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The primate ovarian reserve is established during late fetal development and consists of quiescent primordial follicles in the ovarian cortex each composed of granulosa cells surrounding an oocyte in dictate. As late stages of fetal development are not routinely accessible using human tissues, the current study exploits the evolutionary proximity of the rhesus macaque to investigate follicle formation in primates. Like in humans, the rhesus prenatal ovary develops multiple types of pre-granulosa cells in time and space, with primordial follicles deriving from later emerging pre-granulosa subtypes. In addition, our work shows that activated medullary follicles recruit fetal theca cells to establish a two-cell system for sex-steroid hormone production prior to birth, providing a cell-based explanation for mini puberty.