The ovaries of mammals contain many immature oocytes but their potential for use in in vitro fertilization has been limited because they fail to mature in vitro. In August 1 Nature Yayoi Obata and colleagues from Gunma University, Maebashi, Japan, show that under specific conditions the most primitive murine fetal oocytes can differentiate into competent oocytes with high efficiency (Nature 2002, 418:497-498).

Obata et al. cultured ovaries from murine fetuses at 12.5 days post coitum, with the mesonephroi attached followed by removal of the mesonephroi and further days of culture. They observed that these ovaries had many secondary follicles containing oocytes that are able to complete meiosis and genomic imprinting in vitro. In addition, after serial nuclear transfer the rate of in vitro fertilization of the reconstituted oocytes was normal (89%) and they developed efficiently into blastocysts (89%).

"As well as offering an opportunity...