An in situ study of the hypothalamo-neurohypophysial complex of the freshwater teleost Ompok bimaculatus (Bloch) with a note on the tetrapodan features of its vascularization.

In Ompok bimaculatus the neurosecretory axons of both the pars magnocellularis and pars parvocellularis component of the nucleus preopticus contribute to the formation of the left and right main tracts. At rostral levels, one third of the tracts are loosely built forming a king of curtain, while they become more compact at caudal levels. The caudal two thirds of the main tracts give off several pairs of lateral tracts which join at the midline to form the paired median tracts. The median and the main tract jointly enter the pituitary as a common tract which, on entering the proximal pars distalis, separates into two and enters the pars intermedia. The hypothalamic and the hypophysial arteries take their origin from the internal carotid artery. The former contribute to the formation of the primary capillary plexus of the median eminence, whereas the latter enter the pituitary directly and ramify in the neuroadeno-interface. The primary capillary plexus is in close topographical contact with the neurosecretory axonal complex where the median and common tracts are formed. At this site at least some axons might have their endings on the capillary plexus. Thus, this region my be comparable in structure to the median eminence of the tetrapods. The neurons of the nucleus lateralis tuberis of O. bimaculatus are AF-positive. The infundibular cavity extends into the main neurohypophysial trunk and is lined by ependymal cells like the third ventricle.