Ovulation

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 conducted 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 greatest fertility for a woman.

The ovulation process occurs around the fourteenth day of the normal menstrual cycle (which usually lasts 28 days) and indicates the phase of the ovarian cycle during which a woman is able to conceive.

In order to avoid dangerous misunderstandings, let us 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 tubes. . If not fertilized, the ovum undergoes a rapid disintegration after 24-36 hours from ovulation, therefore from its release in the tubes.
In practice, therefore, a woman with a regular 28-day cycle is fertile for about 4-5 days a month.

Ovulation occurs when a mature oocyte (called ovum or egg cell) is released from the Graafian follicle (at the level of the ovary) to be induced to cross the uterine tubes. Consequently, the mature oocyte becomes available for eventual fertilization.
At the same time, the lining of the uterus thickens in order to be able to receive a fertilized egg. If fertilization and implantation of the zygote do not occur, menstrual flow appears about 14 days after 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 to ovulation is on average 14 days, but there are considerable variations both between women and women, as well as between one cycle and the other 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 brings the follicle to its maximum size and causes a visible swelling on the surface of the ovary. The area where this bulge forms takes on the appearance of a whitish vascular 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 capable of breaking down proteins, carbohydrates, esters, etc.), such as plasmin and collagenase, and it is not an explosive phenomenon, but a gradual one. In fact, the pressure of the liquid follicular reduction progressively 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) are able to capture the "oocyte, which is pushed inside their pavilion.
At the moment of ovulation not only the stigma breaks: the membrane that separated the theca from the granulosa also breaks, determining the vascularization of the latter and forming a clot in the center of the ovulated follicle; this structure begins the transformation into corpus luteum.

The ovulation process is controlled by the hypothalamus, through the release of hormones secreted by the anterior lobe of the pituitary: 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 takes a maximum of 36 hours to complete.

The ovarian cycle can be arbitrarily divided into three phases: preovulatory (follicular), ovulatory and postovulatory (luteal).

Pre-ovulatory phase

Relationship of the follicular phase of the ovarian cycle and the 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 follicogenesis, the process of growth and differentiation of the follicles, some (about ten) reach maturity, but only one of them will ovulate, freeing its oocyte.

Follicogenesis is the fundamental process that predisposes to the ovulatory phase, both from a histological point of view (maturation of the dominant follicle) and from a hormonal point of view. In the pre-ovulatory phase:

Increases the secretion of gonodotropins (especially LH) about 36 hours before ovulation.
The follicle increases its size (the follicular fluid increases).
The oophorus heap secretes hyaluronic acid due to the effect of gonadotropins. Hyaluronic acid disperses the cells of the cumulus oophore and incorporates them into an adhesive cell matrix similar to mucus. This process is called mucification of the cumulus: the extracellular network that is thus formed remains with the egg cell after ovulation and results be necessary for the outcome of fertilization.

Ovulatory phase

Relation of the ovulatory phase of the ovarian cycle and the 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 spermatozoa.If fertilization does not occur, the egg 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 stigma.
The cumulus-oocyte complex leaves the ruptured follicle and exits into the peritoneal cavity, where it is received by the fimbriae at the end of the tube (oviduct).
After entering the oviduct, the cumulus-oocyte complex is pushed through the fallopian tubes by cilia, starting its journey to the uterus.
By this time, the oocyte completed meiosis I, producing two cells: the larger secondary oocyte that contains all the cytoplasmic material and a smaller, inactive one: the first polar body.
Meiosis II follows immediately, but will be arrested in metaphase and will remain so until fertilization, in a state of quiescience.
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)

Post-ovulatory phase relationship ovarian cycle and menstrual cycle

The luteal phase lasts 14 days. During this period, the corpus luteum secretes progesterone, which prepares the uterine lining 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 luteal (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 inwards on itself, transforming itself into the corpus luteum, an endocrine gland that produces estrogen and progesterone.These hormones cause the endometrial glands to start producing proliferative endometrium and later secretory endometrium in order to prepare the site for embryonic growth if fertilization occurs. The action of progesterone increases the basal temperature.
The corpus luteum continues this paracrine (hormone secreting) action for the rest of the menstrual cycle, keeping the endometrium active, before disintegrating into scar tissue during menstruation.