ENDOCHONDRAL OSSIFICATION – 1
1 - Ossification initiates in the area of the perichondrium layer that coats the middle of the cartilage model shaft, an area that will be later on called "bone collar". Mesenchymal cells of th perichondrium, that originally produced the cartilage bone model, begin to give rise to bone-forming cells. Hence, the perichondrium gradually converts into a periosteum.
2 - Bone-forming cells (osteoblasts) settled im the former perichondrium, now periosteum, secrete and become surrounded by the bone non-calcified extracellular matrix also called osteoid. After the matrix calcifies these cells are recognized as osteocytes. The manner of produced this bone is similar to that of the intramembranous ossification. Small bone trabeculae are initially formed in the bone collar. After progressively coalescing and being entirely remodelled they form the compact bone of the diaphysis.
3 - The chondrocytes that populate the inside of the cartilage template under the newly produced bone collar, begin to enlarge and become hypertrophic. This situation is similar to wnat happens in the Zone of hypertrophic cartilage of the epiphyseal disc (the disc does not exist yet at this early stage of bone formation).
4 - The extracellular matrix surrounding each hypertrophic chondrocyte becomes calcified, ensuing their death. This results in the colapse of the inner cartilagineous structure of the bone model.
5 - Meanwhile, branches of periostal blood vessels together with mesenchymal cells including bone-forming cells and hemopoietic stem cells begin to enter the cartilage model and occupy its cavities.
6 - Inside the model, osteoblasts establish on the surfaces of the pieces of remaining calcified cartilage matrix.
7 - The osteoblasts begin producing bone extracelllar matrix, are surrounded by this matrix and become osteocytes, originating a small tridimensional region called primary ossification center. More osteoblasts arrive, repeat this process and bone tissue is progressively added to that previously formed.
8 - Bone trabecules are thus produced inside the bone cavity, as well as bone marrow is formed from mesenchymal cells that penetrated the bone cavities.
9 - This process extends to the epyphises where bone is produced in a similar manner.
10 - Following the initial formation of a bone begins a long and complex process of bone remodelation that depends on the activity of osteoclasts and osteoblasts. The remodelation of bones converts them into the structure presented on the beginning of this chapter, consisting of mature lamellar bone either trabecular In the epiphyses) or as form of flat parallel lamellae combine with Haversian systems in the diaphyses. Remodeling continues during the whole life reacting to the many mechanical forces that act on the bones and as a response to physiological or pathological conditions of the body.
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