From the time an egg begins to develop in one of a woman's ovaries, the granulosa cells serve as a communication system — signaling, for example, when the egg needs nutrients and proteins.
By the end of the follicular phase, the luteinizing hormone surge results in ejection of the oocyte and transforms the remaining follicle into the corpus luteum; small and large luteal cells are formed from theca and granulosa cells, respectively.