Improbable appendages: Deer antler renewal as a unique case of mammalian regeneration.

Deer antlers are periodically replaced cranial appendages that develop from permanent outgrowths of the frontal bones known as pedicles. Antler re-growth is a unique regenerative event in mammals which in general are unable to replace bony appendages. Recent evidence suggests that antler regeneration is a stem cell-based process that depends on the activation of stem cells located in the pedicle periosteum which are presumed to be neural crest-derived. It has been demonstrated that several developmental pathways are involved in antler regeneration that are also known to play a role in the control of skeletal development and regeneration in other vertebrates. However, in contrast to most other natural examples of regeneration of complete body structures, antler regeneration apparently neither depends on a functional nerve supply nor involves a direct contact between wound epithelium and mesenchymal tissue. Antlers thus demonstrate that regeneration of a large bony appendage in a mammal can be achieved by a process that differs in certain aspects from epimorphic regeneration in lower vertebrates.