Developmental Biology: Defining the Roadmap
(funded by NIH R01 AR055580) 10,16-23
The enthesis mineralizes and matures postnatally through a coordinated set of biologic events driven by mechanical loading and genetic cues.24 We have studied the role of muscle loading on enthesis development by paralyzing the rotator cuff muscles at birth in mice.16,18-20,23 A series of experiments using chemical or physical denervation demonstrated that muscle load is necessary for the formation of a functional enthesis. In the absence of load, a cell phenotype gradient failed to form at the developing enthesis, leading to defects in fibrocartilage formation, mineralization, collagen deposition, and a mechanically deficient attachment. Studies of the molecular cues driving enthesis formation led to the identification of a unique population of hedgehog-responsive cells at the developing enthesis (Figure 3).17 Lineage tracing experiments demonstrated that this cell population populates the enthesis through the early post-natal period and controls mineralization of the attachment. Genetic deletion of hedgehog responsiveness in these cells or ablation of these cells led to severe defects in mineralization and attachment mechanical properties. Understanding the biophysical and molecular signals that drive the development of a functional attachment between tendon and bone provides a roadmap for improved healing and for development of tissue engineered replacements.