The presence and synthesis of transthyretin, a major carrier protein of 
thyroxine in rat cerebrospinal fluid, was investigated in choroid plexus 
epithelial cells and ependymal cells by immunocytochemistry, in situ 
hybridization, and analysis by Northern and Western blot using a specific 
oligonucleotide probe and a specific polyclonal antibody to transthyretin. 
Choroid plexus epithelial cells expressed transthyretin at high levels in 
developing rat cerebral hemispheres and in cultured cells. These cells secreted 
transthyretin into the cerebrospinal fluid. In the developing rat brain 
transthyretin was present in the cytoplasm of ependymal cells, in vesicles in 
contact with the apical membrane and in cilia. In ependymal cell cultures this 
protein was particularly abundant in the cilia of these cells. In contrast, 
ependymal cells did not synthesize transthyretin. It is postulated that 
transthyretin is transported to ependymal cells from the cerebrospinal fluid by 
endocytosis.