A possible mechanism for the nucleolar extrusion in nerve cells of the mouse hypothalamus.

The hypothalamus of adult male mice was investigated with the electron microscope. In all hypothalamic nuclei examined the nerve cells frequently contained nucleolus-like inclusion bodies in the cytoplasm. These inclusion bodies appeared to be composed of the same constituent particles as those of the intranuclear nucleolus. This permits consideration of their probable nuclear origin. The accessory nucleolus, tentatively named in the present paper, appearing to consist of nucleolar material with the association of chromatin was frequently encountered attached to the nuclear membrane and sometimes located at evaginated portions of the nucleus. Moreover, near the nucleus in the cytoplasm, inclusion bodies showing a close resemblance in appearance to the accessory nucleolus except for the absence of the association of chromatin particles were observed to be enveloped with a double membrane probably derived from the nuclear envelope. From these results, it was concluded that the direct translocation of the nucleolar substance of accessory nucleoli into the cytoplasm with an investment of a double nuclear membrane may be possible in nerve cells of the mouse hypothalamus.