Antler transformation is advanced by inversion of antlerogenic periosteum implants in sika deer (Cervus nippon)

Deer antlers offer a unique model for the study of tissue-specific stem cells and organogenesis, as antler stem cells are confined to the antlerogenic periosteum (AP), a tissue that can be readily located (overlying a frontal crest) and experimentally manipulated. AP consists of an upper fibrous layer and a lower cellular layer. Tissue transplantation and membrane insertion experiments demonstrated that antler formation is triggered by the interactions between AP and the overlying skin. Interestingly, fairly normal antlers can be induced to grow by an inverted AP implant (the AP cellular layer facing the skin) at an ectopic site, raising the question whether the initial inductive signal is derived from the fibrous layer or cellular layer or both. To answer this question, in this study we used eight sika deer stag calves and selected one side of future antler growth region for implanting inverted AP and the contralateral side for noninverted AP as the control. The results showed that implantation of the AP discs in an inverted orientation generated pedicles with final height (17 ± 5.1 mm), less than half the height of those formed from the noninverted AP implants (45 ± 11.7 mm). Critically, antler transformation was initiated from a shorter pedicle, which was formed from the region where the AP cellular layer was brought in close proximity to the overlying skin. Therefore, the AP cellular layer, as opposed to the AP fibrous layer, is likely to be the main source of the initial inductive molecules for antlerogenesis.