The hypothalamic neurosecretory system of the marine teleost fish, Mugil auratus risso.

The hypothalamic neurosecretory system of the marinefish, Mugil auratus, consists of two nuclei, viz., the nucleus praeopticus and the nucleus laterlis tuberis. Both are paired, and while those of the nucleus praeopticus are vertically arranged as L-shaped bodies, their strand-like counterparts in the nucleus lateralis tuberis extend in an antero-posterior direction. The two constituent bodies of the nucleus praeopticus lie on both sides of the third cerebral ventricle. Each is differentiated into a dorsal pars magnocellularis and a ventral pars parvocellularis. A nervous tract, the hypothalamo-hypophysial tract, extends posteriorly from each body, but it is not until after they penetrate the pituitary gland that they fuse into one structure, the neurohypophysis. Many neurosecretory granules accumulate in the neurohypophysis adjacent to the meta-adenohypophysial region, and fewer scattered granules of varying sizes are also present along the hypothalamo-hypophysial tracts. Inner to these hypothalamo-hypophysial tracts extend the two bodies of the nucleus lateralis tuberis along the infundibular region. Axons from this nucleus extend sideways, and as they merge with those adjacently disposed of the hypothalamo-hypophysial tracts, they enter the pituitary gland as a unified structure.