Programmed cell death in the regenerating deer antler

Antlers are the only mammalian appendages capable of epimorphic regeneration and thus provide a unique model for investigating the mechanisms that underlie mammalian regeneration. Antlers elongate by a modified endochondral ossification process while intramembranous ossification takes place concurrently around the antler shaft. In this study, sites of apoptosis in the growing antler tip were identified by TUNEL staining and related to cell proliferation, as determined by PCNA staining. Bcl-2 and bax were identified by RT-PCR and bax was also immunolocalized in tissue sections. The apoptotic index was high in perichondrium, undifferentiated mesenchymal cells and cellular periosteum but was low in skin. The proliferation index was high in mesenchyme, skin (specifically in hair follicles) and cellular periosteum; it was low in fibrous perichondrium and periosteum, and barely detectable in cartilage. Both bcl-2 and bax were found to be more highly expressed in the perichondrium/mesenchyme and non-mineralized cartilage than in skin and mineralized cartilage. Bax was immunolocalized in mesenchyme cells, chondroprogenitors, chondrocytes, osteoblasts, osteocytes and osteoclasts. In conclusion, this study shows that programmed cell death plays a necessary role in regenerating antlers, as it does during skeletal development, bone growth and bone remodelling. The high level of apoptosis and proliferation in mesenchymal progenitor cells confirms that this represents the antler 'growth zone'. In fact, the percentage of TUNEL-positive cells in the mesenchymal growth zone (up to 64%) is higher than that recorded in any other adult tissue. This extensive cell death probably reflects the phenomenal rate of morphogenesis and tissue remodelling that takes place in a growing antler. The local and/or systemic factors that control the balance between cell growth and apoptosis in antler tissues now need to be determined.