Higher quality rather than superior quantity of oocytes determine the amount of fertilizable oocytes in two outbred Dummerstorf high-fertility mouse lines

Dummerstorf fertility lines FL1 and FL2 represent two models of enhanced fertility characterized by the doubling of the litter size compared with an unselected control population (ctrl line, Dummerstorf FztDU). Both biodiverse FLs managed to reach this goal by increasing the ovulation rate per cycle, even showing decreased pregnancy rate and irregular oestrous cycle and metabolic hormone levels, compared with ctrl. The aim of the present study was to analyse oocytes in terms of quality and quantity by comparing the entire pool of oocytes per ovary, with those from the antral follicles within the same animal. We performed Brilliant Cresyl Blue staining as a non-invasive marker of oocyte quality in combination with an analysis of additional morphological indicators, e.g. cytoplasm clarity, cumulus cell layers, nuclear anatomy, size and shape. We compared our fertility lines with the unselected control population and with another independent line selected from the same founder population, showing lower litter size (DU6P). Our results suggest that fertility lines show decreased number of oocytes per ovary compared with DU6P but increased number of high-quality oocytes before ovulation. Hence, the raise in the ovulation rate and litter size of those super fertile mouse lines are not associated with an increased number of oocytes per ovary but rather with an increased number of higher quality fertilizable oocytes per cycle. In addition, the most conspicuous method to acquire oocytes with the highest quality in our lines is to assess their morphology, rather than their status after staining. All these discoveries together may be of fundamental importance for further studies in livestock farm animals showing some similar characteristics, e.g. irregular cycle or hormonal misbalances, to improve production while lowering costs, and in humans to increase the possibilities of successful pregnancies for couples undergoing in vitro fertilization (IVF).