Describe the structure of cortical bone and an osteon. Detail the steps of endochondral ossification.
The structure of cortical bone, also known as compact bone, is characterized by its dense and solid appearance. It is found on the external surface of bones and forms the majority of the bone tissue. Cortical bone is made up of repeating units called osteons or Haversian systems.
An osteon is a cylindrical structure that consists of concentric rings of bone called lamellae. The lamellae are arranged around a central canal known as the Haversian canal. The Haversian canal contains blood vessels, nerves, and connective tissue. Radiating from the central canal, there are small channels called canaliculi that connect neighboring osteocytes (mature bone cells). These canaliculi allow for the exchange of nutrients and waste products between the osteocytes and blood vessels.
The endochondral ossification is the process by which most of the bones in the body are formed. It involves the transformation of a cartilage model into bone tissue. The steps of endochondral ossification are as follows:
1. Mesenchymal cells differentiate into chondrocytes, which form a cartilaginous model of the future bone.
2. As the cartilage model grows, chondrocytes in the center hypertrophy and produce a calcified matrix.
3. Blood vessels invade the calcified cartilage, bringing in osteoblasts (cells responsible for bone formation) and osteoclasts (cells responsible for bone resorption).
4. Osteoblasts produce new bone matrix over the calcified cartilage, forming a primary ossification center.
5. Osteoclasts remove some of the newly formed bone tissue to create a medullary cavity within the diaphysis (shaft) of the bone.
6. As the bone continues to grow, secondary ossification centers form in the epiphyses (ends) of the bone.
7. The epiphyseal plate, a cartilaginous region between the diaphysis and epiphysis, allows for longitudinal bone growth.
8. In adolescence, the epiphyseal plate undergoes closure, resulting in the fusion of the diaphysis and epiphysis, and the bone reaches its final length.
Overall, through endochondral ossification, the cartilage model is gradually replaced by bone tissue, resulting in the formation and growth of long bones in the body.