MITOCHONDRIAL AND NUCLEAR INTERACTION

One of the things that differentiates cells from "bags of enzymes" is their ability to have the right thing at the right place at the right time. How they do it is one of the central problems of cellular physiology. That their activities are ordered is easily demonstrated, but changes in spatial distribution of cellular components with change in function has not often been easily observable. PoUister, Gettner, and Ward (1) have described the structure of the nuclear membrane of the early oocyte of the tadpole of the green frog, Rana clamitans. The membrane consists of an elaborate net of pore-like rings with processes extending from the nucleus through the rings and a few tenths of a micron into the cytoplasm. This note involves a demonstration of a remarkable relationship which occurs during a limited phase of this early period of development, between these nuclear processes and the mitochondria of the oocyte. These observations represent part of the results of the continuing studies on oocyte maturation that have been going on at Columbia University. This particular relationship became apparent only recently when we began to use rather thick sections for survey and orientation. Fig. 1 shows a phase contrast micrograph of a 1 micron section of an oocyte in this special phase of its development. Numerous mitochondria and cytoplasmic granules can be discerned. Right along two regions of the nuclear membrane, clear cut aggregations of granular material are evident. Most of the remainder of the membrane appears rather thick, but no detail can be made out. Fig. 2 is an electron micrograph of a 0.2 micron section of the same phase of oocyte development. A thick section is shown to include a sufficient number of nuclear membrane processes to make their relationship to the aggregations along the membrane more apparent. The membrane passes somewhat obliquely through the section, and details of a number of the rings may be made out. As can be seen, the processes converge on and seem to "fuse" with the aggregations. These consist in part of groups of mitochondria and denser material with ill defined internal structure, not unlike that of the substance of the processes themselves. Figs. 3 and 4 show portions of the previous field at higher magnification. Here we can see that the mitochondria are closely applied to the denser material which seems to be continuous with the nuclear membrane processes-351