THE POLARITY OF CHOROID-PLEXUS EPITHELIAL-CELLS IN-VITRO IS IMPROVED IN SERUM-FREE MEDIUM

The influence of culture conditions on the development of normal chara cteristics of the choroid plexus epithelium has been investigated in v itro with respect to polarity, barrier properties, transport, and secr etory activity. Withdrawal of serum supplement in the culture medium o f cells grown on filters caused morphologically visible changes by an increased trimming of microvilli at the apical membrane side, which is accompanied by an increased expression of the Na+, K+-ATPase. Moreove r cells under serum-free conditions exhibit structural changes in ligh t junctional zonula occludens protein-1 (ZO-1) organization, a reduced permeability, and a drastically increased electrical resistance from 150 Omega.cm(2) in the presence of serum to 1,500 Omega.cm(2) after se rum withdrawal. Under these conditions, cell monolayers are able to bu ild up a transcellular proton gradient and to secrete fluid into the u pper (apical) filter compartment, which is accompanied by a polarized secretion of proteins like transthyretin. Active transport of the dyes fluorescein and phenol red by the organic anion transporter is found to be driven by the Na+, K+-ATPase. We come to the conclusion that rem oval of serum favors the differentiation process of the plexus epithel ium in vitro, which brings the cell culture model closer to the physio logical situation in vivo. We present preliminary evidence that epider mal growth factor may be one component in serum preventing the proper in vitro differentiation.