The whole reproductive cycle consists of an ovarian cycle and a uterine cycle. Figure illustrates one complete reproductive cycle. You can observe in the figure the - interrelationship among the hypothalamus, pituitary, ovaries and uterus. The average reproductive cycle is spread over 28 days. It commences at puberty and generally continues throughout the reproductive years. The cycle readies the reproductive system for pregnancy. Refer to figure frequently as you read the following discussion. It will assist you to understand the close relationship between the two components of the whole reproductive cycle.
Figure: interrelationship of the hypothalamus, pituitary, ovaries and endometrium. One complete menstrual cycle shown. Note that the cyclic activity of the ovary is intimately linked with the changes in the endometrium.
The hypothalamus synthesizes the gonadotropins releasing hormone (GnRH) which stimulates the anterior pituitary to secrete follicle stimulating hormone (FSH) and luteinizing hormone (LH). The ovarian cycle starts with the phase of follicular growth in the ovary after the end of preceding, menstrual cycle and lasts for about 14 days. In the first half of this period FSH stimulates the growth of 5-12 primary follicles out of that usually only one matures at a time. The maturing follicle secretes estrogen that exerts a feedback control on anterior pituitary inhibiting further FSH production therefore preventing maturation or more follicles. The follicular phase comes to an end along with the maturation of one follicle and its release from the ovary leading to ovulation, on day 14. At the time of ovulation the ovum is in the secondary oocyte stage surrounded by the non-cellular zona pellucida and follicle cells which make the zona radiata (or corona radiata). Following ovulation, from day 15 to 28 production of LH from anterior pituitary stimulates the formation of corpus luteum from the follicle cells left at the back in the ovary. The corpus luteum secretes the hormone progesterone which makes the endometrium for implantation of embryo and later maintenance of pregnancy during early stages. If the ovum is not fertilized, the corpus luteum degenerates and menstruation starts again.