- What is the ovarian cycle?
- Which are the phases of the ovarian cycle?
- Which are the principal hormones involved?
- What happens if the egg is fertilized?
What is the ovarian cycle?
The ovarian cycle takes place alongside the menstrual cycle, lasts about 28 days and consists in a series of periodical changes that happen in the ovaries allowing them to generate ovules and hormones. However, some sources define these phases as proliferative and secretory menstrual phases. Menstrual cycles are counted from the first day of menstrual bleeding.
Which are the phases of the ovarian cycle?
The ovarian cycle is made up of three distinct phases:
- Follicular phase: during which an ovarian follicle that is a small ovarian structure inside which the oocyte is developed, reaches its maturity;
- Ovulatory phase: during which the wall of an ovary breaks at the level of the Graafian follicle, which contains a mature oocyte that is expelled from the ovary;
- Luteal phase: characterised by the transformation of the Graafian follicle into a corpus luteum and by the secretion of progesterone – the hormone that is in charge of preparing the feminine system to a pregnancy – by the temporary endocrine gland; if the egg is not fertilized, the corpus luteum degenerates at the end of the ovarian cycle.
Which are the principal hormones involved?
Through the central nervous system, the hypothalamus releases the gonadotrophin-releasing hormone (GnRH), which regulates the release of LH and FSH, which then subsequently regulate the secretion of oestrogen. The development of primary follicles indicates the beginning of the ovarian cycle with the follicular phase during which 12-20 primary follicles start developing and maturing to form secondary follicles due to the high levels of the follicle-stimulating hormone (FSH) and of oestrogen. Primary follicles are formed from primordial follicles that are developed in the fetus’s ovaries during pregnancy with their maturation ending in a phase of the cell cycle.
About 16 hours before ovulation a peak in LH can be observed; at ovulation, the follicle breaks and the corpus luteum that secretes progesterone and oestrogen is released. About seven days after ovulation, if fertilization hasn’t occurred, the corpus luteum degenerates and the production of hormones starts to decline. At this point, the post-ovulation phase, which lasts on average 14-16 days, begins. By day 9 of the ovarian cycle, only one secondary follicle, known as the dominant follicle, remains. This follicle is responsible of the production of big quantities of oestrogen during the late follicular phase.
On the 14th day of the cycle, during ovulation, a peak in the luteinizing hormone (LH) due to the positive feedback of oestrogen is observed. This allows the development of the secondary follicle into a tertiary follicle that is then released by the ovary.
What happens if the egg is fertilized?
During the development of the tertiary follicle, the primary oocyte completes its first cell division, with subsequent formation of a polar body and a secondary oocyte. The empty follicle forms a corpus luteum that is responsible of progesterone release to maintain a potential pregnancy making the endometrium receptive to the embryo’s implantation. At the same time, to allow a potentially fertilized egg to implant itself in the uterus’s mucosa, the menstrual cycle takes place. This cycle isregulated by the hormones produced by the ovaries and the hypophysis and takes place in all women from puberty to menopause, only interrupted during pregnancy.