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Ovulation is the process in the menstrual cycle by which a mature
ovarian follicle ruptures and discharges an ovum (also known as an
oocyte, female gamete, or casually, an egg) that participates in
reproduction. Ovulation also occurs in the estrous cycle of other
animals, which differs in many fundamental ways to the menstrual cycle.
The process of ovulation is controlled by the hypothalamus of the brain
and through the release of hormones secreted in the anterior lobe of the
pituitary gland, (LH and FSH). In the follicular (pre-ovulatory) phase
of the menstrual cycle, the ovarian follicle will undergo a series of
transformations called cumulus expansion, this is stimulated by the
secretion of FSH. After this is done, a hole called the stigma will form
in the follicle, and the ovum will leave the follicle through this hole.
This release of ovum, ovulation is triggered by a spike in the amount of
FSH and LH released from the pituitary gland. During the luteal (post-ovulatory)
phase, the ovum will travel through the fallopian tubes toward the
uterus, implanting there 6-12 days later if fertilized, or degrading in
the fallopian tubes within 24 hours if not fertilized.
In humans, the few days near ovulation constitute the fertile phase. The
average time of ovulation is the fourteenth day of an average length (twenty-eight
day) menstrual cycle. It is normal and common for the day of ovulation
to vary from the average, however.
Cycle length alone is not a reliable indicator of the day of ovulation.
While in general an earlier ovulation will result in a shorter menstrual
cycle, and vice versa, the luteal (post-ovulatory) phase of the
menstrual cycle may vary by up to a week between women.
The start of
ovulation can be detected by various signs. Because the signs are not
readily discernable by people other than the woman herself, humans are
said to have a concealed ovulation.
Women near ovulation experience changes in the cervix, in mucus produced
by the cervix, and in their basal body temperature. Furthermore, many
women also experience secondary
fertility signs including Mittelschmerz
(pain associated with ovulation) and a heightened sense of smell.
Many women experience heightened sexual desire in the several days
immediately before ovulation. One study concluded that women subtly
improve their facial attractiveness during ovulation.[3
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For ovulation to be
successful, the ovum must be supported by both the
corona radiata and cumulus oophorous granulosa cells.
The latter undergo a period of proliferation and
mucification known as cumulus expansion. Mucification
refers to the secretion of a hyaluronic acid-rich
cocktail that disperses and suspends the cumulus cell
network in a sticky matrix around the ovum. This network
stays with the ovum after ovulation and have been shown
to be necessary for fertilization.
An increase in cumulus cell number causes a concomitant
increase in antrum fluid volume that can swell the
follicle to over 20mm in diameter. It forms a pronounced
bulge at the surface of the ovary called the blister.
The majority of hormonal contraceptives and conception
boosters focus on the ovulatory phase of the menstrual
cycle because it is the most important determinant of
fertility. Hormone therapy can positively or negatively
interfere with ovulation and can give a sense of cycle
control to the woman.
Follicle stimulating hormone, gonadotropin releasing
hormone (GnRH), and estradiol have been purified in the
laboratory. Chemical analogues of estradiol and
progesterone have also been synthesized. Recall that
GnRH is an upstream inducer of both FSH and LH secretion.
Generally, administered FSH or GnRH can induce ovulation
by rapidly accelerating the pace of folliculogenesis,
allowing for conception. Estradiol and progesterone,
taken in various forms including combined oral
contraceptive pills, mimics the hormonal levels of the
menstrual cycle and engage in negative feedback of
folliculogenesis and ovulation.
Ovulation induction is a promising assisted reproductive
technology for patients with polycystic ovary syndrome (PCOS),
oligomenorrhea, endometriosis or male
factor infertility.
Medications that are commonly prescribed to induce
ovulation include Clomid, Gonal-F/Follistim AQ,
Metformin, Bravelle, Menopur and Repronex.
All ovulation-inducing medications carry the risk of
side effects. A recent study has raised the possibility
of a link between ovulation-inducing agents and an
increased risk of ovarian carcinoma.