Astrocytes, ependymal cells, and radial glia are developmentally related cell types and formed a subtree in the dendrogram (Figure 3). This taxon included two specialized secretory cell types: the hypendymal cells (HYPEN), which are specialized ependymal-like cells of the subcommissural organ that secrete SCO-spondin (encoded by Sspo) into the cerebrospinal fluid to form Reissner’s fiber, and the choroid plexus epithelial cells (CHOR), which are an extension of the ependymal lining of the ventricular surfaces that envelop branching capillaries protruding into the ventricles and secrete the extremely abundant thyroxine and retinol transport protein Transthyretin (encoded by Ttr).
