Electron microscopic studies of antlerogenic cells from five developmental stages during pedicle and early antler formation in red deer (Cervus elaphus).

Previous studies using light microscopy have revealed that histogenesis of deer pedicle and antler has four ossification stages. The first of these stages is the development of the permanent pedicle. Initial development of the pedicle is from the cellular layer cells of the antlerogenic periosteum and these cells have been termed initial antlerogenic cells (IACs). Apart from the IACs, it has also been shown that the cellular layer cells of the apical periosteum/perichondrium, the peripheral periosteum of pedicles or antlers, and the marginal periosteum surrounding the pedicles are also capable of either partially or fully generating a pedicle or an antler. Therefore, these cells can all be considered antlerogenic cells and called apical antlerogenic cells (AACs), peripheral antlerogenic cells (PACs), and marginal antlerogenic cells (MACs), respectively. The aim of this study was to examine the ultrastructure of these antlerogenic cells, and to determine whether there were ultrastructural correlates with the changes of these antlerogenic cells and ossification stages. The ultrastructure of each type of antlerogenic cells was systematically examined using transmission electron microscopy, at each stage of pedicle and first antler growth. At the first ossification stage, the IACs were spindle-shaped and inactive. The most obvious feature was the presence of abundant intracellular glycogen. The MACs were similar to the IACs. During the early second stage, most of the AACs changed in appearance from preosteoblasts to prechondroblasts. Much less heterochromatin was found in the AACs than in the IACs. The most striking attribute of the AACs was the existence of intracellular collagen fibers. The MACs showed abnormal dilation of the rough endoplasmic reticulum (RER). During the late second stage, the majority of the AACs were prechondroblasts. AAC nucleoli were clearly discernible and the cisternae of the RER were arranged in parallel. The MACs contained a greater proportion of abnormally-dilated RER. During the third stage, the AACs were all prechondroblasts. The Golgi apparatus in these cells was well developed. Many free ribosomes in rosettes were scattered in the cytoplasm. Most cytoplasm of the majority of the MACs was occupied by abnormally-dilated RER (the lumen of the RER was extremely dilated and appeared electron-lucent). During the fourth stage, the AACs were similar to their counterparts from the third stage, but the boundaries of some AACs were ill-defined. Some MACs were found to be undergoing apoptosis. The PACs were becoming less and less active from distal to proximal along the shaft of the antler. It is a novel finding that antlerogenic cells change in appearance and subcellular content from preosteoblasts to prechondroblasts prior to the transition from intramembranous to endochondral ossification during pedicle formation. Therefore, the differentiation process from antlerogenic cells to chondroblasts is a matter of maturation from prechondroblasts to chondroblasts. The fact that the antlerogenic cells are rich in glycogen makes them more like embryonic cells. The local membrane deficiency of some AACs at the fourth stage and the presence of mature collagen fibrils within the AACs may reflect the unusually high demand for collagen fibrils during the period of rapid antler growth.