The Ultrastructures On The Surface Of Hippocampus In Lateral Ventricle Of The Rats: Scanning Electron Microscopic Observations

The ependyma is a single-layered epithelium that lines the ventricles of the brain, mesencephalic aqueduct and the central canal of the spinal cord. It is composed of cuboidal, columnar or flattened epithelial cells. It has been elucidated that some specialized ultrastructures exist on the ependymal surface, which include cilia, microvilli, blebs, the dendrites of distal CSF-contacting neurons, supraependymal cells and supraependymal fibers. These fine structures together with the ependymal cells form the structural base of the brain-CSF neurohumoral circuit. In present study, the morphological and distributional features of the ultrastructures on the hippocampal surfaces of nine normal rats were observed by scanning electron microscope and the correlative functions were discussed, which provided the morphological data for further comprehension of the brain-CSF neurohumoral circuit. Dense clustered cilia, sparse single cilia, thick microvilli and some scattered blebs were observed on the surfaces of hippocampal fimbria. Clustered cilia were seen the most compact on the anterior and lateral surfaces, and became sparse on the caudal and medial surfaces. However, microvilli were the main ultrastructural feature on the surface of hippocampal body, and only sparse single cilia existed on the anterior surface, which formed a distinct boundary with hippocampal fimbria coated with dense clustered cilia. In the nine rats, three were observed that there was remnant choroid plexus of lateral ventricle on the surface of hippocampal fimbria or the junction between hippocampal fimbria and body. Numerous microvilli and many different-sized blebs were observed on the folds of the plexus. Besides cilia and microvilli, many supraependymal cells and nerve fibers were observed on the surface of hippocampus. Supraependymal nerve fibers with different diameters existed not only on the ciliated surface of hippocampal fimbria, but also on the middle and posterior surfaces of hippocampal body. Although some of the fibers appeared singly, the majority of them interwinded to form nerve bundles or networks. Supraependymal cells observed on the hippocampal surface included phagocyte-like cells, neuron-like cells and epiplexus cells. The number of phagocyte-like cells that mainly existed on the ciliated surface of hippocampal fimbria was the most; the number of neuron-like cells that scattered on the middle and posterior surfaces of hippocampal body was the least. The epiplexus cells groveled on the microvilli of choroid plexus, which protruded many pseudopods and filamentous processes.