Each ovary is whitish in color and located alongside the lateral wall of theuterus in a region called theovarian fossa. The ovarian fossa is the region that is bounded by theexternal iliac artery and in front of theureter and theinternal iliac artery. This area is about 4 cm x 3 cm x 2 cm in size.[3][4]
The ovaries are surrounded by a capsule, and have an outer cortex and an inner medulla.[4] The capsule is of dense connective tissue and is known as thetunica albuginea.[5]
Usually, ovulation occurs in one of the two ovaries releasing an egg eachmenstrual cycle.
Micrograph of the ovarian cortex from a rhesus monkey showing several round follicles embedded in a matrix of stromal cells. A secondary follicle sectioned through the nucleus of an oocyte is at the upper left, and earlier stage follicles are at the lower right. The tissue was stained with the dyeshematoxylin andeosin.
Follicular cells are flat epithelial cells that originate from surface epithelium covering the ovary. They are surrounded bygranulosa cells that have changed from flat to cuboidal and proliferated to produce a stratified epithelium.
Atpuberty, the ovary begins to secrete increasing levels of hormones. Secondary sex characteristics begin to develop in response to the hormones. The ovary changes structure and function beginning at puberty.[2] Since the ovaries are able to regulate hormones, they also play an important role in pregnancy andfertility. When egg cells (oocytes) are released from the fallopian tube, a variety of feedback mechanisms stimulate the endocrine system, which cause hormone levels to change.[10] These feedback mechanisms are controlled by the hypothalamus and pituitary glands. Messages or signals from the hypothalamus are sent to the pituitary gland. In turn, the pituitary gland releases hormones to the ovaries. From this signaling, the ovaries release their own hormones.
The process of ovulation and gamete production, oogenesis, in a human ovary
The ovaries are the site of production and periodical release ofegg cells, the female gametes. In the ovaries, the developing egg cells (oroocytes) mature in the fluid-filledfollicles. Typically, only one oocyte develops at a time, but others can also mature simultaneously. Follicles are composed of different types and number of cells according to the stage oftheir maturation, and their size is indicative of the stage of oocyte development.[11]: 833
When an oocyte completes its maturation in the ovary, a surge ofluteinizing hormone is secreted by thepituitary gland, which stimulates the release of the oocyte through the rupture of the follicle, a process calledovulation.[12] The follicle remains functional and reorganizes into acorpus luteum, which secretesprogesterone in order to prepare theuterus for an eventualimplantation of the embryo.[11]: 839
At maturity, ovaries secreteestrogen,androgen,[13][14]inhibin, andprogestogen.[15][16][2] In women before menopause, 50% of testosterone is produced by the ovaries and released directly into the blood stream. The other 50% of testosterone in the blood stream is made from conversion of the adrenal pre-androgens ( DHEA and androstenedione) to testosterone in other parts of the body. Estrogen is responsible for the appearance ofsecondary sex characteristics for females atpuberty and for the maturation and maintenance of thereproductive organs in their mature functional state. Progesterone prepares the uterus for pregnancy, and the mammary glands for lactation. Progesterone functions with estrogen by promotingmenstrual cycle changes in theendometrium.[medical citation needed]
As women age, they experience a decline in reproductive performance leading tomenopause. This decline is tied to a decline in the number of ovarian follicles. Although about 1 million oocytes are present at birth in the human ovary, only about 500 (about 0.05%) of these ovulate, and the rest do not ovulate. The decline in ovarian reserve appears to occur at a constantly increasing rate with age,[17] and leads to nearly complete exhaustion of the reserve by about age 52. As ovarian reserve and fertility decline with age, there is also a parallel increase in pregnancy failure and meiotic errors resulting in chromosomally abnormal conceptions. The ovarian reserve and fertility perform optimally around 20–30 years of age.[18] Around 45 years of age, the menstrual cycle begins to change and the follicle pool decreases significantly.[18] The events that lead to ovarian aging remain unclear. The variability of aging could include environmental factors, lifestyle habits or genetic factors.[18]
Women with an inherited mutation in the DNA repair gene BRCA1 undergo menopause prematurely,[19] suggesting that naturally occurring DNA damages in oocytes are repaired less efficiently in these women, and this inefficiency leads to early reproductive failure. The BRCA1 protein plays a key role in a type of DNA repair termed homologous recombinational repair that is the only known cellular process that can accurately repair DNA double-strand breaks. Titus et al.[20] showed that DNA double-strand breaks accumulate with age in humans and mice in primordial follicles. Primordial follicles contain oocytes that are at an intermediate (prophase I) stage of meiosis. Meiosis is the general process in eukaryotic organisms by which germ cells are formed, and it is likely an adaptation for removing DNA damages, especially double-strand breaks, from germ line DNA (seeMeiosis andOrigin and function of meiosis).[21] Homologous recombinational repair is especially promoted during meiosis. Titus et al.[20] also found that expression of 4 key genes necessary for homologous recombinational repair of DNA double-strand breaks (BRCA1, MRE11, RAD51 and ATM) decline with age in the oocytes of humans and mice. They hypothesized that DNA double-strand break repair is vital for the maintenance of oocyte reserve and that a decline in efficiency of repair with age plays a key role in ovarian aging. A study identified 290 genetic determinants of ovarian ageing, also found thatDNA damage response processes are implicated and suggests that possible effects of extending fertility in women would improve bone health, reduce risk of type 2 diabetes and increase the risk of hormone-sensitive cancers.[22][23]
A variety of testing methods can be used in order to determine fertility based on maternal age. Many of these tests measure levels of hormones FSH, and GnrH. Methods such as measuring AMH (anti-Müllerian hormone) levels, and AFC (antral follicule count) can predict ovarian aging. AMH levels serve as an indicator of ovarian aging since the quality of ovarian follicles can be determined.[24]
If the egg fails to release from the follicle in the ovary anovarian cyst may form. Small ovarian cysts are common in healthy women. Some women have more follicles than usual (polycystic ovary syndrome), which inhibits the follicles to grow normally and this will cause cycle irregularities.
