Histochemical assessment for osteoblastic activities in modeling and remodeling regions in osteoclast-lacking mice

Objectives: Bone remodeling is characterized by matrix replacement in which osteoblasts deposit new bone onto the bone surface where osteoclasts are previously resorbed, while bone modeling is the change of bone shape and extension, which is caused by osteoblastic bone formation independent of the coupling from the osteoclasts. C-fos deficient (c-fos^-/-) mice have no osteoclast, providing an animal model for defective bone remodeling, but they can still show bone modeling. To better understand what activates osteoblasts in bone modeling, we have examined osteoblastic bone formation at the bone modeling and remodeling sites in wild-type mice and c-fos^-/- mice.
Methods: Eight weeks-old wild-type mice and c-fos^-/-mice were injected with calcein before fixation, and then fixed with a 4% paraformaldehyde solution. Femora were employed for immunohistochemistry of tissue-nonspecific alkaline phosphatase (ALPase), endomucin, EphB4, ephrinB2, calcein labeling (bone mineral deposition), and transmission electron microscopic (TEM) observation to assess the osteoblastic activities in the primary (modeling area) and secondary trabeculae (bone remodeling area), respectively.
Results: In the primary trabeculae, wild-type mice and c-fos^-/- mice demonstrated ALPase-positive plump osteoblasts with well-developed rough endoplasmic reticulum and Golgi apparatus, as well as calcein labeling, i.e., calcified bone deposition. However, in the secondary trabeculae, wild-type mice showed ALPase-positive mature osteoblast accompanied with calcein labeling in bone, while c-fos^-/- mice merely revealed inactivated, flattened bone lining cells and no calcein labeling. In the primary trabeculae, endomucin-positive bone-specific blood vessels possessing EphB4 were in contact with or in the close proximity of ephrinB2-immunoreactive osteoblasts, and TEM observation demonstrated cell-to-cell contact with osteoblasts and blood vessels, indicating the reciprocal signaling of EphB4/ephinB2 between the osteoblast and vascular endothelial cells.
Conclusions: In bone modeling, osteoblastic activation appears to be independent of osteoclastic bone resorption, but at least in part, to be associated with bone-specific blood vessels.