An electron microscopic investigation of osteogenesis in the embryonic chick

Abstract

Abstract Cell‐matrix and bone‐matrix relationships have been studied by electron microscopy in the diaphysis of chick embryo tibio‐tarsus. Osteoblasts within the periosteum possess cytoplasmic machinery characteristic of cells involved in protein synthesis. They contain abundant ergastoplasm, and an extensive Golgi apparatus. Small vesicles are associated with the juxtanuclear Golgi apparatus as well as with the extensive ergastoplasm. This suggests that the mechanism of protein synthesis and its transfer to the external milieu may be similar to that of other protein secreting cells, the fibroblast, the odontoblast and the chondroblast. A rather wide variation in collagen fibril diameter is seen in those fibrils located between the osteoblasts and the initial ossification area. The first nucleation of apatite crystals is seen as random aggregations distinct and separate from the collagen fibrils. This appearance is independent of section orientation since there appeared to be no difference between sections oriented at right angles to or parallel with the diaphysis' long axis. The observations of this study are consistent with proposed mechanisms for the nucleation phenomenon involving the ground substance and collagen fibrils. In addition, the observations suggest a concept of cellular control over the extracellular environment effecting the initial crystal nucleation. The morphologic evidence indicates that the molecular arrangement of collagen as fibrils probably does not provide the mechanism for apatite crystal nucleation.