Histological and histochemical study of the caudal neurosecretory system of the freshwater teleost Ompok bimaculatus (Bloch) with a note on its response to hypophysectomy and osmotic stress.

The caudal neurosecretory system in well developed in Ompok bimaculatus. It extends from 8th to the last caudal vertebra. In the last vertebra the spinal cord forms a distinct, lobate and highly vascular ventral urophysis where the neurosecretory fibers have perivascular endings forming a neurohaemal organ. In the anterior segment near the 8th preterminal vertebra the Dahlgren cells are large and sparsely distributed. In the posterior segment close to the urophysis they are small and densely arranged. The neurosecretory material (NSM) has affinity for acid stains but reacts negatively to Gomori's stain or to a reagent demonstrating SS/SH group. The Dahlgren cells have high RNA content in the cytoplasm indicating active protein synthesis. Histological evidence suggests that the caudal neurosecretory system of this species did not respond to hypertonic osmotic stress. However, immersion in deionized water is accompanied by hypertrophy of Dahlgren cells and depletion of NSM from the caudal neurosecretory system. Hypophysectomy results in initial hypertrophy of the Dahlgren cells followed by atrophic changes and depletion of NSM. Ninety days after hypophysectomy this system gradually reverts to normal.