Ovulation

Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle  ruptures and discharges an ovum (also known as an oocyte, female  gamete, or casually, an egg). Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from  the menstrual cycle. The time immediately surrounding ovulation is referred to as the ovulatory  phase or the periovulatory period.

Overview
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, 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 this is done, a hole called the stigma  will form in the follicle, and the ovum will leave the  follicle through this hole. 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. If fertilized by a sperm, it may perform implantation there 6–12 days  later.

Ovulation occurs when a mature egg is released from the ovary into the abdominal cavity. Afterwards, it will eventually become available to be fertilized. Concomitantly, the lining of the uterus is thickened to be able to receive a fertilized egg. If no conception occurs, the uterine lining as well as blood will be shed in menstruation.

In humans, the few days near ovulation constitute the fertile phase. The time from the beginning of the last menstrual period (LMP) until ovulation is, on average, 14.6 days, but with substantial variation both between women and between cycles in any single woman, with an overall 95% prediction interval of 8.2 to 20.5 days.

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.

Details
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:  preovulatory, 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 Graafian follicle or mature tertiary follicle)  contains an oocyte  arrested in prophase of meiosis I  surrounded by a layer of 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 oophorus  granulosa cells (or simply cumulus cells) connects the corona-ovum complex to  the mural granulosa  cells.

The granulosa cells engage in bidirectional messaging with the cells and the oocyte  to facilitate follicular function.

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. In contrast to the other phases of the menstrual cycle, estrogen release in late follicular phase  has 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 preovulatory phase.

Preovulatory 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 is the secretion of a hyaluronic acid-rich cocktail that disperses and gathers the cumulus cell network in a sticky matrix  around the ovum. This network stays with the ovum after ovulation and has 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 20 mm in diameter. It forms a pronounced bulge at the surface of the ovary called the blister.

Ovulatory phase
Through a signal transduction cascade initiated 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 cumulus-oocyte complex (COC) 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. corpora lutea), a  steroidogenic cluster of cells that produces estrogen and progesterone. These hormones induce the endometrial glands to begin production of the proliferative endometrium and later  into secretory endometrium, the site of embryonic growth if  fertilization occurs. The action of progesterone increases basal body temperature by one-quarter to one-half degree Celsius (one-half  to one degree Fahrenheit). The corpus luteum continues this paracrine action for the remainder of the menstrual cycle, maintaining the  endometrium, before disintegrating into scar tissue during menses.

Ovulation is the release of a single, mature egg from a follicle that developed in the ovary. It usually occurs regularly, around day 14 of a 28-day menstrual cycle. Once released, the egg is capable of being fertilized for 12 to 48 hours before it begins to disintegrate. Although there are several days of the month in which a woman is fertile, she is most fertile during the days around ovulation.

Clinical presentation
The start of ovulation can be detected by various signs. Because the signs are not readily discernible by people other than the woman herself, humans are said to  have a concealed ovulation. In many animal species there are distinctive signals indicating the period when the female is fertile. Several explanations have been proposed to explain concealed ovulation in humans.

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 and period.

Symptoms related to the onset of ovulation, the moment of ovulation and the body's process of beginning and ending the menstrual cycle vary in intensity with each individual woman but are fundamentally the same. The charting of such symptoms, primarily Basal Body Temperature, Mittelschmerz and cervical position is referred to as the Sympto-Thermal method of Fertility Awareness, which serves to allow auto diagnosis by a woman of her state of ovulation. Once training has been given by a suitable authority, fertility charts can be completed on a cycle by cycle basis to chart ovulation giving the possibility of using the data to predict fertility for both natural contraception and pregnancy planning.

The moment of ovulation has been photographed.

Follicular waves
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. 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 gene Lrh1 appears to be essential in regulating ovulation.

Induced ovulation
Ovulation induction is a promising assisted reproductive technology  for patients with conditions such as polycystic ovary syndrome (PCOS)  and oligomenorrhea. It is also used in in vitro fertilization to make the follicles mature prior to egg retrieval. Usually, ovarian stimulation is used in conjunction with ovulation induction to stimulate the formation of multiple oocytes. Some sources include ovulation induction in the definition of ovarian stimulation.

A low dose of human chorionic gonadotropin (HCG) may be injected after completed ovarian stimulation. Ovulation will occur between 24–36 hours after the HCG injection.

Suppressed ovulation
Contraception can be achieved by suppressing the ovulation.

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.

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.

Animals

 * In cats, rabbits, and camelids, 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.
 * After ovulation, oocytes remain in Meiosis I. The oocyte proceeds to Meiosis II before fertilization.