Spatial and temporal expression of molecular markers and cell signals during normal development of the mouse patellar tendon.

Tendon injuries are common clinical problems and are difficult to treat. In particular, the tendon-to-bone insertion site, once damaged, does not regenerate its complex zonal arrangement. A potential treatment for tendon injuries is to replace injured tendons with bioengineered tendons. However, the bioengineering of tendon will require a detailed understanding of the normal development of tendon, which is currently lacking. Here, we use the mouse patellar tendon as a model to describe the spatial and temporal pattern of expression of molecular markers for tendon differentiation from late fetal life to 2 weeks after birth. We found that collagen I, fibromodulin, and tenomodulin were expressed throughout the tendon, whereas tenascin-C, biglycan, and cartilage oligomeric protein were concentrated in the insertion site during this period. We also identified signaling pathways that are activated both throughout the developing tendon, for example, transforming growth factor beta and bone morphogenetic protein, and specifically in the insertion site, for example, hedgehog pathway. Using a mouse line expressing green fluorescent protein in all tenocytes, we also found that tenocyte cell proliferation occurs at highest levels during late fetal life, and declines to very low levels by 2 weeks after birth. These data will allow both the functional analysis of specific signaling pathways in tenocyte development and their application to tissue-engineering studies in vitro.