The primary female reproductive organs, or gonads, are the two ovaries. Each ovary is a solid, ovoid structure about the size and shape of an almond, about 3.5 cm in length, 2 cm wide, and 1 cm thick. The ovaries are located in shallow depressions, called ovarian fossae, one on each side of the uterus, in the lateral walls of the pelvic cavity. They are held loosely in place by peritoneal ligaments.

Structure

The ovaries are covered on the outside by a layer of simple cuboidal epithelium called germinal (ovarian) epithelium. This is actually the visceral peritoneum that envelops the ovaries. Underneath this layer there is a dense connective tissue capsule, the tunica albuginea. The substance of the ovaries is distinctly divided into an outer cortex and an inner medulla. The cortex appears more dense and granular due to the presence of numerous ovarian follicles in various stages of development. Each of the follicles contains an oocyte, a female germ cell. The medulla is loose connective tissue with abundant blood vessels, lymphatic vessels, and nerve fibers.

Oogenesis

Female sex cells, or gametes, develop in the ovaries by a form of meiosis called oogenesis. The sequence of events in oogenesis is similar to the sequence in spermatogenesis, but the timing and final result are different. Early in fetal development, primitive germ cells in the ovaries differentiate into oogonia. These divide rapidly to form thousands of cells, still called oogonia, which have a full complement of 46 (23 pairs) chromosomes. Oogonia then enter a growth phase, enlarge, and become primary oocytes. The diploid (46 chromosomes) primary oocytes replicate their DNA and begin the first meiotic division, but the process stops in prophase and the cells remain in this suspended state until puberty. Many of the primary oocytes degenerate before birth, but even with this decline, the two ovaries together contain approximately 700,000 oocytes at birth. This is the lifetime supply, and no more will develop. This is quite different than the male in which spermatogonia and primary spermatocytes continue to be produced throughout the reproductive lifetime. By puberty the number of primary oocytes has further declined to about 400,000.

Beginning at puberty, under the influence of follicle-stimulating hormone, several primary oocytes start to grow again each month. One of the primary oocytes seems to outgrow the others and it resumes meiosis I. The other cells degenerate. The large cell undergoes an unequal division so that nearly all the cytoplasm, organelles, and half the chromosomes go to one cell, which becomes a secondary oocyte. The remaining half of the chromosomes go to a smaller cell called the first polar body. The secondary oocyte begins the second meiotic division, but the process stops in metaphase. At this point ovulation occurs. If fertilization occurs, meiosis II continues. Again this is an unequal division with all of the cytoplasm going to the ovum, which has 23 single-stranded chromosome. The smaller cell from this division is a second polar body. The first polar body also usually divides in meiosis I to produce two even smaller polar bodies. If fertilization does not occur, the second meiotic division is never completed and the secondary oocyte degenerates. Here again there are obvious differences between the male and female. In spermatogenesis, four functional sperm develop from each primary spermatocyte. In oogenesis, only one functional fertilizable cell develops from a primary oocyte. The other three cells are polar bodies and they degenerate.

Ovarian Follicle Development An ovarian follicle consists of a developing oocyte surrounded by one or more layers of cells called follicular cells. At the same time that the oocyte is progressing through meiosis, corresponding changes are taking place in the follicular cells. Primordial follicles, which consist of a primary oocyte surrounded by a single layer of flattened cells, develop in the fetus and are the stage that is present in the ovaries at birth and throughout childhood.

Beginning at puberty follicle-stimulating hormone stimulates changes in the primordial follicles. The follicular cells become cuboidal, the primary oocyte enlarges, and it is now a primary follicle. The follicles continue to grow under the influence of follicle-stimulating hormone, and the follicular cells proliferate to form several layers of granulose cells around the primary oocyte. Most of these primary follicles degenerate along with the primary oocytes within them, but usually one continues to develop each month. The granulosa cells start secreting estrogen and a cavity, or antrum, forms within the follicle. When the antrum starts to develop, the follicle becomes a secondary follicle. The granulose cells also secrete a glycoprotein substance that forms a clear membrane, the zona pellucida, around the oocyte. After about 10 days of growth the follicle is a mature vesicular (graafian) follicle, which forms a "blister" on the surface of the ovary and contains a secondary oocyte ready for ovulation.

