Ovulation corresponds to the moment in which the oocyte is released from the Graafian follicle at the level of the ovary. The egg thus released is led into the uterine tube, where it can be fertilized by the spermatozoa, and reach the uterus.
The ovulatory phase includes many different aspects, and, more generally, corresponds to the days of greater fertility for a woman.
The process of ovulation takes place around the fourteenth day of the normal menstrual cycle (whose duration is generally 28 days) and indicates the phase of the ovarian cycle during which a woman is able to conceive.
In order to avoid dangerous misunderstandings, we immediately clarify that a woman is able to conceive even by consuming one or more sexual intercourse in the days preceding ovulation, since spermatozoa can survive even for 3-4 days inside the uterine cavity and the tubes . If not fertilized, the egg undergoes a rapid breakdown after 24-36 hours from ovulation, then from its release in the tubes.
In practice, therefore, a woman with a regular cycle of 28 days is fruitful for about 4-5 days a month.
Ovulation occurs when a mature oocyte (called an egg or egg cell) is released from the Graafian follicle (at the level of the ovary) to be induced to pass through the uterine tubes. As a result, the mature oocyte becomes available for possible fertilization.
At the same time, the lining of the uterus thickens, to be able to receive a fertilized egg. If fertilization and implantation of the zygote does not occur, the menstrual flow appears after about 14 days from ovulation.
In humans, the few days preceding the impending ovulation constitute the fertile phase . The time that elapses from the beginning of the menstrual period until ovulation is on average 14 days, but there are considerable variations both between women and women, and between one cycle and another in the same woman. On the contrary, the period between ovulation and the onset of subsequent menstruation is less variable and is equal to 14 days.
What happens during ovulation
The growth that occurs during the pre-ovulatory phase leads the follicle to reach its maximum size and causes a visible swelling on the surface of the ovary. The area where this bulge is formed takes on the appearance of a vascular, whitish area, known as a stigma. At that point the follicle is separated from the peritoneal cavity only by a thin layer of cells.
The rupture of the follicle is due to the direct action of lytic enzymes (enzymes able to split proteins, carbohydrates, esters, etc.), such as plasmin and collagenase, and is not an explosive phenomenon, but a gradual one. In fact, the pressure of the follicular fluid is progressively reduced during the action of enzymes on the stigma: the oocyte is slowly expelled from the ovary together with the viscous fluid of the antrum. The fimbriae of the uterine tubes (oviducts that connect the ovaries with the uterus) manage to capture the oocyte, which is pushed into their pavilion.
At the time of ovulation not only does the stigma break: even the membrane that separated the theca from the granulosa breaks, causing the vascularization of the latter and forming a clot in the center of the ovulated follicle; this structure begins the transformation into a corpus luteum .
The ovulation process is controlled by the hypothalamus, through the release of hormones secreted by the anterior pituitary lobe: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In the pre-ovulatory phase of the menstrual cycle, the ovarian follicle will undergo a series of transformations called cumulus expansion, which is stimulated by FSH. After its maturation, the ovulatory phase requires a maximum of 36 hours to complete.
The ovarian cycle can be arbitrarily divided into three phases: preovulatory ( follicular ), ovulatory and postovulatory phase (luteinic) .
Pre-ovulatory phase
Phase follicularization of the ovarian cycle and menstrual cycle
- The pre-ovulatory phase (understood as the follicular phase) lasts about 14 days.
- Menstruation coincides with the beginning of the follicular phase.
- During the follicogenesis, the process of growth and differentiation of the follicles, some (around ten or so) reach maturity, but only one of them will undergo ovulation, freeing its own oocyte.
Follicogenesis is the fundamental process that predisposes to the ovulatory phase, both from the histological point of view (maturation of the dominant follicle) and from the hormonal one. In the pre-ovulatory phase:
- Increases gonodotropin secretion (especially LH) about 36 hours before ovulation.
- The follicle increases its size (the follicular fluid increases).
- Cumulus oophorus secretes hyaluronic acid due to gonadotropins. Hyaluronic acid disperses the cumulus oophorus cells and incorporates them into an adhesive cellular matrix similar to mucus. This process is called cumulation of the heap : the extracellular network that is thus formed remains with the egg cell after ovulation and appears to be necessary for the outcome of fertilization.
Ovulatory phase
Relationship ovulatory phase of the ovarian cycle and menstrual cycle
During ovulation, the secretion peak of FSH (follicle-stimulating hormone) and LH (luteinizing hormone) is reached. The intervention of these hormones causes the release of the oocyte from the follicle, which is dragged into the uterine tubes, where it can be fertilized by the spermatozoa . In the event that fertilization does not occur, the oocyte will degenerate within approximately 24-32 hours after ovulation.
- Through a signal transduction cascade initiated by LH, proteolytic enzymes are secreted from the follicle which degrade the follicular tissue producing the stigma .
- The cumulus-oocyte complex leaves the broken follicle and exits into the peritoneal cavity, where it is received by the fimbriae at the end of the tuba ( oviduct ).
- After entering the oviduct, the cumulus-oocyte complex is pushed by cilia through the tubes, starting its journey towards the uterus .
- From this moment, the oocyte completed meiosis I, producing two cells: the largest secondary oocyte containing all the cytoplasmic material and a smaller, inactive one: the first polar body .
- Meiosis II follows immediately, but will be stopped in a metaphase and so will remain until fertilization, in a state of quiescence.
- The spindle apparatus of the second meiotic division appears at the time of ovulation.
- The mucous membrane of the uterus has reached its maximum size and the endometrial glands have developed, although they are not yet secreting.
Post-ovulatory phase (luteal phase)
Report post-ovulatory ovarian cycle and menstrual cycle
The luteinic phase lasts 14 days. During this period, the corpus luteum secretes progesterone, which prepares the uterine mucosa for fertilization. Once released, the egg is able to be fertilized for 12 to 48 hours before it begins to degenerate.
Ovulation is the release of a single mature egg from a follicle that has developed in the ovaries. It usually occurs regularly, around the fourteenth day of a 28-day menstrual cycle.
During the luteinic (post-ovulatory) phase, the egg will travel through the fallopian tubes to the uterus: the egg can implant itself there, if fertilized by a sperm, 6-12 days later.
- Without the egg, the follicle folds inward, on itself, turning into a corpus luteum, an endocrine gland that produces estrogen and progesterone . These hormones induce the endometrial glands to begin production of proliferative endometrium and later of secretory endometrium, in order to prepare the site of embryonic growth, if fertilization occurs. The action of progesterone increases the basal temperature.
- The corpus luteum continues this paracrine action (secretes hormones) for the rest of the menstrual cycle, keeping the endometrium active, before disintegrating into scar tissue during menstruation.
