After spongy bone is formed in the diaphysis, osteoclasts break down the newly formed bone to open up the medullary cavity. Ossification continues from this center toward the ends of the bones. This forms a primary ossification center. Osteoblasts penetrate the disintegrating cartilage and replace it with spongy bone. At the same time, the cartilage in the center of the diaphysis begins to disintegrate. The osteoblasts form a collar of compact bone around the diaphysis. During the third month after conception, the perichondrium that surrounds the hyaline cartilage "models" becomes infiltrated with blood vessels and osteoblasts and changes into a periosteum. In this process, the future bones are first formed as hyaline cartilage models. These bones are called endochondral bones. Most of the bones of the skeleton are formed in this manner. Endochondral OssificationĮndochondral ossification involves the replacement of hyaline cartilage with bony tissue. When the osteoblasts are surrounded by matrix they are called osteocytes. Osteoblasts migrate to the membranes and deposit bony matrix around themselves. The future bones are first formed as connective tissue membranes. They include certain flat bones of the skull and some of the irregular bones. Bones formed in this manner are called intramembranous bones. Intramembranous ossification involves the replacement of sheet-like connective tissue membranes with bony tissue. There are two types of ossification: intramembranous and endochondral. Osteoblasts are bone-forming cells, osteocytes are mature bone cells and osteoclasts break down and reabsorb bone. Osteoblasts, osteocytes and osteoclasts are the three cell types involved in the development, growth and remodeling of bones. Even after adult stature is attained, bone development continues for repair of fractures and for remodeling to meet changing lifestyles. By the end of the eighth week after conception, the skeletal pattern is formed in cartilage and connective tissue membranes and ossification begins.īone development continues throughout adulthood. Parts of the skeleton form during the first few weeks after conception. The terms osteogenesis and ossification are often used synonymously to indicate the process of bone formation. Acknowledgements Bone Development & Growth
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