Ossification center

The first step in ossification is that the chondrocytes at this point become hypertrophic and arrange themselves in rows.

A deposit of calcareous material now takes place in this matrix, between the rows of cells, so that they become separated from each other by longitudinal columns of calcified matrix, presenting a granular and opaque appearance.

A primary ossification center is the first area of a bone to start ossifying.

[2] At the end of the formation of the secondary ossification center, the only two areas where the cartilage remains is at the articular cartilage covering the epiphysis and at the epiphyseal plate between the epiphysis and diaphysis.

[3] This article incorporates text in the public domain from page 93 of the 20th edition of Gray's Anatomy (1918)

Section of fetal bone of cat. ir. Irruption of the subperiosteal tissue. p. Fibrous layer of the periosteum. o. Layer of osteoblasts. im. Subperiosteal bony deposit.
A schematic for long bone endochondral ossification: [ 4 ]
  1. Primary endochondral ossification begins with the formation of a chondrocyte template. Afterwards, chondrocytes undergo hypertrophy beginning from the mid- diaphysis , eventually extending to the epiphyseal poles, vasculature invades the forming bone transporting mesenchymal stromal cells and hypertrophic cells undergo apoptosis. Mesenchymal stromal cells differentiate into osteoblasts and then osteocytes.
  2. Secondary ossification occurs at the epiphysis post-natally and bone formation initiates at the center and extends peripherally.