Ultrastructural, enzyme-, lectin, and immunohistochemical studies of the erosion zone in rat tibiae.

To clarify the process of endochondral ossification, we used ultrastructural, enzyme-, lectin-, and immunohistochemical techniques to study perivascular cells located in the erosion zones of rat tibiae. In growth plate erosion zones, perivascular cells directly connected to blood capillaries were seen invading cartilage. These cells contained a well-developed rough endoplasmic reticulum and Golgi apparatus in their cytoplasm and formed finger-like cytoplasmic processes toward uncalcified transverse cartilage walls. These processes were seen to stretch as far as the degenerated chondrocytes located in the calcified layer of the growth plate. Interestingly, these perivascular cells showed neither alkaline phosphatase activity nor tartrate-resistant acid phosphatase activity. Lectin histochemistry revealed specific staining by Dolichos Biflorus agglutinin (DBA) on the perivascular cells. No reactivity for DBA was detected on either endothelial cells, osteoblasts, chondroclasts, or osteoclasts. In addition, immuno-histochemical studies showed that the perivascular cells neither expressed CD44, which was localized on the plasma membrane of chondroclasts, osteoclasts, and osteocytes, nor were surrounded by laminin. These results suggest that the perivascular cells in the erosion zone are distinct from endothelial cells, osteoblasts, chondroclasts, and osteoclasts; that they may resorb uncalcified cartilage matrix and degenerated chondrocytes; and that perivascular cells may play an important role in the capillary invasion during the process of endochondral ossification.