The last issue we learned about the chemical composition and physical properties of bone.

On this issue, we would like to talk about membranous bone and chondrogenic bone.

Bone occurs in the mesodermal mesenchyma. From the 8th week of the embryo, the mesenchyma is membranous and being ossified gradually, which is called membranous bone. Or the mesenchyma first develop into cartilage, and continue ossification, which is called chondrogenic bone.

1. Some of the cells in the mesenchymal membrane differentiate into osteoblasts and produce osteofibrils and matrix, in which calcium gradually deposit to form bone. The site where the bone is initialized is called the ossification point, from which it radiates outward to form spongy bone. The mesenchymal membrane surrounding the new bone is called periosteum. The osteoblasts under the periosteum constantly form new bone and thicken the bone. New bone is constantly formed at the edge of the ossification point, making the bone widen. At the same time, the osteoclasts destroy and absorb the bones as planned, and the osteoblasts then transform and rebuild, finally shaping the shape of the adult bone.

2. Chondrocytes are first formed in the mesenchyma, and perichondrium is formed in the mesenchyma. Some cells under the mesenchyma differentiate into osteoblasts, and bone collar is formed around the middle of the cartilage body. The perichondrium in the bone collar is called periosteum. When bone collar is formed, blood vessels flow through the center of cartilage body, and mesenchyma follows to form red marrow. The incoming mesenchymal cells differentiate into osteoblasts and osteoclasts and initiate osteogenesis.

All in all, the initiation and growth of bones are crucial to achieve movement for bones acting as a framework to support our body under the influence of gravity.