Integrin expression and extracellular matrix adhesion of septoclasts, pericytes, and endothelial cells at the chondro‐osseous junction and the metaphysis of the proximal tibia in young mice

We previously reported that septoclasts, which are uncalcified growth plate (GP) cartilage matrix‐resorbing cells, are derived from pericytes surrounding capillary endothelial cells. Resorption of the GP is assumed to be regulated synchronously by septoclasts, pericytes, and endothelial cells. To reveal the contribution of the extracellular matrix (ECM) to the regulatory mechanisms of septoclastic cartilage resorption, we investigated the spatial correlation between the cells and the ECM in the GP matrix and basement membrane (BM) and investigated the expression of integrins—ECM receptors—in the cells. Septoclasts attached to the transverse septa containing collagen‐II/‐X at the tip of their processes and to the longitudinal septa containing collagen‐II/‐X at the spine‐like processes extending from their bodies and processes. Collagen‐IV and laminin α4 in the BM were sparsely detected between septoclasts and capillary endothelial cells at the chondro‐osseous junction (COJ) and were absent in the outer surface of pericytes at the metaphysis. Integrin α1/α2, integrin α1, and integrin α2/α6 were detected in the cell membranes of septoclasts, pericytes, and endothelial cells, respectively. These results suggest that the adhesion between septoclasts and the cartilage ECM forming the scaffolds for cartilage resorption and migration is provided by integrin α2–collagen‐II/‐X interaction and that the adhesions between the BM and pericytes or endothelial cells are mediated by integrin α1–collagen‐IV and integrin α2/α6–laminin interaction, respectively. The adhesion mediated by integrin–extracellular matrix interactions spatially supports septoclasts to enable the resorption of transverse septa and to provide the scaffold for migration along the longitudinal septa at the chondro‐osseous junction of the growth plate. The partial lack of the basement membrane between endothelial cells and septoclasts or pericytes suggests the angiogenetic activity of capillaries at the chondro‐osseous junction and metaphysis.