Scanning electron microscopy of the mammalian cerebral-ventricular system

Abstract

This investigation deals with correlative scanning-transmission electron microscopic analysis of the mammalian circumventricular organ system CVO. This specilized system of ancient midline regions throughout the third and fourth cerebral ventricle of the brain share many common structural similarities with respect to their vascular, neuronal and glial organization. Areas of analysis of this investigation have included the median eminence, the neural lobe, the organum vasculosum of the lamina terminalis, the subfornical organ and the area postrema. Ample evidence now suggests that these regions of the brain may play an integrative role between blood, brain and cerebrospinal fluid and because they share so many common features, circumventricular organs have been analogized as ‘windows of the brain’. Regions of the cerebral ventricular wall that do not possess neuroendocrine (either real or suspected), possess a dense nap of cilia throughout their mural walls. However, lining ependymal cells of CVO, the so-called ‘tanycyte’, does not exhibit a major population of cilia and instead exhibits a felt work of microvilli organized in a mosaic pattern. These microvilli appear most prominent along the interface of apically oriented cells that constitute the lining of the family of structures collectively referred to as circumventricular organs. Unlike surrounding brain tissue, circumventricular organs do not exhibit the classical blood-brain barrier but possess instead fenestrated capillaries and specialized epennymal cells (tanycytes) that possess tight junctions at their apical (ventricular) sites. Hence in these specialized regions the blood-brain barrier has been shifted from the vascular to the ventricular side and becomes instead a blood-CSF barrier. Coupled with the fundamental differences in the anatomical appearance of the ventricular lumen of circumventricular organs, numerous individual clusters of cells can be seen to reside upon the ventricular surfaces of circumventricular organs. These cells are identified as either populations of quiescent histiocytes or large dense populations of neurons with extensive neuritic arborizations which extend over the ventricular lumen and appear to represent a system of interconnecting axonal processes that may serve to integrate the various circumventricular organs of the third and fourth cerebral ventricle. This investigation deals with the possible role that supraependymal CSF-contacting neurons may play in the synthesis and secretion of neuropeptides into the brain and into the adjacent cerebrospinal fluid.