Endochondral ossification occurs during the embyonic development using a template or model of preformed cartilagineous bones. At the end of the ossification, most the cartilage tissue is replaced by bone. It is a complex process that goes through several stages with simultaneous events in different parts of the cartilaginous template, which will be summarized below.
1 - The cartilage cells that occupy the middle of the template enlarge, become hypertrophic, similarly to wnat happens in the Zone of hypertrophic cartilage cells of the Growth plate or Epiphyseal plate.
2 - Almost simultaneously, the chondroblasts that occupy the collar of perichondrium that surrounds the middle of the shaft of the cartilagenous template are substituted by osteoblasts.
3 - The hypertrophic cartilage cells present in the middle of the cartilage model undergo a process of calcification and die in consequence of this.
4 - As the perichondrium is substituted by periosteum around the middle of the shaft, it is in this region that ossification begins. It is initiated inside and/or beneath the periostal layer, forming a bony collar by way of intramembranous ossification.
5- Blood vessels accompanied by connective tissue containing osteoblasts penetrate the area of dead cartilage cells at the center of the bone. Osteoblasts placed over the remains of the walls of cartilagenous matrix begin to produce bone extracelllar matrix, are surrounded by this matrix and become osteocytes. New layers of osteoblasts arrive and repeat this process and bone tissue is progressively produced.
6- the initial bone is produced as spicules, very thin and short pieces of bone, that progressively grow forming bone trabeculae. Follows an intense process of bone remodelation (thanks to the activity of osteoclasts) ending finally in the development of compact and spongy bone.
7-37 Bone
ENDOCHONDRAL OSSIFICATION – 1