Osteoblasts synthesize and secrete the osteoid, and participate in the calcium wid phosphate regulation in the osteogenesis. Osteoblasts are derived from osteoprogenitor cells, which are mesenchymal stem cells committed to differentiate into the bone forming cells. There are, however, different opinions concerning development of osteoblasts from osteoprogenitor cells and bone forming processes involved by osteoblasts.
Author has undertaken an experimental work to elucidate the mechanism by which the bone has been formed from subperiosteal osteoblasts.
The experiment consisted by 32 Korean rabbits. Animals in each group received bilateral ostectomies of the femoral shaft of 5mm in length, and controlateral bone shaft was transplanted. A chronologic observation was made in relation to the activity of alkaline phosphatase and electron microscopic feature of the subperiosteal osteoblasts. From gross, histologic, enzyme histochemical and electron microscopic observations, following results were obtained.
I. Osteoblasts derived from the peristeum adjacent to the ostectomy site began to proliferate on the day 4. Features of osteoprogenitor cells were indistinguishable from those of fibroblasts.
2. Alkaline phosphatase activity on the proliferating osteoblasts was markedly in-creased throughout the experimental period. Both osteoprogenitor cell and fibroblasts showed no activity on alkaline phosphatase.
3. The osteoblasts became cuboidal in shape and had cytoplasmic processes. The cytoplasm were filled with extensive Golgi complex, rough endoplasmic reticulum, and secretory vacuoles. Poor cytoplasmic organelles were present in both fibroblasts and osteoprogenitor cells.
4. The osteoid began to lay down on the day 7 around osteoblasts. Osteoblasts embedded in osteoid, woven bone, and/or lamellar bone transformed into osteocytes. Alkaline phosphatase activity was weakly positive in osteocytes. Woven bone and lamellar bone became apparent on the day 21 and 30, respectively.
In summary from the results obtained, osteoblasts play a most significant role in the osteogenesis during fracture repair. Alkaline phosphatase activity is remarkably prominent in the osteoblasts, the finding of which is well correlated to the amount
i of lysosomes observed in the electron microscopic study. Both histochemical and electron microscopic features of osteoprogenitor cells and fibroblasts are not distinctive between the two cell types.
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