Final answer:
The follicular phase involves rising levels of FSH and estrogen, leading to the development of ovarian follicles and the regeneration of the endometrial lining in the uterus, preparing for potential pregnancy.
Step-by-step explanation:
The Follicular Phase in the Ovarian and Menstrual Cycles
During stage 1 of the ovarian and menstrual cycles, known as the follicular phase, there is a series of events involving the ovaries, Hypothalamic-Pituitary-Gonadal (HPG) axis, and the uterus. Follicle-Stimulating Hormone (FSH) levels slowly rise, stimulating the growth of multiple follicles on the surface of the ovaries. Out of these, typically only one matures into a Graafian follicle, poised to release an egg during ovulation. Concurrently, the maturing follicles increase estrogen production which has a few direct impacts on the reproductive cycle:
- Estrogen helps to rebuild the endometrium of the uterus, preparing it for possible pregnancy.
- The rise in estrogen eventually leads to a decrease in FSH due to negative feedback, preventing the maturation of other potential follicles.
- The estrogen surge eventually triggers an LH surge as a result of positive feedback, setting the stage for ovulation.
It's important to note that the initial days of the follicular phase overlap with menstruation, where the functional layer of the endometrium is sloughed off. After this, the proliferative phase of the menstrual cycle begins, characterized by the regrowth of the endometrial lining.