It exists in two forms, central and primary. Central hypogonadism is a condition that is a result of improper function from thehypothalamus andpituitary gland.
Cryopreservation of ovarian tissue, often calledovarian tissue cryopreservation, is of interest to women who want to preserve their reproductive function beyond the natural limit, or whose reproductive potential is threatened by cancer therapy,[48] for example in hematologic malignancies or breast cancer.[49] The procedure is to take a part of the ovary and carry out slow freezing before storing it in liquid nitrogen whilst therapy is undertaken. Tissue can then be thawed and implanted near the fallopian, either orthotopic (on the natural location) or heterotopic (on the abdominal wall),[49] where it starts to produce new eggs, allowing normal conception to take place.[50] A study of 60 procedures concluded that ovarian tissue harvesting appears to be safe.[49] The ovarian tissue may also be transplanted into mice that are immunocompromised (SCID mice) to avoidgraft rejection, and tissue can be harvested later when mature follicles have developed.[51]
Aristotle referred toovarectomy inHistoria Animalium, in sows to stimulate growth and fatness, and in femalewar camels to sterilize them. However he believed that embryos are formed when the seed of a male (produced by thesperm ducts, rather than the testicles) plants into the soil of a female (probably referring tomenstrual blood), and did not subscribe the gonads of significance in heredity. He also observed that spayed females cease to go intoestrus.[52]: 1–3, 21
Herophilus was the first to describe the ovaries and theoviducts. He called the ovaries the "female testes", and gave the female equal credit with the male for producing the fetus. He thought the ovaries produced female seeds, and the testicles produced male seeds. The female seeds travel through the fallopian tubes and combine with the male seed in the uterus (not the fallopian tubes), thus producing an embryo.[53]Soranus of Ephesus gave a detailed description of the ovaries.[52]: 5 Galen inOn Semen argued that both the testicles and the ovaries contribute to embryo, but the testicles contribute more since they are larger.
De Humani Corporis Fabrica: first representation of the ovaries. The ovaries are clearly shown as bumpy, representing the ovulation stigmas.
Vesalius, by dissecting women of reproductive age, discovered the follicles with fluid within "white and like a milky serum", as well as yellow stuff likely to be the corpus luteum. He also noted the ovaries are bumpy withovulation stigmas.Fabricius observed that in the hen,yolk is formed in the ovary, while thewhite and theshell are formed during the transit through the oviduct. He also made the first illustration of the corpus luteum inDe Formato Foetu (1604) and called it "numerous conjoined little glands".Carlo Ruini noted that the ovaries of fetal mare are up to 4 times as large as the mother's.[52]: 6–12
Nicolas Steno in 1667 hypothesized that the ovaries of mammals are analogous to the ovaries of birds, and thus they too contain eggs (rather than the "female semen"). In 1675 he described the ovaries of various animals, and hypothesized that she-mules are infertile because their ovaries do not produce eggs.[54]De Graaf made a full-length study of the female reproductive system.[55] He thought the whole follicle was an egg, and proved it by cooking and eating the liquid inside follicles and noting that it had the same color, taste, and consistency as cooked egg-white. He stated that the number of corpus luteum is the same as the number of fetuses, and it only occurs after coitus (he was observing rabbits, who haveinduced ovulation).[56] He also thought the corpus luteum is the scar tissue left behind after the egg is shed from the ovary. He observed that the corpus luteum is yellow in cows, red in sheep red, and ashen in others.Drelincourt in 1685[57] suggested that the follicle is not the egg, and that the true egg is much smaller, and within the follicle.Malpighi speculated that the follicle never leaves the ovary, but remains within. The true egg is much smaller and within the follicle, and leaves the ovary. He also coined the word "corpus luteum" [Latin for "yellow body"] because he worked with the cow's ovaries, in which the corpus luteum is yellow.Boerhaave clearly stated the currently accepted theory that the ovum escapes from the ovary, leaving a scar tissue of corpus luteum, and pass down the fallopian tube, be fertilized by a sperm within the tube, then enters the uterus.[52]: 15–21
