The hypothalamic neurosecretory cells of the rats, both the supraoptic and paraventricular nuclei were observed with the electron microscope.
It is interesting to note that the neuroscretory cells in both the nuclei frequently contain nucleolus-like bodies in the cytoplasm of their perikarya. The outline of such an inclusion body is much smoother and more regularly round or oval than that of the intranuclear nucleolus, and the diameter of the body is variable, reaching the maximal size of about 2.5μ, which is approximately the same as the maximal diameter of the true mucleolus. The ultrastructure of the inclusion body is quite akin in appearance to the intranuclear nucleolus, that is, the seeming constituent particles are lined up in beaded filaments or cross-striated bands. A close similarity between the ultrastructure of the inclusion bodies and that of the nucleolus strongly suggests the possibility that the inclusion bodies might be derived from the nucleolus. That corresponds to the light microscopic findings suggesting the extrusion of nucleoli in certain neurosecretory cells previously reported by many authors. The functional significance of the occurrence of such inclusion bodies is not clarified in the present study.