| Abstract: | The hypothalamo-neurohypophysial (HN) complex of Macrognathus aculeatus is described with the help of in situ staining technique and tissue sections. Bulk stained preparations provide a three-dimentional view of the entire HN complex. The nucleus preopticus (NPO) which is oval in the ventral view appears boat-shaped in the cross sectional view of the in toto stained brain. But it is in the form of an inverted L in median parasagittal section. Both pars magnocellularis and parvocellularis contribute fine fibers towards the formation of the right and the left main tracts which are composed of groups of loosely set bundles of axons. Unlike the earlier reported cases the main tract in this species runs lateral to the pituitary axis and independently enters it at the level of the proximal pars distalis without forming a common neurosecretory tract which is a feature not yet well known. In situ demonstration of the neurosecretory system in M. aculeatus has clearly revealed certain complex features which are difficult or impossible to understand through reconstruction studies of tissue sections. Extension of such in situ staining techniques to other species of teleosts may provide a wide variety of variations in the distributional patterns of their neurosecretory tracts which might throw additional light on the function and evolution of this system. |
