An electron microscope study on the cytoplasmic and nuclear components of rat primary oocytes

Summary

This paper reports on the structure of rat primary oocytes, as observed with the electron microscope. Four main components are described in the cytoplasm: Golgi apparatus, centrioles, mitochondria and multivesicular bodies.

The components of the Golgi apparatus are forming a single mass confined to a limited region of the cytoplasm and the centrioles were found located in a clear zone sited in the middle of this mass. Mitochondria are scattered at random in the cytoplasm. Multivesicular bodies are elements integrated by an enveloping membrane containing a varied number of tiny vesicles. They are generally found associated with a short number of small free vesicles. Only one two groups of this kind are found per oocyte. This contrast with what has been observed previously in full-grown rat oocytes, where the groups are numerous and constituted by many units.

Two components were described for the oocyte nucleus: nucleoli and chromosomes. Nucleoli are constituted by a tangled thread whose elemental component is a fine fibrous material of high electron density.

At the age studied on this paper, primary oocytes are undergoing meiotic prophase, chromosomes have at this time the same components observed by different authors in primary spermatocytes. These are two thick ribbon-like threads helically twisted around a thinner medial filament. Each tripartite group is attached by one end to the nuclear membrane. It was actually seen tripartite groups incompletely organized; the images recorded of such groups suggest that the medial filament is the first to appear in the nucleoplasm. The possible significance of these filaments in respect to the meiotic phase called chromosome pairing is discussed.