Ultrastructural aspects of cartilage formation, mineralization, and degeneration during primary antler growth in fallow deer (Dama dama)

Due to their rapid growth, regular replacement and easy accessibility, deer antlers are considered a useful model for the study of cartilage and bone differentiation and mineralization in mammals. The present study describes, for the first time, the cellular and extracellular matrix changes associat ed with cartilage formation, mineralization and degeneration in primary ant lers on the ultrastructural level. Growing primary antlers of 3 to 4 cm len gth were obtained from six fallow bucks, aged about 10 months. It was shown that the chondroblasts were derived from progenitor cells of the antler pe richondrium and differentiated into mature chondrocytes that subsequently u nderwent hypertrophic changes. Concomitant with cell hypertrophy, formation of a lacunar and a perilacunar extracellular matrix was observed, the latt er containing numerous collagenous fibers. Mineralization of the extracellu lar matrix occurred via matrix vesicles and the formation of apatite crysta ls at distinct sites of the collagenous fibers. The hypertrophic chondrocyt es of the mineralized cartilage then degenerated, a process that was also o ccasionally observed in more distally located cells surrounded by still unm ineralized matrix. No morphological indications of a transdifferentiation o f hypertrophic chondrocytes into bone forming cells, i.e., co-occurrence of a degenerating chondrocyte and a viable osteogenic cell in intact lacunae, were found. The cellular and extracellular matrix changes seen in primary antlers resemble those described for secondary antlers. Our results further indicate that the hypertrophic chondrocytes of primary antlers eventually undergo apoptosis, thereby providing further evidence that metaplastic conv ersion of cartilage into bone does not play a role in antler growth.