Intramembranous osteogenesis and angiogenesis in the chick embryo.

Membrane bone formation was examined in the developing chick frontal bone of Stage 32-36 embryos using alizarin red whole mounts, wax and resin histology, transmission electron microscopy and India ink injection techniques. Mineralised bone was seen at Stage 36, but was presaged by the appearance of twin mesenchymal condensations at Stage 35. The condensations appeared dorsolateral to the upper edge of the interorbital septum near its centre. The mesenchymal cells of the condensation were fusiform in shape and initially exhibited a close-packed, layered configuration which was avascular. Consequent differentiation of the central cells first into preosteoblasts and then into definitive osteoblasts, accompanied by secretion of bone matrix, coincided with blood vessel invasion of the frontal anlagen. It is suggested that these vascular changes associated with the onset of bone cell differentiation indicate that osteogenic cells may secrete an angiogenesis factor.