Morphological and morphometric studies on the terminal boutons on the neurosecretory cells of the rat paraventricular nucleus.

Ultrathin sections of the male rat hypothalamus, including the paraventricular nucleus prepared by perfusion fixation, were observed with a transmission electron microscope. The neurosecretory cells were classified into four types based on the difference in the structures of cell organelles and in the number and appearance of secretory granules present. This was reported elsewhere (Yukitake et al. Cell Tis. Res. 177: 1-8, 1977). Morphological and morphometric analyses on the terminal boutons abutting on the surface of the neursecretory cells were performed. The boutons were classified into three types: first type, having only small clear vesicles; second type, having large cored vesicles; and third type, having closely packed small vesicles. The incidence of the occurrence of each type of terminal bouton as well as the ratios of the surface of the perikarya of the neurosecretory cells covered by the terminal boutons were almost constant in all types of neurosecretory cells. As the stage of the secretory cycle proceeded, the numerical densities of the small synaptic vesicles in the terminal boutons clearly decreased. These results were consistent with the idea that each type of neurosecretory cell represented a different stage in the cell specific secretory cycle rather than a difference in neuron grouping, and that the terminal boutons probably transmitted some information regulating the secretory function of the cells.