The mineralized osteocyte: a living fossil

We report here on an enigmatic and biologically mysterious event in which a single cell, the osteocyte, mineralizes in vivo and in this process the cell's organelles, cytoskeleton and membrane, are mineralized in a dying state. That the bony lacuna in which the lone osteocyte resides becomes infilled with mineral in vivo is not a new observation and was noted by early microscopists. This study has applied scanning and transmission electron microscopy to modern, archaeological, and fossil bone to investigate the mineral and organic structure and content of this cell. The results from this study revealed that within this mineral lies a visibly identifiable cell, which has an apoptotic-like morphology. The mechanisms by which this cell mineralizes are so intimate chemically that remnant cell organelles, membranes, cytoskeleton, and potentially nucleic bodies are morphologically identifiable. We have further identified mineralized osteocytes surviving in archaeological and fossil mammal bone up to 5 million years BP. The significance of our findings demonstrates that a single cell may itself mineralize in vivo via an unknown set of biochemical events. Importantly, the location and survival of extra cellular and cellular proteins, including nuclear and mitochondrial DNA in bone after death, has been an area of some speculation, and this unique fossil cell provides a preservation locus within human and mammalian bone, which might be fruitfully targeted in future biomolecular studies.