The primordial follicles consist of an oocyte surrounded by flat cells. These cells are called pregranulosa cells.
These follicles form in the fetal period of a woman in large numbers. However, many of them will die from atresia. Only 1-2 million primordial follicles will remain at the time of a woman's birth, 500,000 at puberty and will reach exhaustion at menopause.