Ovulation

Ovulation, prompted by luteinizing hormone from the anterior pituitary, occurs when the mature follicle at the surface of the ovary ruptures and releases the secondary oocyte into the peritoneal cavity. The ovulated secondary oocyte, ready for fertilization is still surrounded by the zona pellucida and a few layers of cells called the corona radiata. If it is not fertilized, the secondary oocyte degenerates in a couple of days. If a sperm passes through the corona radiata and zona pellucida and enters the cytoplasm of the secondary oocyte, the second meiotic division resumes to form a polar body and a mature ovum

After ovulation and in response to luteinizing hormone, the portion of the follicle that remains in the ovary enlarges and is transformed into a corpus luteum. The corpus luteum is a glandular structure that secretes progesterone and some estrogens. Its fate depends on whether fertilization occurs. If fertilization does not take place, the corpus luteum remains functional for about 10 days then it begins to degenerate into a corpus albicans, which is primarily scar tissue, and its hormone output ceases. If fertilization occurs, the corpus luteum persists and continues its hormone functions until the placenta develops sufficiently to secrete the necessary hormones. Again, the corpus luteum ultimately degenerates into corpus albicans, but it remains functional for a longer period of time.

Referrence:Ovary(Chinese Version)

ovaries

The two organs that store eggs in a woman's body. Ovaries also produce hormones, including estrogen, progesterone, and testosterone.

Ovary From Wikipedia, the free encyclopedia.

Ovaries are a part of the female mammalian reproductive system. They contain many small follicles which produce the eggs which will merge with the sperm to make a embryo. Also they produce various hormones controlling amongst many other things the menstruation cycle.

Normally, a female will have two ovaries, each containing a fixed number of eggs. In humans, an egg launched from an ovary has to traverse a slight space before entering the fallopian tube and moving gradually down to the uterus where it implants itself into the lining and develops as the pregnancy continues. If the egg settles into the fallopian tube instead of the uterus an ectopic pregnancy will result. If the egg fails to release from the follicle in the ovary an ovarian cyst may form. These cysts are a major sign of a disease called Polycystic ovarian syndrome.

Ovaries http://www.vh.org/adult/provider/anatomy/AnatomicVariants/OrganSystem/Text/Ovaries.html

Ronald A. Bergman, PhD

Adel K. Afifi, MD, MS

Ryosuke Miyauchi, MD

Peer Review Status: Internally Peer Reviewed

Abnormalities in the sex organs of the female are, for the most part, due to developmental deviations. Anomalies of the reproductive tract are more frequent in women than men (1.5-2.0/1).

Incomplete or modification of its descent from the abdomen affect the final position of the ovary. It may retain its original suprapelvic position and lie above or upon the psoas major muscle. If the female gubernaculum (ovarian ligament) fails to adhere to the uterus the ovary may follow the round ligament and pass partly or entirely through the inguinal canal into the labium majus.

The adult ovary may present marked deviations from its typical form, sometimes being unusually long, spheroidal, flattened, triangular, crescentric, or otherwise irregular.

Supernumerary ovaries, varying in size, are not infrequent, occurring in 2-5% of women. In one study of 500 autopsies (Testut), supernumerary ovaries were found 23 times (or 4.6%).

Their usual location is variable in parous women, because they are displaced in the first prequency and probably never return to their original location. Also variably mobile, they may change position according to intestinal activity. In nulliparous women, in the upright position, their long axis is verticle; it has lateral and mesial surfaces, tubal and uterine extremities and mesovarian and free borders. They occupy the ovarian fossae on the lateral pelvic wall.

Isolation of ovarian anlage result in normal follicle-bearing ovarian tissue.

The ovaries may themselves may be the site of displaced adrenal or thyroid tissue.

Absence of one or both ovaries is extremely rare.

An ovary may become involved in inguinal hernia; the displacement of the ovaries into the rectouterine pouch (of Douglas) has also been reported.

By the third month of fetal life, the ovaries have fully developed and descended to their permanent pelvic position.

Remnants of the primitive genital system provide lateral supporting attachments to the uterus; in the mature female, these supporting structures evolve into the round and suspensory ligaments.

