Final answer:
The proliferative phase of the menstrual cycle is regulated by estrogen, which is produced by the granulosa and theca cells of the ovary, stimulating the regrowth of the uterine lining after menstruation.
Step-by-step explanation:
Estrogen produced by the granulosa and theca cells of the ovary controls the proliferative phase of the menstrual cycle. During this phase, estrogen stimulates the rebuilding of the endometrial lining of the uterus following menstruation.
The menstrual cycle is composed of three phases with the proliferative phase being the second one. It comes after the menses phase and before the secretory phase. Once menstrual flow ceases, the granulosa and theca cells of the tertiary follicles in the ovaries start producing increased levels of estrogen. This rise in estrogen facilitates the regrowth of the endometrium, preparing it for potential implantation should fertilization occur.
These biological events are part of the complex coordination between the ovarian and menstrual cycles governed by the hormones of the hypothalamus, pituitary, and ovaries. The female reproductive system functions in a cycle averaging 28 days, where the ovarian cycle involves preparing eggs for release, and the menstrual cycle prepares and maintains the uterine lining accordingly.