Answer:
Endochondrial ossification is one of the two major pathways through which the skeletal structure is formed. While the other one, intramembranous ossification, is used to form the flat bones mostly of the skull, face, and others, endochondrial ossification is pretty unique because it depends on the presence of hyaline cartilage as a blueprint during fetal development, to then initiate the process of bone formation.
Endochondrial ossification is the process followed by long bones, like the femur. This ossification process initiates about 6 to 8 weeks into fetal development and it is not finished until adolescence. As such, the correct steps of the process of endochondrial ossification would be:
3. Chondrocytes (cells responsible for the formation of the hyaline cartilage) enlarge and calcify.
1. Blood vessels invade the perichondrium. This perichondrium appears over the hyaline cartilage and covers it before calcification and with the arrival of blood vessels, come osteogenic cells, some of which will mature later on into osteoblasts.
5. The perichondrium is converted into a periosteum and the inner layer produces bone. This happens as the matrix attracts mineral salts and traps them into the matrix. The periosteum is the hard covering of bones and calcification and ossification will continue from the middle of the long bone towards the distal parts.
4. Osteoblasts replace calcified cartilage with spongy bone. This type of bone which emerges as the newly specialized osteogenic cells (osteoblasts) replace the cartilage for it, is specific to the epyphysis of long bones, or the heads, of these bones.
2. Osteoclasts create a marrow cavity. Osteoclasts are the other specialized type of osteogenic cells and their purpose is to digest bone to remodel it. During the skeletal formation of long bones, blood vessels will require spaces where they can interact with the bone tissues, especially the spongy parts, and these spaces are known as the marrow cavities.
