Infoldings of blood vessels of the pia mater, projecting into the cerebral ventricles, covered with ependymal cells, and secreting cerebrospinal fluid.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 735 (https://doi.org/10.1515/pac-2015-1202)