The developing tendon and enthesis are hypoxic and rely on hypoxia-inducible factor 1a during postnatal development.

The enthesis is a specialized tissue crucial for transmission of muscle force from tendon to bone, yet its postnatal development is not fully understood. In this study, we investigated if and how hypoxia inducible factor 1 alpha (HIF-1α) regulates enthesis cell adaptation, e.g. cell survival and extracellular matrix (ECM) deposition. We investigated the spatial and temporal dynamics of hypoxia in the murine Achilles tendon enthesis and found that, whereas neonatal tendons rapidly resolve hypoxia after birth, the enthesis maintains a hypoxic niche until postnatal day 5, mirroring a hypoxic gradient observed in growth plates. Genetic disruption of Hif1a in mouse tendon/enthesis-resident cells (cKO) resulted in pronounced deficits in grip strength, abnormal tendon-bone attachment morphology, disrupted calcaneal architecture, impaired mineralization and significant ECM disorganization. We found persistent cell death and loss of collagen organization in cKO entheses. In vitro, Hif1a-deficient tendon fibroblasts exhibited blunted transcriptional responses to hypoxia, altered metabolic gene expression and disrupted ECM deposition. Collectively, our findings illuminate hypoxia as a sustained niche in the postnatal enthesis, with HIF-1α required for cell survival, ECM organization and structural integrity of entheses.