THE VENTRICULAR SYSTEM OF THE PIGEON BRAIN - A SCANNING ELECTRON-MICROSCOPE STUDY

The fine structural features and regional differences of the ependyma in adult pigeons have been investigated by scanning electron microscop y. Pigeons of either sex were fixed with buffered glutaraldehyde (3%) and formaldehyde (0.5%) by intravascular perfusion. The brain was diss ected using section planes adequate to expose each part of the ventric ular system. The specimens were then dehydrated, critical point dried and sputtered with gold. Depending upon the distribution of cilia, mic rovilli and single cilia, different areas were recognised in the 4 ven tricles. The topographic locations of these areas were determined usin g the atlas of Karten & Hodos (1967). The medial surfaces of the 1st a nd 2nd lateral ventricles are more densely ciliated than the lateral s urfaces. In the floor of the 4th ventricle the medial part is less cil iated than the lateral parts. The circumventricular organs (subseptal organ, organum vasculosum of the lamina terminalis, infundibulum, chor oid plexus, subcommissural organ, area postrema) show very characteris tic surfaces and are surrounded by a transitional zone with the nonspe cialised ependyma. In contrast, in the paraventricular organ the trans ition to the nonspecialised ependyma is rather abrupt. The ependyma co vering the trochlear nucleus appears densely ciliated, differing from that of the classic circumventricular organs. Finally, the existence o f openings in the caudal medullary velum, which represent direct commu nications between the ventricles and the subarachnoid space, was demon strated.