l produce red bone marrow. Additional osteoblasts that are formed from osteochondral progenitor cells will gather into the inner surface of the trabeculae thus forming the endosteum. The endosteum contains osteoblasts, osteoclasts, and osteochondral progenitor cells. The trabeculae at the surface continue to calcify until the empty spaces between them are filled in. This process converts the spongy bone into compact bone. No we have formed the general structure of flat bone, which contains spongy bone sandwiched by compact bone.
             Endochondral ossification starts from a hyaline cartilage model. It produces the bones of the base of the skull, part of the mandible, the medial epiphysis of the clavicles, and the remaining bones of the skeletal system. It begins around the sixth week of development and continues into a person's twenties. Endochonral ossification starts with mesenchymal cells becoming chondroblasts. These chondroblasts produce the hyaline cartilage model. This model is surrounded by perichondrium except at sites of articulation. Blood vessels will invade the outer surface of
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