Prediction of Developmentally Competent Chromatin Conformation in Mouse Antral Oocytes

Abstract

DASZKIEWICZ R., SZYMONIAK M., G¥SIOR £., POLAÑSKI Z. 2016. Prediction of developmentally competent chromatin conformation in mouse antral oocytes. Folia Biologica (Kraków) 64: 59-65.Mouse prophase oocytes isolated from antral follicles may possess two alternative types of chromatin configuration: NSN configuration represents more dispersed chromatin and is characteristic mainly for growing oocytes whereas SN configuration, attained upon oocyte growth, comprises more condensed chromatin with a significant fraction concentrated around the nucleolus. Importantly, fully grown oocytes isolated from antral follicles represent a non-homogenous population in which some oocytes posses NSN-type and others SN-type of chromatin conformation. From these two, only oocytes with SN configuration are able to complete full development upon fertilization. We show that among mouse oocytes isolated from antral follicles, those surrounded by cumulus cells were larger and more frequently possessed SN chromatin than oocytes lacking the complete cumulus cell layer. Females primed with PMSG gave a higher number of oocytes with a complete layer of cumulus cells and the frequency of oocytes with SN chromatin was also elevated. Within the whole population of isolated antral oocytes, we observed subtle variation in size which allowed fractionation of oocytes under a stereomicroscope into groups representing oocytes of slightly different sizes. The occurrence of SN chromatin configuration was highly dependent on the oocyte size and its frequency increased gradually in subsequent size groups reaching 95-100% in the group representing the largest oocytes. These findings demonstrate that the subtle differences in the size of antral oocytes allow prediction of the status of their chromatin, thus providing a simple, fast, non-invasive and non-expensive way to select good quality oocytes for ART purposes in mammals.

A transition phase in late mouse oogenesis impacts DNA methylation of the early embryo

“…When comparing cellular sizes of the three types, we detected a small but significant increase in the oocyte volume in SN-GVOs compared to NSN-GVOs (Fig. 2a ), similar to previous observations 6 , 29 . INT-GVOs are of similar size as NSN-GVOs and are significantly smaller than SN-GVOs (Fig.…”

“…4c, Supplementary Movie 1 and 2). Hence, the timeline of IVM might be useful as a non-invasive marker to discriminate between SN-and NSN-GVOs, as suggested previously 6,21,45 , combined with other non-invasive markers such as size, perivitelline space, and position of the germinal vesicle 29,[46][47][48] .…”