Lisääntymiskiertokulku

Lisääntymiskiertokulku refers to the cyclical biological processes that prepare a female mammal's body for potential pregnancy and, if pregnancy does not occur, lead to menstruation or a similar shedding of the uterine lining. This cycle is primarily regulated by hormones produced by the hypothalamus, pituitary gland, and ovaries. The key phases are typically follicular, ovulatory, and luteal.

During the follicular phase, follicle-stimulating hormone (FSH) stimulates the growth of ovarian follicles, one of which will eventually mature. As the follicle grows, it produces estrogen, which thickens the uterine lining (endometrium). As estrogen levels rise, they trigger a surge in luteinizing hormone (LH), initiating the ovulatory phase. Ovulation is the release of a mature egg from the ovary.

Following ovulation, the luteal phase begins. The ruptured follicle transforms into the corpus luteum, which produces progesterone. Progesterone further prepares the endometrium for implantation of a fertilized egg and inhibits the release of FSH and LH. If fertilization and implantation do not occur, the corpus luteum degenerates, leading to a drop in progesterone and estrogen levels. This hormonal decline causes the shedding of the thickened endometrium, resulting in menstruation. The cycle then restarts with the next follicular phase. The length and specific characteristics of the reproductive cycle can vary between species and individuals.