NEUROSECRETION IN TELEOST FISHES: THE CAUDAL NEUROSECRETORY SYSTEM

Abstract

The term urophysis spinalis is introduced in order to stress the relative position of the storage and release organ of the caudal neurosecretory system. The spinal neurosecretory cells show a species specific distribution. The neurosecretory axons and processes enter the urophysis and terminate at a vascular reticulum. The urophysis spinalis in most species is of a ventral type. The neurosecretory processes enter the organ and form a central region, the medulla. Glial cells and ependymal and glial fibers also enter the organ from the spinal cord. Herring - body like structures known from other neurosecretory systems, are frequent in the dorsal region of the urophysis and are also found in more central parts of the organ. Large neurosecretory cells are found in the transverse sections of the spinal cells of the American eel at the level of the 5th vertebra from the caudal and of the spine. The cells are Gomori- negative but show affinity for stains as acid fuchsin and azocarmine. Unlike other neurosecretory systems, the spinal cells react negatively to PAS [positive acid Schiff] and Alcian blue. The cells react positively for ninhydrin-Schiff andaloxan-Schiff. Methods for demonstration of RNA have indicated that considerable amounts are present in the cells. A very intimate relationship between capillaries and the spinal neurosecretory cells have been observed, suggesting a strong rate of exchange between the neurosecretory cells and the blood. Herring - body like structures are usually formed in the ventral region of the spinal cord. Swollen processes containing secretory material are seen. Accumulation of material are frequently formed along the tracts, probably due to the fact that the tracts are easily expandable. A polymorphic nucleus is frequent in the spinal neurosecretory cells. The neuronal character of the spinal cells is attested by the presence of elements such as dendrites, neurofibrils and Nissl- bodies. The spinal neurosecretory cells seem at the same time to be glandular, elaborating microscopically visible granules or "droplets" which pass along the processes or axons towards the nerve terminals, ending in an organ or area where the material is stored and released. The neurosecretory granules are below the resolving power of the light microscope (1000-2000 A). The neurosecretory material has nevertheless been described as aggregations clusters or colloid droplets. The formation of such structures is apparently influenced by fixation and staining procedures. The urophysis spinalis and the caudal neurosecretory system develop considerably later in the ontogeny than the hypothalamic system. A sagittal section of the newly hatched Fundulus heteroclitus showed no signs of a urophysis. The electronmicrographs show nerve endings with neurosecretory granules (vesicles) of 2 distinct sizes (approx. 1000 A and 1800 A in diameter. Nerve terminals may contain both neurosecretory vesicles and synaptic vesicles, 200-300 A in diameter. Great similarities seem to exist between the general organization of all known neurosecretory systems and this similarity extends to the fine structure of the individual components. It should be noted that the caudal neurosecretory system is Gomori- negative which indicates that the staining response is of minor importance in the discussion of the concept of neurosecretion, the definition of which must be based on morphologic characters, the cytology, histochemical reactions and first of all studies of the fine structure. The functional studies may in addition indicate hormonal components. Studies on the function of the caudal system have so far given inconclusive results.