Immunohistochemical and Morphometric Assessment on the Biological Function and Vascular Endothelial Cells in the Initial Process of Cortical Porosity in Mice With PTH Administration

To clarify the cellular mechanism of cortical porosity induced by intermittent parathyroid hormone (PTH) administration, we examined the femoral cortical bone of mice that received 40 µg/kg/day (four times a day) human PTH (hPTH) (1–34). The PTH-driven cortical porosity initiated from the metaphyseal region and chronologically expanded toward the diaphysis. Alkaline phosphatase (ALP)-positive osteoblasts in the control mice covered the cortical surface, and endomucin-positive blood vessels were distant from these osteoblasts. In PTH-administered mice, endomucin-reactive blood vessels with TRAP-positive penetrated the ALP-positive osteoblast layer, invading the cortical bone. Statistically, the distance between endomucin-positive blood vessels and the cortical bone surface abated after PTH administration. Transmission electron microscopic observation demonstrated that vascular endothelial cells often pass through the flattened osteoblast layer and accompanied osteoclasts in the deep region of the cortical bone. The cell layers covering mature osteoblasts thickened with PTH administration and exhibited ALP, α-smooth muscle actin (αSMA), vascular cell adhesion molecule-1 (VCAM1), and receptor activator of NF-κB ligand (RANKL). Within these cell layers, osteoclasts were found near endomucin-reactive blood vessels. In PTH-administered femora, osteocytes secreted Dkk1, a Wnt inhibitor that affects angiogenesis, and blood vessels exhibited plasmalemma vesicle-associated protein, an angiogenic molecule. In summary, endomucin-positive blood vessels, when accompanied by osteoclasts in the ALP/αSMA/VCAM1/RANKL-reactive osteoblastic cell layers, invade the cortical bone, potentially due to the action of osteocyte-derived molecules such as DKK1.