Cytoplasmic fine-structure

Abstract

In any attempt to reach an integrated conception of the cytoplasm in variation and development, a study of the fine-structure of the cytoplasm and its relation to the nucleus must take its proper place. It is the object of our paper to survey, as adequately as we are able in a limited space, selected data on cytoplasmic fine-structure and we hope that this will provide the Discussion with a background against which to consider a morphological basis for that variation which genetical studies may show to be due to changes in the organization of the cytoplasm. It is possibly too early as yet to hope that examination of the morphology of cells by means of the electron microscope will reveal cytoplasmic differences between organisms which differ from one another in the characteristics studied in genetical experiments; it would be useful to the future study of the problem of Cytoplasmic change, however, to know within what limits speculation must be reasonably confined by the nature of the fine-structure of the cytoplasm. It is now becoming apparent that though cells of organisms widely separated phylogenetically have basic similarities, cellular specialization has led to some diversity in the fine-structure. In the first part of the paper we shall briefly consider the development of electron-microscope methods, e.g. the thin-sectioning procedures, which have made it possible to examine biological material at a resolution which allows comparatively small macromolecular units to be distinguished (10 to 50 Å); at the same time we shall emphasize the danger of overestimating the significance of the observations that have been made. In the second part we shall consider certain selected features of the cell in some detail; in view of the large body of literature on cell fine-structure that is now available (publications numbered over 100 during the last 6 months of 1956) no attempt will be made to review all the findings which have been published during the last few years. Rather we will consider, in general terms, the structure of each component, then compare the variations in structural form noted in different cell types and indicate where there is direct disagreement in the findings of various authorities.