['To understand the mechanism of premature ovarian failure (POF).', 'The ultrastructural (electron microscopy) analysis of primordial ovarian follicles in Nobox deficient mice.', 'We studied, for the first time, the fate of oogonia in embryonic (prenatal) mouse ovaries and showed that the abolishment of the transition from germ cell cysts to primordial follicles in the ovaries of Nobox deficient mice is caused by defects in germ cell cyst breakdown, leading to the formation of syncytial follicles instead of primordial follicles.'] Is premature ovarian failure in nobox-deficient mice caused by defects in somatic cell invasion and germ cell cyst breakdown?