Bones replace the cartilaginous models from the end of the embryonic period onward, through processes of perichondral and endochondral ossification. In perichondral ossification, a perichondrium forms at the periphery of the cartilaginous model with differentiating osteoblasts. The osteoblasts form a periosteal collar all around the middle part of the cartilaginous model. This periosteal collar expands in proximal and distal directions. The perichondrium becomes the periostium.
Endochondral ossification also begins at the level of the osseous sheath. Chondrocytes hypertrophy and induce mineralization of the cartilaginous matrix. Starting from the periostium, blood vessels penetrate into the cartilaginous matrix through the osseous sheath. They are accompanied by mesenchymal cells, which are precursors for both osteoblasts and osteoclasts. Osteoclasts dissolve a part of the matrix, which leads to the formation of the primitive medullary cavity. A part of the matrix remains. It is used by  the osteoblasts as a scaffold for the formation of the primary spongy substance (primary ossification centre). Endochondral ossification of the diaphysis extends to both ends in parallel with the formation of the osseous sheath. The primitive medullary cavity becomes bounded proximally and distally by a transversely oriented ossification zone. The proximal and distal end is formed by the cartilaginous epiphysis.