EARLY HISTOLOGIC AND ULTRASTRUCTURAL-CHANGES IN MICROVESSELS OF PERIOSTEAL CALLUS

Objective: To document early histological and ultrastructural changes in periosteal fracture callus blood vessels. Design: Rabbit control an d fractured ribs, after healing for three, six, and twelve hours and d aily for seven days, were evaluated by light and electron microscopy. Results: Control periosteal microvessels were formed mainly by endothe lial cells and occasionally by pericytes. Only these cells displayed b asal lamina within the periosteum. Three to twelve hours postfracture, periosteal microvessels were little changed. By two days postfracture , dramatic increases in size and population of microvesssel cells resu lted in a smaller lumen and thicker wall. Microvessel cells, while ret aining their basal lamina, had transformed to mesenchymal cells. Trans formed pericytes, as evidenced by their basal lamina, had ex travasate d. Three to four days postfracture, additional transformed pericytes h ad extravasated. Within the distal periosteal callus, a close spatial relationship among transformed microvessels, extravascular mesenchymal cells (some with basal lamina), and osteoblasts was present. Four to five days postfracture, within the proximal periosteal callus, a close spatial relationship among transformed microvessels (rapidly disappea ring because of continued extravasation), extravascular mesenchymal ce lls (some with basal lamina), and chondroblasts (some with basal lamin a) was present. Conclusions: New evidence showed that after fracture, peri osteal microvessel endothelial cells and pericytes increased in p opulation and transformed to mesenchymal cells. These changes, their s ubsequent extravasation as mesenchymal cells, and their development in to chondroblasts were verified by basal lamina evidence. New evidence also suggested that continued extravasation of transformed microvessel cells rendered the fracture callus cartilage avascular.