Ovarian follicles. Light micrograph of a primary (lower left) and secondary (upper right) ovarian follicles containing oocytes (ova) at their centres. At birth there are tens of thousands of primordial follicles, the precursors to primary follicles, present in the ovaries. Once sexual maturity is reached, primordial follicles are triggered to start developing into primary follicles every menstrual cycle. In the secondary follicle stage the chromosome-containing oocyte enlarges and is surrounded by follicular cells (dark brown dots). One secondary follicle per month becomes a Graafian follicle, releasing its egg at ovulation.