Development of migrating tendon-bone attachments involves replacement of progenitor populations

Abstract

Tendon-bone attachment sites, called entheses, are essential for musculoskeletal function. They are formed embryonically by <i>Sox9</i>+ progenitors and continue to develop postnatally, utilizing <i>Gli1</i> lineage cells. Despite their importance, we lack information on the transition from embryonic to mature enthesis and on the relation between <i>Sox9</i>+ progenitors and the <i>Gli1</i> lineage. Here, by performing a series of lineage tracing experiments in mice, we identify the onset of <i>Gli1</i> lineage contribution to different entheses. We show that <i>Gli1</i> expression is regulated embryonically by SHH signaling, whereas postnatally it is maintained by IHH signaling. During bone elongation, some entheses migrate along the bone shaft, whereas others remain stationary. Interestingly, in stationary entheses <i>Sox9</i>⁺ cells differentiate into the <i>Gli1</i> lineage, but in migrating entheses this lineage is replaced by <i>Gli1</i> lineage. These <i>Gli1⁺</i> progenitors are defined embryonically to occupy the different domains of the mature enthesis. Overall, these findings demonstrate a developmental strategy whereby one progenitor population establishes a simple embryonic tissue, whereas another population contributes to its maturation. Moreover, they suggest that different cell populations may be considered for cell-based therapy of enthesis injuries.