THE ORGANIZATION OF NUCLEI AND CHROMOSOMES IN HONEYBEE EMBRYONIC CELLS

Electron microscopy of thin-sectioned material has provided little insight into the macromolecular organization of nuclei and chromosomes. Recently a "whole mount" technique, originally used by Kleinschmidt' with bacteria and viruses, has been applied by several authors2-4 to nucleated cells. In this technique, entire cells are spread on the air-water interface of a Langmuir trough, picked up directly on a coated grid, dehydrated, and dried by the critical point method of Anderson.5 Such preparations, which may or may not be fixed and stained, make possible direct examination of the fine structure of entire metaphase chromosomes and of whole interphase nuclei. Examination of a wide range of material by this method2 4 has shown that both nuclei and chromosomes contain a system of long, irregularly organized fibers, whose diameter according to most authors varies around 250 A; the occurrence of this approximate dimension across a range of both vertebrates and invertebrates has been impressive. Similar fibers have recently been demonstrated in thinsectioned calf thymus nuclei,6 and evidence is accumulating that the chromosomal fibers are nucleoprotein in nature, 6 that they are the sites of messenger RNA synthesis,6 and that they are organized as a central core surrounded by a sheath of distinctly different physical and chemical properties.2' 4 In the work reported here, the Kleinschmidt technique has been applied to embryonic cells of the honeybee.7 This material is especially suitable for whole mount electron microscopy, due to the remarkably small size of the chromosomes (whose lengths and diameters are less than those of most vertebrate mitochondria) and to the correspondingly low concentration of chromatin in the interphase nuclei.