Ovulation
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 animals, which differs in many fundamental ways to the menstrual cycle.
Note: This article deals primarily with human ovulation; nonhuman animal ovulation is touched on briefly at the conclusion.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 pituatary 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 menstural cycle, and vice versa, the luteal (post-ovulatory) phase of the menstrual cycle may vary by up to a week between women.
Strictly defined, the ovulatory phase spans the period of hormonal elevation in the menstrual cycle. The process requires a maximum of thirty-six hours to complete, and it is arbitrarily separated into three phases: periovulatory, ovulatory, and postovulatory.
Prerequisite events
 |
Histology of the preovulatory follicle |
Through a process that takes approximately 375 days, or thirteen menstrual cycles, a large group of undeveloped
primordial follicles dormant in the
ovary is grown and progressively weaned into one
preovulatory follicle. Histologically, the preovulatory follicle (also called a mature Graffian follicle or mature tertiary follicle) contains an oocyte arrested in prophase of
meiosis I surrounded by a layer corona radiata granulosa cells, a layer of mural granulosa cells, a protective basal lamina, and a network of blood-carrying capillary vessels sandwiched between a layer of theca interna and theca externa cells. A large sac of fluid called the antrum predominates in the follicle. A "bridge" of cumulus oophorous granulosa cells (or simply cumulus cells) connects the corona-ovum complex to the mural granulosa cells.
Simply stated, the granulosa cells engage in bidirectional messenging with the theca cells and the oocyte to facilitate follicular function. Research is elucidating the specific factors used in follicular messenging at a rapid pace, but such discussion is beyond the scope of this article.
By the action of
luteinizing hormone (LH), the preovulatory follicle's theca cells secrete
androstenedione that is aromatized by mural granulosa cells into
estradiol, a type of
estrogen. High levels of estrogen have a stimulatory effect on
hypothalamus gonadotropin-releasing hormone (GnRH) that in turn stimulates the expression of
pituitary LH and
follicle stimulating hormone (FSH).
The building concentrations of LH and FSH marks the beginning of the periovulatory phase.
Periovulatory phase
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.
Ovulatory phase
Through a signal transduction cascade kicked off by LH,
proteolytic enzymes are secreted by the follicle that degrade the follicular tissue at the site of the blister, forming a hole called the
stigma. The ovum-cumulus complex leaves the ruptured follicle and moves out into the
peritoneal cavity through the stigma, where it is caught by the fimbriae at the end of the
fallopian tube (also called the oviduct). After entering the oviduct, the ovum-cumulus complex is pushed along by
cilia, beginning its journey toward the
uterus.
By this time, the oocyte has completed
meiosis I, yielding two cells: the larger
secondary oocyte that contains all of the cytoplasmic material and a smaller, inactive first polar body.
Meiosis II follows at once but will be arrested in the
metaphase and will so remain until fertilization. The
spindle apparatus of the second meiotic division appears at the time of ovulation. If no fertilization occurs, the oocyte will degenerate approximately twenty-four hours after ovulation.
The
mucous membrane of the
uterus, termed the
functionalis, has reached its maximum size, and so have the endometrial glands, although they are still non-secretory.
Postovulatory phase
The follicle proper has met the end of its lifespan. Without the ovum, the follicle folds inward on itself, transforming into the
corpus luteum (pl. corpus lutea), a steriodogenic cluster of cells that produces
estrogen and
progesterone. These hormones induce the endometrial glands to begin production of the
proliferative endometrium, the site of embryonic growth if fertilization occurs. The corpus luteum continues this paracrine action for the remainder of the menstrual cycle, maintaining the endometrium, before disintegrating into scar tissue during menses.
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.
Beyond these physiological signs, a study suggests that women's preferences in men shift during ovulation, appealing toward a more primitive drive to find a suitable mate. Another study has concluded that women subtly improve their facial attractiveness during ovulation. The significance and accuracy of these studies have been hotly debated among the medical and scientific professions.
Research spearheaded by Baerwald
et al. suggests that the menstrual cycle may not regulate follicular growth as strictly as previously thought. In particular, the majority of women during an average twenty-eight day cycle experience two or three "waves" of follicular development, with only the final wave being ovulatory. The remainder of the waves are
anovulatory, characterized by the developed preovulatory follicle falling into atresia (a major anovulatory cycle) or no preovulatory follicle being chosen at all (a minor anovulatory cycle).
The phenomenon is similar to the follicular waves seen in
cows and
horses. In these animals, a large cohort of early tertiary follicles develop consistently during the follicular phase of the menstrual cycle, suggesting that the endocrine system does not regulate folliculogenesis stringently.
While seen as a revelation by some in the medical community, researchers of
fertility awareness or
natural family planning methods discovered follicular waves in the 1950s. These methods of family planning have always taken multiple follicular waves into account, and this research does not challenge their effectiveness.
The majority of
oral 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 the form of
oral contraceptives, mimics the hormonal levels of the menstrual cycle and engage in negative feedback of folliculogenesis and ovulation.
Some interesting aspects can be noted here:
*Ovulation in
camels is induced by male
pheromones. In
caravans without bulls female camels don't have an
estrus.
*In
cats ovulation is induced mechanically by the male through
copulation.
*
Chickens have an ovulation almost every day.
*The
embryos of some
Marsupial species enter
embryonic diapause (or
delayed implantation) after
fertilization.
*
Anovulatory cycle*
Corpus luteum*
Folliculogenesis*
Menstrual cycle*
Oogenesis* Baewald AR, Adams GP, Pierson RA. 2004.
A new model for ovarian follicular development during the human menstrual cycle. Fertil Steril 80:116-122 (
Abstract)
* Chabbert Buffet N, Djakoure C, Maitre SC, Bouchard P. 1998.
Regulation of the human menstrual cycle. Front Neuroendocrinol 19:151-86. (
Abstract)
* Fortune JE. 1994.
Ovarian follicular growth and development in mammals. Biol Reprod: 50:225-232
* Guraya SS,
Dhanju CK. 1992.
Mechanism of ovulation -- an overview. Indian J Exp Biol 30:958-967
* Roberts SC, Havlicek J, Flegr J, Hruskova M, Little AC, Jones BC, Perrett DI, Petrie M. 2003.
Female facial attractiveness increases during the fertile phase of the menstrual cycle. Proc Biol Sci: 271 Suppl 5 (
Abstract)
