, the antlers are red because there is still an small amount on blood in the velvet. After the velvet is gone, the antlers can't grow anymore. When antler growth begans again in mid winter, it causes the existing antlers to drop off or shed, allowing the new antlers to grow.