There was a controversy during the period about how reproduction works. Some likeHarvey agreed with Aristotle that the ovaries are unimportant. Some likeWharton,Descartes, andLe Grand, agreed with Galen that the ovaries produced female semen that mixes with male semen to produce an embryo. The new "ovism" school like Steno and de Graaf thought the ovaries in mammals worked like the ovaries in birds: The entire follicle is an egg. The egg is fertilized in the ovary. Only fertilized eggs can leave the ovary. The new "animalculism" school believed only the sperm contained the generative principle.[58][52]: 17–18
John Huner experimentally removed one ovary from a sow and compared her with a sow with two ovaries. He noted that the half-spayed sow became infertile after producing 76 piglets, but still continued to go into estrus. The other sow produced 162 piglets, thus he proposed that each ovary could produce a fixed number of offsprings.[59]Percival Pott reported in 1775 that he removed two herniated ovaries from a woman, after which she became thinner, lost her large breasts, and ceased menstruation. This was the first accurate description of the effect of ovarectomy in humans. In 1827,von Baer first described and illustrated the mammalian egg within the follicle.[58][52]: 23–27
Albert Peuch in 1873 stated that spaying young sows prevented subsequent growth in uterus.Emil Knauer showed that transplanting ovaries into the abdominal cavity in spayed gods, rabbits, and guinea pigs prevented this uterineatrophy, similar to the testicle transplant experiment ofBerthold in 1849.[60]Prenant in 1898[61] concluded by histology that the corpus luteum was an endocrine gland.[62]Ludwig FraenkelVilhelm Magnus both showed independently in 1901 that spaying pregnant rabbits or removing their corpora lutea bythermocautery resulted in abortion or resorption of the embryos.Leo Loeb noted in 1906[63] that traumatizing the endometrium of a nonpregnant guinea pig or rabbit resulted in formation ofplacenta ordeciduoma, but this did not happen if corpora lutea were removed. In 1915, Edmund Herrmann, an Austrian obstetrician, showed that a lipid extract of pig's corpora lutea produced a fully progestational endometrium when injected into immature rabbits. This line of work was continued byCorner andAllen, resulting in the discovery ofprogesterone.[64] Leo Loeb also showed in 1911 that removing the corpora lutea hastened the next ovulation.[65] (Makepeaceet al, 1937)[66] showed specifically that injecting progesterone inhibited ovulation in rabbits. This eventually resulted inhormonal contraception.[52]: 29–34
Ovary of a marine fish and its parasite, the nematodePhilometra fasciati
Birds have only one functional ovary (the left), while the other remains vestigial. In mammals including humans, the female ovary ishomologous to the maletesticle, in that they are bothgonads andendocrine glands. Ovaries of some kind are found in the female reproductive system of many invertebrates that employsexual reproduction. However, they develop in a very different way in most invertebrates than they do in vertebrates, and are not truly homologous.[67]
Many of the features found in human ovaries are common to all vertebrates, including the presence of follicular cells, tunica albuginea, and so on. However, many species produce a far greater number of eggs during their lifetime than do humans, so that,in fish and amphibians, there may be hundreds, or even millions of fertile eggs present in the ovary at any given time. In these species, fresh eggs may be developing from the germinal epithelium throughout life. Corpora lutea are found only in mammals, and in someelasmobranch fish; in other species, the remnants of the follicle are quickly resorbed by the ovary. In birds, reptiles, andmonotremes, the egg is relatively large, filling the follicle, and distorting the shape of the ovary at maturity.[67]
Amphibians and reptiles have no ovarian medulla; the central part of the ovary is a hollow,lymph-filled space.[68]
The ovary ofteleosts is also often hollow, but in this case, the eggs are shed into the cavity, which opens into theoviduct.[67] Certainnematodes of the genusPhilometra are parasitic in the ovary of marine fishes and can be spectacular, with females as long as 40 cm (16 in), coiled in the ovary of a fish half this length.[69] Although most female vertebrates have two ovaries, this is not the case in all species. In most birds and inplatypuses, the right ovary never matures, so that only the left is functional. (Exceptions include thekiwi and some, but not allraptors, in which both ovaries persist.[70][71]) In some elasmobranchs, only the right ovary develops fully. In the primitivejawless fish, and some teleosts, there is only one ovary, formed by the fusion of the paired organs in the embryo.[67]
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