Synovial lining macrophages form during early postnatal life via fetal-derived CX3CR1(+) cells and AQP1(+) intermediates.

Fetal-derived CX3CR1(+) macrophages and AQP1(+) intermediates drive postnatal formation of the synovial lining in mice, with macrophages developing during the first weeks of life.

• Why it matters: Understanding how the synovial lining develops is crucial because its disruption leads to inflammatory joint diseases, yet its developmental mechanisms remain poorly characterized.
• What they did: Using genetic mouse models, imaging, and single-cell transcriptomics, the study tracked the maturation of the synovial lining, revealing a CSF1-dependent process involving fetal macrophages and intermediates during early postnatal life.
• The result: This work shows that the synovial lining matures postnatally through specific cellular and transcriptional changes, highlighting a critical window for joint health that could inform future therapeutic strategies.