Eimitsu Tamura

Studies on Japanese chars of the genus Salvelinus. IV. The caudal neurosecretory system of the Nikkô-iwana, Salvelinus leucomaenis pluvius (Hilgendorf).

During a one-year period, seasonal changes in histology of the caudal neurosecretory system in a Japanese char, the Nikko-iwana, Salvelinus leucomaenis pluvius, from mountain tributaries were examined to possibly elucidate its function. The vascular supply to this system was also traced after injection of Chinese ink. The urophysis (neurophysis spinalis caudalis=neurohemal organ) of the Nikko-iwana is elongated and poorly differentiated. The neurosecretory material is stained principally by acidic dyes, but not by Gomoris chromalum hematoxylin or paraldehyde fuchsin. There are no clearcut seasonal changes in the apparent activity of the secretory cells or in the amount of secretory material stored in the neurohemal areas in adult fish. In larval fish, immediately after hatching the caudal neurosecretory system is not developed (there is neither a differentiated neurosecretory cell or a vascular plexus in the presumptive area of the urophysis). On the contrary, fuchsinophilic material is already visible in the hypothalamo-hypophysial neurosecretory system of the same specimens. An early indication of formation of the caudal neurosecretory system is found in parr-stage fish, which begin active swimming about 6 months after insemination. A possible role of the urophysial system, which is peculiar to the actinopterygian fishes with a homocercal tail, is discussed from phylogenetic, ontogenetic, and ecological aspects.

Fine Structure of the Sterile Testis of Hybrid Threespine Stickleback between Marine and Landlocked Forms

The process and cause of male sterility of a hybrid between female marine (M) and male landlocked (L) forms (M ⧫ × L ♂) of the threespine stickleback were studied by examining the testes of this hybrid and backcross offspring between M ⧫ and F1 (L ⧫ × M ♂) (MOLM ♂). In the M form numerous spermatids were elaborated and contained in the apical processes of the Sertoli cells, whereas in the ML and MOLM forms there were a number of macrophages in the lobular cavity with numerous projected pseudopodia engulfing voraciously the spermatids. As a result of this heavy phagocytosis the seminiferous lobules were quite vacant. Curiously, the Sertoli cells also actively project its processes and pseudopodia to catch the spermatids. In addition to morphometrical, behavioral and biochemical-genetic differences, these findings of sterility seem to suggest that the M form and L form represent independent species.