Relationships between endothelial cells, pericytes, and osteoblasts during bone formation in the sheep femur following implantation of tricalciumphosphate‐ceramic

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Abstract
Background: The origin of osteoblasts is still controversial. Whereas several authors consider the stromal fibrablast of the bone marrow as the osteoprogenitor cell, others propose that the osteoblasts can be derived from the “capillary system.” The present study examines the replacement of tricalciumphosphate (TCP)-ceramic implanted into an artificial bone defect by newly formed bone. The results support the hypothesis that osteogenic cells can be derived from invading blood vessels. Methods: The spongiosa of the trochanter major of sheep was removed and the defect was filled with TCP-ceramic. Two months after surgery the ceramic implants together with the surrounding bone were removed and processed for transmission electron microscopy. Serial ultrathin sections of three newly formed osteons were examined. Results: The osteons contain one or two small sprouting capillaries, a peripheral layer of osteoblasts, and in between, a network of glycogen-rich cells. Some of the glycogen-rich cells are completely or partly surrounded by the endothelial basal lamina and are thereby characterized as pericytes. Weibel-Palade bodies, which are considered to be a marker of endothelial cells, were occasionally observed in glycogen-rich pericytes. Conclusions: Since pericytes differentiate into osteoblasts under in vivo and in vitro conditions, and have thus been regarded as osteoprogenitor cells, the presence of Weibel-Palade bodies in these cells suggest that osteogenic cells can be derived from endothelial cells.