The fine structure of the proximal growth plate and metaphysis of the avian tibia: endochondral osteogenesis.

The ultrastructure of endochondral osteogenesis from the proximal end of tibias of 7 weeks old normal White Leghorn and broiler chickens is described. Little, if any, ultrastructural differences exists between these two strains of birds. In the growth plate, proliferation of chondrocytes and their matriceal production is approximately matched by resorption. Erosion of the metaphyseal aspect of the growth plate is accomplished under two sets of circumstances: firstly, when mineralization of the cartilaginous matrix is so scanty as to be discernible only by electron microscopy, and secondly, when calcification is gross and easily appreciated by light microscopy. The former process appears to be accomplished in the main by perivascular mononuclear cells and the latter, to a great extent, by chondroclasts. It appears that metaphyseal blood vessels expand, by saccular protrusions, into pre-existing spaces or those created by cellular erosion. Cones or plates of calcified cartilage, which extend into the metaphysis, act as a template for initial deposition of bone. Osteoblasts recently surrounded by osteoid have adjacent to their plasmalemma a pericellular sheath which is composed of ill-defined clumps of amorphous material as well as some incompletely aggregated collagenous fibrils. Beyond this sheath collagenous fibrils and fibres, with distinct periodicities (ranging from 57 to 69 nm) are observed. Towards the diaphysis the range of the periodicity of fibres in osteoid narrows to 64-69 nm.