ULTRASTRUCTURE OF MATRIX VESICLES AND MINERAL IN UNFIXED EMBRYONIC BONE

Femurs of seven-day-old chick embryos were prepared by frozen thin-sectioning and conventional embedding methods and examined by electron microscopy. A collar of bone mineral on the cartilage anlage and a layer of osteoid containing mineral deposits were observed in the frozen thin-sections but not in conventionally prepared tissue. Numerous vesicles and nodules were present in the osteoid and they contained mineral in both the amorphous and crystalline forms. Vesicles and nodules in many stages of mineralization were present and it was possible to arrange them in a sequence of maturation from early, slightly mineralized matrix vesicles to heavily mineralized nodules. This may reflect the sequence of events which occurs in vivo. Crystals within vesicles were frequently found in parallel arrangement. There was no evidence of crystal formation along collagen fibers at this early stage or mineralization. Qualitative electron microprobe analysis of nodules revealed the presence of sodium, magnesium, phosphorus, chlorine, potassium and calcium while adjacent regions of matrix contained the same elements but were deficient in phosphorus. © 1978.