Remnants that do not evolve may form cysts, which may become symptomatic later in life.

Oogenesis is the process of generation of ova by mitotic division that begins at the sixth week of fetal life.

These primitive germ cells ultimately provide the 1 to 2 million oocytes that are present in the ovaries at birth.

At puberty, this number is reduced through cell death to approximately 300,000.

The neonate's ovaries are smooth, pale, and elongated.

They become shorter, thicker, and heavier before the onset of menarche, which is initiated by pituitary influence.

The initial hormonal stimulus for this development is believed to come from ovarian rather than systemic estrogen.

In the adult, the ovaries are flat, almond-shaped structures that are 3 to 5 cm long and weigh 2 to 3 g.

They are located on either side of the uterus below the fimbriated ends of the two oviducts, or fallopian tubes.

The ovaries are attached to the posterior surface of the broad ligament and to the uterus by the ovarian ligament.

They are covered with a thin layer of surface epithelium that is continuous with the lining of the peritoneum.

The integrity of this covering is periodically broken at the time of ovulation.

The ovaries, like the male testes, have a dual function: they store the female germ cells, or ova, and produce the female sex hormones, estrogen and progesterone.

Unlike the male gonads, which produce sperm throughout a man's reproductive life, the female gonads contain a fixed number of ova at birth that diminishes throughout a woman's life.

Structurally, the mature ovary is divided into a highly vascular inner medulla, which contains supporting connective tissue and an outer cortex of stroma and epithelial follicles (i.e., vesicles), which contain the primary oocytes, or germ cells.

After puberty, the pituitary gonadotropic hormones-follicle-stimulating hormone (FSH) and the luteinizing hormone (LH)-stimulate the primordial fol- licles to develop into mature graaflan follicles.

The graafian follicle produces estrogen, which begins to stimulate the development of the endometrium in the uterus.

Although several follicles begin to develop during each ovulatory cycle, only one or two complete the entire developmental process and rupture to release a mature ovum.

After ovulation, the follicle becomes luteinized; as the corpus luteurn, it produces estrogen and progesterone to support the endometrium until conception occurs or the cycle begins again. ?

OVARIAN FOLLICLE DEVELOPMENT AND OVULATION

The tissues of the adult ovary can be conveniently divided into four compartments, or units: the stroma, or supporting tissue; the interstitial cells; the follicles; and the corpus luteum.

The stroma is the connective tissue substance of the ovary in which the follicles are distributed.

The interstitial cells are estrogen-secreting cells that resemble the Leydig's cells, or interstitial cells, of the testes.

Beginning at puberty, a cyclic rise in the anterior pituitary hormones FSH and LH stimulates the development of several graafian, or mature, follicles. Follicles at all stages of development can be found in both ovaries, except in menopausal women.

The primary follicles constitute an inactive pool of follicles from which all the ovulating follicles develop.

High levels of estrogen exert a negative feedback effect on FSH, inhibiting multiple follicular development and causing an increase in LH levels.

This represents the follicular stage of the menstrual cycle.

As estrogen suppresses FSH, the actions of LH predominate, and the mature follicle (measuring approximately 20 mm) bursts; the oocyte, along with the corona radiata, is ejected from the follicle.

The ovum normally is then picked up and transported through the fallopian tube toward the uterus.

After ovulation, the follicle collapses, and the luteal stage of the menstrual cycle begins.

The granulosa cells are invaded by blood vessels and yellow lipochrome-bearing cells from the theca layer.

A rapid accumulation of blood and fluid forms a mass called the corpus luteum.

Leakage of this blood onto the peritoneal surface that surrounds the ovary is thought to contribute to the mittelschmerz ("middle [or intermenstrual] pain") of ovulation.

During the luteal stage, progesterone is secreted from the corpus luteum.

If fertilization does not take place, the corpus luteum atrophies and is replaced by white scar tissue called the corpus albicans; the hormonal support of the endometrium is withdrawn and menstruation occurs.

In the event of fertilization, a hormone called human chorionic gonadotropin is produced by the trophoblastic cells in the blastocyst.

This hormone prevents luteal regression.

The corpus luteum remains functional for 3 months and provides hormonal support for pregnancy until the placenta is fully functional.