Immunocytochemical identification of serotonergic supraependymal nerve fibers in the third ventricle of the house musk shrew, Suncus murinus.

Supraependymal fibers of the house musk shrew (Suncus murinus) were examined by conventional scanning electron microscopy (SEM), backscattered electron (BSE) imaging of enzyme immunohistochemistry and by immunotransmission electron microscopy in the dorsal part of the third ventricular wall. In this region, ependymal cells were not so heavily ciliated and conventional SEM studies showed two main categories of supraependymal fibers. The first type consisted of long fibers fasciculated which were distributed over the ventricular surface between the anterior commissure and the subfornical organ. The second category was a thin fiber which was observed on the ependymal luminal surface. Some of these fibers had varicoses or terminal-like swellings. This type of supraependymal fiber seemed to originate in the first type of fiber bundles. To confirm the nature and the distribution of serotonin-immunoreactive supraependymal fibers, BSE imaging using immunohistochemical reactions was used. Serotonin-immunoreactive structures were shown as highlighted structures by means of a backscattered electron mode. These investigations revealed that the majority of both types of supraependymal fibers observed by conventional SEM contained serotonin. A moderate number of serotonergic supraependymal fibers was observed on the ventricular surface of the subfornical organ. Immunohistochemical studies using Vibratome sections of identical ventricular regions revealed the presence of serotonin-immunoreactive processes, with the use of light- and electron-microscopy. They were distributed in in the third ventricle just adjacent to the ependymal luminal surfaces. These fibers contained immunoreactive large cored vesicles and immunonegative small clear vesicles.