Ultrastructure of the caudal portion of the fourth ventricular roof in the mouse.

A scanning and transmission electron microscopic study was done of the caudal portion of the fourth ventricular roof in the adult mouse. The caudal portion of the fourth ventricular roof was divided into two parts: choroid plexus and nonchoroid plexus roof. The plexus epithelial cells had numerous microvilli and were connected with each other with a tight junction, gap junction, or zonula adherens. The nonchoroid plexus roof was a thin neuroglial membrane with a continuous basement membrane, which we designated as "the membranous roof." The membranous roof was mostly composed of a single layer of paving ependymal cells supported by discontinuous pial cells. The ependymal cells were characterized by their squamous cell configuration, various numbers of microvilli and cilia, and a continuous basement membrane. They were connected with each other by gap junctions or zonulae adherentes, but they lacked tight junctions. Neither a cytoplasmic pore nor a wide intercellular cleft was identified. Neuronal and glial cell processes were occasionally observed. The aperture that is supposed to exist in the caudal, median portion of the fourth ventricular roof of the adult mammalian brains, called the foramen of Magendie in the human brain, was not found. Our observations suggest that the membranous roof itself would permit passage of the cerebrospinal fluid containing macromolecules via the interependymal cell clefts, implying that the foramen of Magendie would not be an anatomical aperture but a functional channel for the outflow of the cerebrospinal fluid.