Persistent and transient senescent cells contribute to brain-barrier development

Establishment of the blood-brain barrier (BBB) and blood-cerebrospinal fluid (CSF) barrier requires precise coordination between diverse cell types to protect and nourish the brain. Here, we identify developmentally programmed p21⁺ senescent cells that exhibit divergent senescence-associated features across these two brain interfaces in mice. In the choroid plexus (ChP), epithelial cells adopt a lifelong, non-inflammatory senescent state associated with CSF production and blood-CSF barrier integrity. In contrast, vascular endothelial cells and brain-resident macrophages transiently exhibit pro-inflammatory senescence profiles during brain vascularization, with reciprocal signaling linked to angiogenic patterning and extracellular matrix assembly. The ablation of p21⁺ cells during mid-gestation disrupts brain vascular patterning and ChP integrity, which results in hemorrhage, impaired CSF production, and ventricular collapse. These findings indicate that embryonic senescent cells adopt divergent transient and long-lived states that support brain-barrier formation and homeostasis, thus reframing the prevailing view of persistent senescence beyond solely a pathological state.