Electron microscope study on meiosis

Summary

The cycle of the nucleolus and sex chromosome was studied with the electron microscope during the following stages of spermatogenesis and spermiogenesis of Gryllus argentinus: (1) spermatogonia; (2) prophase cyte I, leptotene, part of pachytene, and end of diplotene till breakdown of the nuclear envelope; (3) division I, metaphase and anaphase; (4) cyte II, prometaphase; (5) division II, metaphase, anaphase and telophase; (6) early and late spermatids. Some observations were also carried out in the primary oocyte until beginning of the growth period.

It was found that nucleolus and sex chromosome are associated, at first without mixture of their components (leptotene) and afterwards interchanging components (pachytene). The interchange takes place by the passage from one element to another of filamentous units ot low electron density, similar in appearance to those existing in the medial plane of tripartite groups (synaptinemal complexes).

At pachytene the primary results of interchange are: (1) the nucleolus contains filaments of chromosomal nature; (2) the nucleolus emits a long rod-like prolongation containing a cylindrical bundle of filaments, and an axial unit of the same nature; two equidistant clear spaces separate the axial unit from the cylindrical bundle and the latter from the dark wall of nucleolar material. At the end of diplotene these components are found organized in two bodies and a “prolongation”. One of the bodies is formed by a number of alternatively dark and light bands, the other by a pack of tubular units each showing the structure of the former nucleolar prolongation. The “prolongation” is either formed as in the preceding stage or it is composed of five ribbons, two dark ones outside and three light ones between them. It is supposed that both bodies are united by the “prolongation” but no definite proof was obtained. It is assumed that the complex thus constituted represents the sex chromosome.

The sex chromosome was found at any phase of both divisions as well as at the intermediate stages between them; at the division phase the “chromosome” is separated from the autosomes and moves independently of them.

The element could not be traced at telophase II but it reappears within the reorganized nuclei of spermatids. Amorphous nucleolar-like material and chromosome-like material are found associated at this stage with banded complexes like those seen at the end of prophase I. All these components undergo involution during spermatid maturation. At the final step of maturation no traces of them are found.

A similar association of nucleolus and chromosome was found at prophase of primary oocytes of the same species. The associated body is of the same structure as that described for primary spermatocytes. The structures existing in the primary oocytes disorganize at the beginning of growth. At this time the nucleolus has developed into a large body containing masses of chromatin-like material.