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.

Overview

In humans, the period when ovulation occurs is called the ovulatory phase, and it occupies the fourteenth day of an idealized twenty-eight day menstrual cycle. Because of the variable nature of the menstrual cycle in vivo, however, normal ovulation can occur as early as day 8 or as late as day 20 and beyond. A woman with a particularly short cycle will ovulate earlier while a woman with a long cycle will ovulate later. In humans, concealed ovulation means that the female shows few visible signs of being fertile, compared to most species. There are evolutionary analyses of how this could evolve in terms of evolutionary game theory.

Before ovulation, the ovarian follicle will undergo a series of transformations that are necessary for the ovum to survive afterwards. This process is called cumulus expansion. After this is done, a hole called the stigma will form in the follicle, and the ovum will leave the follicle through this hole. It will then enter the fallopian tubes and travel toward the uterus, implanting there 6-12 days later if fertilized, or degrading in the fallopian tubes within 24 hours if not fertilized.

The ovulatory phase is preceded by the follicular phase, where an immature follicle-encased ovum completes growing, and followed by the luteal phase, where the uterus is prepared to receive the fertilized ovum. The complete process of follicle growth leading up to ovulation is known as folliculogenesis.

Scientific investigations like ^* have indicated that the olfactory acuity or the sense of smell is greatest during ovulation in women.

A closer look at the process

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

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.

Clinical presentation

The start of ovulation can be detected by various signs. Because the signs are not readily discernable by people other than the woman herself, however, humans are said to have a concealed ovulation.

Ovulating women experience a rise in body temperature, on average one-half to one degree Fahrenheit (one-quarter to one-half degree Celsius). The temperature elevation persists until approximately the beginning of menstruation, normally 10-16 day later. Furthermore, some women may experience novel discomfort or pain in their lower abdomen during ovulation termed Mittelschmerz ("midpain") that is most likely caused by irritation of the abdominal wall from the blood and fluid escaping from the ruptured follicle. Lastly, the chemical composition of a woman's cervical mucus will shift up to several days before ovulation, (the stringy characteristics are often perceived as stretching by women). The most fertile type of mucus may resemble raw egg white. Cervical mucus reverts to infertile or less fertile forms shortly after ovulation occurs.

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.

New research: follicular waves

Emerging 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 a typical 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.

Induced ovulation and contraception

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.

Ovulation in animals

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.

References

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)

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