Electron microscopic observations of bone tissue prepared by ultracryomicrotomy

Freshly dissected bone from embryonic chicks and young postnatal mice has been prepared for electron microscopy by ultracryomicrotomy. Nuclei, ribosomes, endoplasmic reticulum, and a large number of dense mitochondrial granules within osteogenic cells are observed in unfixed, undecalcified, and unstained sections, obtained with dry knives in the temperature range of -60 to -80°C. The presence of granules in mitochondria from tissues not exposed to solvents strongly suggests that the granules contain a solid phase of calcium phosphate in vivo. Small clusters of lathe-shaped and dense needle-like particles are observed in the extracellular tissue space closest to the osteoblasts. Tilting of unstained frozen sections indicates that the needle-like particles are platelets viewed obliquely or on edge. The rapid and massive changes in the amount of mineral deposited arise principally by an increase in the number of mineral particles, rather than their growth in size. Sections prepared by alternative methods combined with ultracryomicrotomy present similar features.