The developing cortical surfaces of long bones are sculpted and modeled by periosteal osteoclasts and osteoblasts. These surfaces also receive the insertions of tendons and ligaments, and these insertion sites too are modeled to form the root systems that anchor them into the cortical bone. The regulatory molecules that control modeling are poorly understood, but recent evidence suggests that parathyroid hormone-related protein (PTHrP) participates in this process. PTHrP functions principally as a paracrine regulatory molecule, and is known to be induced by mechanical loading in a number of sites. The most curious example of developmental modeling of the cortex is the migration of insertion sites such as that of the medial collateral ligament (MCL) along the bone surface during long-bone growth. We report here the mechanisms that mediate MCL migration using a combination of genetic, imaging and histological techniques. We describe a MCL migratory complex that comprises two components. The first is the MCL insertion site itself, which is a prototypical fibrous insertion site with coupled osteoclast and osteoblast activities, and its key feature is that it is anchored early in development, well before initiation of the long-bone growth spurt. Above the insertion site the periosteum is excavated by osteoclasts to form a migratory tract; this is mediated by wholly uncoupled osteoclastic bone resorption and remains as an unmineralized canal on the cortical surface in the adult. Load-induction of PTHrP appears to regulate the osteoclastic activity in both the insertion site and migratory tract.
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