A three-dimensional ultrastructural study of osteoid-osteocytes in the tibia of chick embryos

Morphology and ultrastructure of osteoid-osteocytes were studied in serial thin sections (700-800 A thick) of periosteal woven bone in tibiae of 15-day-old chick embryos. The three-dimensional shapes of 21 partially, and of one fully sectioned cell were reconstructed manually and by means of a computer-assisted image analyser. Osteoid-osteocytes are active cells engaged in organic matrix secretion and calcification. Like osteoblasts, their activity seems to be polarized towards the mineralization front, as shown by the presence of cytoplasmic processes on their mineral-facing side and by the position of the nucleus toward the vascular side of the cytoplasm. Cellular processes directed towards blood vessels appear only at a later stage, i.e. when the mineralization starts to spread all round the cell. The asynchrony in formation, together with the observed differences in morphology suggest the hypothesis that the cellular processes of the mineral-facing side are mainly involved in bone formation and those of the vascular side in cell nutrition.