Replacement of cartilage with bone (indirect osteogenesis)

Stage of perichondral and endochondral ossification in diaphysis

(hematoxylin and eosin)

This fragment of a tubular bone rudiment in a mammal depicts an epiphysis and a part of diaphysis.

At this ossification stage, the epiphysis is formed by hyaline cartilage under the perichondrium; there are no ossification centers. The diaphysis is limited by the periosteum from the outside. There is a perichondral bone collar beneath the periosteum (the collar builds up in thickness towards the epiphyses).

The formation of perichondral bone starts with differentiation of osteoblasts, which are derived from osteogenic cells of the inner perichondral layer (the perichondrium transforms later into periosteum). The cells produce osseous intercellular substance via direct osteogenesis. The peripheral parts of oxyphilic woven bone trabeculae contain osteoblasts and osteoclasts, whereas the bone matrix contains numerous osteocytes in lacunae.

Endochondral ossification centers are being formed in the central diaphyseal region as a result of osteoblast differentiation from osteogenic cells around blood vessels growing deep from the periosteum. Hyaline cartilage cells become dystrophic (matrix vesicles-carrying chondrocytes) and then die while the matrix becomes calcified. Thus, it is well stained by basic dyes.

Unlike perichondral ossification, oxyphilic endochondral bone matrix retains some basophilic stripes of calcified cartilage #1, #2. The cartilage disintegrates, and the new cavities are filled then by hematopoietic centers.