Patterns of gene activity during ovum formation in the mouse

Abstract

We have examined the transcriptional activities of the mouse oocyte genome during follicle growth in order to obtain information on the stages at which the synthesis of particular RNA species becomes important. The effects of gonadotrophins on these patterns of transcription have also been investigated. RNA synthesis in oocytes was measured at different stages of growth using a cytochemical application of the standard biochemical assay for DNA-dependent RNA polymerase. The presence of endogenous nucleolar and nucleoplasmic polymerases was detected by autoradiography and their relative activities were quantitated by grain counts. Both activities were relatively low in oocytes of the resting pool of small follicles but increased as growth commenced, reaching a peak as the oocytes approached maximum size. Thereafter RNA polymerase activities declined rapidly and were very low in the oocytes of large, antral follicles. Administration of PMSG to mice had no significant effect on the polymerases of oocytes of small follicles or during the early stages of follicles growth when compared with saline-treated controls. However, in oocytes which had attained maximum size, a statistically significant increase in nucleolar labelling was detected after PMSG treatment. The elevated levels of RNA polymerase activity were confined to oocytes within this growth compartment. These observations are discussed in relation to the way the production of gene transcripts is regulated during the growth of the oocyte, both with respect to the storage of informational molecules and the development of meiotic maturation competence.