Surface ultra-architecture of supraependymal elements in the vicinity of the area postrema of rhesus monkeys

The caudal part of the fourth ventricle in adult rhesus monkeys, of both sexes, revealed the area postrema (AP) to be composed of bilateral spindle-shaped elevations located ventromedial to the gracile tubercles. Supraependymal (SE) elements, comprised of SE cells and/or fibre processes could be seen near the medial margin of a narrow oligociliated strip separating the nonciliated AP from the profusely ciliated vagal triangle (VT). In general, these consisted of multilobed ganglion-like cell clusters interconnected by fasciculated fibre processes with varicosities along their lengths. Thinner processes radiating from the cell clusters and varicosities traversed towards the VT for short distances and often appeared to penetrate the underlying ependyma. The thicker fibre processes which exhibited a fasciculated configuration extended between raised nodule-like areas in the ventricular floor. The majority of such areas exhibited an uneven surface due to the presence of numerous membranous folds over them. Near the periphery of nodular areas as well as along the fasciculated fibre processes interconnecting them, a few bouton-like protrusions were discernible. The SE elements observed in the caudal portion of the fourth ventricular floor are suggestive of comprising some integrative pathways between the ventricular cerebrospinal fluid and subependymal functional centres in the vicinity of the AP, a circumventricular organ having diverse functions.