Ossification in the human calcaneus: a model for spatial bone development and ossification

Perichondral bone, the circumferential grooves of Ranvier and cartilage can als are features of endochondral bone development. Cartilage canals contain ing connective tissue and blood vessels are found in the epiphysis of long bones and in cartilaginous anlagen of small and irregular bones. The patter n of cartilage canals seems to be integral to bone development and ossifica tion. The canals may be concerned with the nourishment of large masses of c artilage, but neither their role in the formation of ossification centres n or their interaction with the circumferential grooves of Ranvier has been e stablished. The relationships between cartilage canals, perichondral bone a nd the ossification centre were studied in the calcaneus of 9 to 38-wk-old human fetuses, by use of epoxy resin embedding, three-dimensional computer reconstructions and immunhistochemistry on paraffin sections. We found that cartilage canals are regularly arranged in shells surrounding the ossifica tion centre. Whereas most of the shell canals might be involved in the nour ishment of the cartilage, the inner shell is directly connected with the pe richondral ossification groove of Ranvier and with large vessels from outsi de. In this way the inner shell canal imports extracellular matrix, cells a nd vessels into the cartilage. With the so-called communicating canals it i s also connected to the endochondral ossification centre to which it delive rs extracellular matrix, cells and vessels. The communicating canals can be considered as inverted 'internal' ossification grooves. They seem to be re sponsible for both build up intramembranous osteoid and for the direction o f growth and thereby for orientation of the ossication centre.