What are various types of bones based on origin and development?
The process of bone formation is called osteogenesis or bone ossification. Bone formation begins at the sixth or seventh week of the embryonic development and continues after birth until about twenty five years of age.
The origin of bones is the mesenchymal tissues. Intramembranous and endochondral are the two methods of bone formation, both having the mesenchymal origin.
The mesenchymal tissues in the intramembranous method are directly converted into bones. In this method, the mesenchymal tissues differentiate to form the osteoblasts, which causes the bone-formation. The cluster of osteoblasts form the ossification center. The osteoblasts are responsible for secreting osteoid that binds to calcium to harden the bone matrix and entraps more osteoblasts. This causes the transformation of osteoblasts into osteocytes and eventually forms the structure of the bone. For example, the flat bone of the clavicle and skull.
In the endochondral mechanism, the hyaline cartilage is transformed into the bone. The mesenchymal cells differentiate to form chondrocytes that undergo proliferation and secretion of extracellular matrix. The cartilage structure is formed by this extracellular matrix. The process of calcification begins when the chondrocytes undergo hypertrophy and adds more fibronectin and collagen X to the bone matrix. The external matrix calcification causes the death of the internal chondrocytes, forming a cavity that allows blood vessels to enter into the space. It also carries osteoblasts that causes the bone to thicken and form a compact structure. Thus the cartilaginous structure is replaced by the bone formation. For example, long bones and axial skeleton.
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