- Endochondral ossification: all bones with the exception of those following the intramembranous ossification, are formed this way. Endochondral ossification begins during the late second gestational month, and proceeds until the second decade of postnatal life. In this osteogenic process, bones are modeled in hyaline cartilage that is after
- replaced by osseus tissue. In order to understand the first stages of this process remember that cartilage tissue is not vascularized and that nutrients reach chondroblast by diffusion. Endochondral ossification can be broken down into 5 stages (description refers to a long bone):
1_ The endochondral bone is first modeled by a hyaline cartilage structure. Around the 8th gestational week, the perichondrion is invaded by blood vessels that brings osteoblasts. The perichondrion becomes a periosteum as osteoblasts secrete osteoid that is quickly mineralized. This way osteoblasts deposit a collar of bone tissue along the diaphysis.
2_ The presence of a collar bone prevents the nutrients to diffuse into the diaphysis. Chondroblasts and chondrocytes will die, and the matrix, as a consequence, will calcify and deteriorate leaving a cavity in the middle of the diaphysis (cavitation of diaphysis).
- 3_ The newly formed cavity in the diaphysis is invaded by a periosteal bud (vessels, artery and vein) that brings nutrients, osteoblasts, osteoclasts. Osteoclasts remove the calcified cartilage, and osteoblasts start secreting osteoid that
- mineralizes. This way spongy bone is deposited in the middle of the diaphysis. This osseous tissue is called the primary ossification center and it is located in the diaphysis.
- 4_ Secondary ossification centers are formed in the epiphysis. Cartilage in the center of the epiphysis calcifies and deteriorates. Vessels invade this area and bring osteoblasts (that remove the calcified cartilage)
- and osteoblasts (that deposit osteoid). This way new bone tissue is formed. These are the secondary ossification centers.
5_ Ossification of the epiphysis continue until the entire structure is substituted by osseous tissue. Hyaline cartilage is still present at the level of the articular cartilage and of the epiphyseal plates. Epiphyseal plates are structures that will ensure the lengthening of the long bone until the second decade of life.
Short bones arise from a single ossification center. Irregular bones develop from several ossification centers. Short long bones have only 1 secondary ossification center.