SIGNIFICANCE OF CELL PARTICULATES AS SEEN BY ELECTRON MICROSCOPY

I t is becoming increasingly common these days to encounter reports of electron microscope observations on tumor cells or fractions of tumor cell^.^-^ Almost without exception, emphasis is placed on the denser particulates that may be evident, with an implication that they are of viral character and involved in the malignancy of the cell showing them. The majority of the published micrographs show clearly that the fine structure of the cells has suffered from fixation and other procedures used to prepare them for microscopy and little, if any, attention is given to other components of the cell or to micrographs of comparable normal cells. Special interest in these dense particulates derives, of course, indirectly from the existence of chicken tumors4 and the mouse mammary tumor,5 in which the activity of a virus-like agent can be demonstrated. This excuse, notwithstanding the present particle hunt, is reminiscent of the search for a cancer microbe in vogue some 50 years ago6. These particulates and inclusions of an earlier time uniformly failed to induce tumors when tested experimentally, and a similar fate may be the lot of the virus-like bodies of the present day. Be that as it may, a steady increase in interest and a parallel improvement in the results continues in electron microscopy of tissue cells. The volume of observations by such means will certainly grow, therefore, and will make electron microscopy a standard laboratory procedure in the study of cells. With such prospects in the offing, it becomes the responsibility of those making the observations to seek, first, to interpret the electron microscope image of normal cells and to determine which submicroscopic structures are common to all cells and to what extent these vary in the normal. Studies which do not recognize these considerations seem likely to have only a very limited value. In an attempt to live up to this ideal, we have sought in our own studies to define, first, the fundamental fine structure of tissue cell cytoplasm. We intend to review this quickly and give special attention, in keeping with the occasion and the times, to a certain species of particulate which appears as one of the more interesting submicroscopic components. The observations have been made for the most part on various types of cells grown in vitro. of tissues permits equivalent and even broader studies of cell structure, but thus far we have used such preparations only to confirm findings made in the cultured cells. The cells are grown by standard culture techniques on surfaces previously coated with a resin (Formvar). When a satisfactory collection of thinly spread cells has grown out, they are Recent improvements in the fixation7 and sectioning88