Kondrychyn, I.; Chen, Y.; Kumar, R.; Chen, G.; Kawakami, K.; McEvoy, E.; Phng, L.-K.

Haematopoietic stem and progenitor cells (HSPCs) arise from specialised haemogenic endothelial cells (HECs) located at the ventral wall of the dorsal aorta (VDA) through a process known as endothelial-to-haematopoietic transition (EHT). A hallmark of EHT is a morphological change in which elongated HECs undergo actomyosin-driven rounding prior to extrusion. Here, we identify an osmo-hydraulic mechanism that enables HECs to adapt to and withstand intrinsic contractile forces associated with this shape transition. We show that Piezo1 activation induces Ca2+ transients in HECs of the VDA. Elevated intracellular Ca2+ promotes cell swelling and activates volume-regulated anion channels (VRAC), resulting in Cl- and water efflux and consequent volume reduction. Disruption of aquaporin-mediated water efflux leads to excessive cell swelling and rupture, ultimately reducing HSPC production. Together, our findings establish aquaporins as pressure-relief valves that preserve mechanical robustness during EHT and demonstrate that failure to dissipate intracellular hydraulic pressure compromises HEC survival and definitive haematopoiesis.