Policy information about studies involving animals; ARRIVE guidelines recommended for reporting animal research Laboratory animalsC57Bl/6 male mice, aged (19 months from NIA rodent colony), young (3 months from Charles River or Jackson Labs) Wild animalsThis study did not involve wild animals.Field-collected samples This study did not involve field-collected samples. Ethics oversightInstitutional Animal Care and Use Committee at Stanford University Note that full information on the approval of the study protocol must also be provided in the manuscript. Human research participantsPolicy information about studies involving human research participants Population characteristicsMale and female aged (58-93 years old) cognitively normal and clinically-diagnosed Alzheimer's disease patients. Cognitively normal patients do not display atypical vascular pathologies. Patients are mixed in APOE genotype. RecruitmentSubjects were not recruited specifically for this study. Samples are derived from a brain bank maintained by the Stanford/ VA/ NIA Aging Clinical Research Center (ACRC) from patients that provide consent for broad, de-identified data sharing under Institutional Review Board (IRB) approval. Ethics oversightStanford/ VA/ NIA Aging Clinical Research Center (ACRC) Note that full information on the approval of the study protocol must also be provided in the manuscript.

Summary Extrinsic signals controlling generation of neocortical neurons during embryonic life have been difficult to identify. In this study we demonstrate that the dorsal forebrain meninges communicate with the adjacent radial glial endfeet and influence cortical development. We took advantage of Foxc1 mutant mice with defects in forebrain meningeal formation. Foxc1 dosage and loss of meninges correlated with a dramatic reduction in both neuron and intermediate progenitor production and elongation of the neuroepithelium. Several types of experiments demonstrate that retinoic acid (RA) is the key component of this secreted activity. In addition, Rdh10 and Raldh2 expressing cells in the dorsal meninges were either reduced or absent in the Foxc1 mutants and Rdh10 mutants had a cortical phenotype similar to the Foxc1-null mutants. Lastly, in utero RA treatment rescued the cortical phenotype in Foxc1 mutants. These results establish RA as a potent, meningeal-derived cue required for successful corticogenesis.

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Highlights d scRNA-seq shows transcriptional profiles corresponding to E14 meningeal layers d Regional heterogeneity in E14 meningeal fibroblast gene expression d Temporal-spatial maturation of telencephalic meningeal layers d Conservation of meningeal layer enriched markers in human fetal meninges