Fig. 3. The ultrastructure of the R-rings is indistinguishable from that of the nucleolar channel system. (A) Immunoelectron micrograph of an R-ring induced by NoppR in COS cells and labeled with Nopp140 antibodies as in Fig. 2G. (B) Electron micrograph of a nucleolar channel system (NCS) induced by progesterone in a human endometrial cell in the secretory phase. Reproduced from The Journal of Cell Biology (Terzakis, 1965) by copyright permission of The Rockefeller University Press. A section of the nuclear envelope (nuclear membrane, NM) is visible. Note the granular matrix that embeds both tubular membrane systems and the halo-like appearance of their surrounding nucleoplasm. Bars, 0.2 µm.
