Fig. 9. Multiple steps are required for brain ventricle formation. Three steps have been identified during initial brain ventricle formation and occur independently of circulation. The Nagie oko protein helps maintain epithelial polarity and/or integrity, which is required for normal ventricle morphogenesis. Atp1a1a.1 is essential to inflate the ventricular space with fluid, and localized cell proliferation also appears to be necessary. Later brain ventricle expansion requires circulation. A, anterior; F, forebrain ventricle; H, hindbrain ventricle; M, midbrain ventricle; P, posterior.