INFLUENCE OF THE CHOROID-PLEXUS ON CEREBELLAR DEVELOPMENT - ANALYSIS OF RETINOIC ACID SYNTHESIS

The choroid plexus of the fourth ventricle is conspicuous both in loca tion and size: it protrudes over the outer hindbrain, closely apposed to the caudal external surface of the cerebellum, and it is disproport ionately large early on. While the developing cerebellum is known to r espond to retinoic acid (RA), it does not express significant levels o f RA synthesizing enzyme. Retinaldehyde dehydrogenase levels in the ch oroid plexus, however, are very high, with maxima during the pre- and postnatal periods of cerebellar morphogenesis. Explants assays demonst rate release of a neurite-outgrowth promoting activity from the choroi d plexus, whose levels parallel the levels of RA synthesizing enzyme h ere, and which can be mimicked by RA. These observations characterize the choroid plexus as a paracrine, growth-promoting organ for the deve loping cerebellum, with the effects mediated through temporally regula ted RA production.