The floor plate of the hindbrain is a highly specialized gland. Immunocytochemical and ultrastructural characteristics.

Although the participation of the floor plate (FP) in the differentiation of neurons in the ventral spinal cord is well established, the function of the FP at the level of the hindbrain is not known. The present investigation was focused on the FP of the rat hindbrain. We used an antibody specifically labeling the rat FP (FP4) and an antiserum raised against the secretory glycoproteins of the subcommissural organ that also reacts with an intracellular material in the FP. Immunocytochemical and ultrastructural evidence was obtained indicating that the rostralmost end of the FP, at the metencephalon, is a highly specialized gland which differentiates early and undergoes regression before birth. The material(s) secreted by these FP cells appear to be transported downward into their basal processes, and upward into ventricular protrusions. It is proposed that the former would participate in the differentiation of the serotonergic neurons, and the latter could be released into the ventricle and reach distant targets. Furthermore, evidence is presented supporting the occurrence of regional and temporal specializations of the FP.