Final answer:
At stage 0 of the ovarian and menstrual cycles, the ovaries are in the follicular phase beginning to develop follicles under the regulation of FSH; the HPG axis initiates the cycle via hormone signaling; and the uterus is in the menstruation stage, shedding the endometrial lining unless fertilization occurs.
Step-by-step explanation:
At stage 0 in the ovarian and menstrual cycle, the ovaries are in the initial part of the follicular phase, where the levels of Follicle Stimulating Hormone (FSH) slowly begin to rise, causing the growth of follicles on the surface of the ovary and preparing the egg for ovulation. As the follicles develop, they start releasing estrogens. Concurrently, the Hypothalamic-Pituitary-Gonadal (HPG) axis function is initiated by the hypothalamus that sends Gonadotropin-Releasing Hormone (GnRH) to the anterior pituitary gland, which releases FSH and Luteinizing Hormone (LH). The uterus, on the other hand, is in the menstruation phase, shedding the functional layer of its endometrium. If a fertilized egg does not implant in the uterus, the corpus luteum degenerates, and hormone levels of estrogen and progesterone decrease, causing the endometrium to slough off and initiating the next menstrual cycle.