Conversion of mesenchyme into bone (direct osteogenesis)

Fetal jawbone of a mammal

(hematoxylin and eosin)

You may find here some sites of osteogenesis at different stages in the mesenchyme and blood vessels.

At the stage of ossification center formation, there are mesenchymal cell agglomerations where some of them divide mitotically #1, #2.

At the stage of ossification center development where cells differentiate and form an organic preosseous matrix, the organic matrix islands (osteoid) are produced. The islands are surrounded by proliferating osteoblasts #1, #2 derived from the mesenchyme.

At the stage of osteoid calcification (the most common stage to be distinguished on slides), trabeculae of woven bone #1, #2 appear first. They are homogenous and stained intensely by eosin (collagen fibers are chaotic and not easily seen).

Cellular elements of a growing bone:

• osteoblasts #1, #2 that are cuboidal or prismatic cells with eccentric nuclei and basophilic cytoplasm. They form a single layer at the edge of growing bone trabeculae;
• osteocytes #1, #2 are found inside the calcified osteoid of bone trabeculae, namely in thin lacunae. They have a smaller size, flattened cell bodies, and narrow rims of cytoplasm;
• osteoclasts #1, #2 are large (up to 200 μm) multinucleated cells with a slightly basophilic cytoplasm. They are found as solitary cells or in small groups on trabecular surfaces (in erosion lacunae).