Replacement of cartilage with bone (indirect osteogenesis)

Stage of epichondral ossification in epiphyses

(hematoxylin and eosin)

This fragment of a tubular bone rudiment in a mammal shows two epiphyses #1, #2 and a part of diaphysis of a bone. The bone is surrounded by skeletal muscles that are developing now.

At this ossification stage, vascular ingrowth and differentiation of osteogenic cells result in the formation of endochondral ossification centers #1, #2 in central epiphyseal regions (the process resembles the earlier one in diaphyses). Numerous hematopoietic centers are found between the bone trabeculae #1, #2, which contain some remnants of calcified cartilage. .

Hyaline cartilage of epiphyses is only retained in the areas of articulating surfaces and between ossification regions of epiphyses and diaphysis. It is called physeal growth plate (it is responsible for longitudinal growth of tubular bones). The physis includes four zones:

• resting or germinal zone,
• zone of chondrocyte columns (proliferative zone of cartilaginous cells),
• hypertrophic zone (includes rounded degenerating chondrocytes),
• calcification zone is a site for cartilage destruction and replacement with endochondral bone.

The diaphysis shows zones of perichondral and endochondral ossification. Its central part contains a cavity to form the red bone marrow #1, #2.