Ovaries

Anatomy

Anatomy or system affected: All

Specialties and related fields: Biochemistry, embryology, endocrinology, genetics, gynecology, obstetrics

Definition: The ovaries produce eggs and sex hormones in females.

Key terms:

atresia: the programmed process of cell death

corpus luteum: the structure that develops from an emptied ovarian follicle after ovulation

Fallopian tubes or oviducts: tubular structures attached at their lower ends to the uterus; the passageways for ova following ovulation

follicle: a structure composed of an oocyte and surrounding granulosa cells

hormone: a chemical messenger secreted by one cell type and acting on another to cause a predictable response

oocyte: a female germ cell that differentiates to become a mature ova

ovum (pl. ova): an egg cell

Structure and Functions

Ovaries develop from undifferentiated gonadal tissue in the absence of a Y chromosome. They are ductless glands located in the female pelvis, attached on either side of the uterus by the ovarian ligaments. Each is a flattened lumpy oval about 5 centimeters (cm) in length, 2.5 cm wide, and less than 1 cm in thickness. They are often described as being about the size and shape of an almond. There are three regions to each ovary: An outer cortex contains the developing oocytes, an inner medulla produces steroid hormones, and the hilum serves as the point of attachment and entry of blood vessels and nerves.

A principal function of the ovaries is gametogenesis, the production of ova through meiosis. This process begins during fetal life, and about 1 to 2 million immature oocytes are present in the cortex of the ovaries at birth. By puberty, this number has been reduced to about 300,000 through the process of atresia, and only about 400 mature ova are actually ovulated during life. The number of lifetime ovulated oocytes may be reduced if oral contraceptives are used.

Research has found that it may be possible to increase oocyte numbers during life through the implantation of stem cells that are capable of mitosis. These stem cells can be isolated from the ovaries of reproductive age women. Studies done on mice have isolated germline stem cells from ovaries that can actively divide and produce oocytes when implanted in a human body.

Within the mature ovarian cortex are many follicles, each containing an oocyte. At any given time, there are oocytes in all stages of development, and only a small percentage of available oocytes ever undergo ovulation; the rest undergo atresia and are recycled by the body. This occurs because groups of oocytes are activated during ovulation, and only one makes it to the luteal stage, while the others undergo atresia.

Immature oocytes remain dormant until puberty. During this time, the hypothalamus region of the brain releases a surge of hormone, the gonadotropin-releasing hormone (GnRH), that initiates the release of hormones from the pituitary gland, which will drive the process of ovulation. During puberty, eggs mature successively, and one breaks through the ovarian wall each cycle in the process of ovulation. This continues until menopause, or cessation of reproductive functioning in the female. After release from the ovary, the ovum passes through the Fallopian tube and into the uterus. If the ovum is fertilized, then pregnancy ensues and the ovum (now a blastocyst) implants in the uterine lining.

Following ovulation, the empty follicle left in the ovary becomes the corpus luteum. It appears as a yellow body (because of lipid droplets in the cells) on the surface of the ovary and secretes progesterone to prepare the uterine lining for implantation. The corpus luteum deteriorates after ten to twelve days if conception does not occur, and the decrease in progesterone allows deterioration of the uterine lining, and thus, menstruation begins.

The ovarian medulla is also responsible for hormonogenesis. The primary steroids produced here are estradiol (estrogen) and progesterone. Androgens (particularly androstenedione and testosterone) are also secreted, but most of them are converted to estradiol within the ovary. Estrogens are essential for the development of ova and female body characteristics (including breasts, body shape, fat deposition, and body hair distribution) and for implantation and pregnancy maintenance. They also initiate mammary gland maturation and contribute to bone mineralization. Progesterone prepares the uterine lining for implantation and pregnancy, regulates the release of luteinizing hormone (LH) from the pituitary, and contributes to the maturation of mammary gland alveoli.

In addition to the steroid hormones, peptide hormones are produced by the ovaries. Relaxin is secreted from the corpus luteum and induces relaxation of the pelvic bones and ligaments, inhibits myometrial motility, softens the cervix, and induces uterine growth. Activins stimulate the release of follicle stimulating hormone (FSH) from the pituitary, and inhibin causes decreasing FSH output, although the mechanism is not yet understood. The ovary has been found to produce a variety of neuropeptides such as b-endorphin, adrenocorticotropin, α-melanocyte-stimulating hormone, vasopressin, and oxytocin. The physiologic roles that these peptides may play in the ovary are uncertain.

Human ovaries display a regular cycle in reproductively mature females. This cycle, known as menstruation, includes a follicular phase, during which FSH promotes growth and maturation of the granulosa cells, partial maturation of oocytes, synthesis of proteins, and estrogen secretion. Next is the ovulatory phase, during which one mature oocyte and the surrounding cells are discharged. Lastly is the luteal phase, in which progesterone is secreted. The average ovarian cycle is twenty-eight days, with a normal range between twenty-four and thirty-two days in length.

Disorders and Diseases

The most common ovarian disorder is polycystic ovary syndrome (PCOS), or Stein-Leventhal syndrome, affecting 6 to 10 percent of all women. The ovarian stroma becomes enlarged and produces excessive amounts of androgens. Atretic follicles accumulate and large numbers of cysts (fluid-filled sacs) may form; most are harmless and require no treatment. Ovarian follicle development is incomplete, and menstrual cycles are usually irregular. Symptoms can be alleviated with combination contraceptives and antiandrogens. Ovulation can sometimes be induced by clomiphene.

Ovulatory dysfunction is one of the causes of infertility. Most often this is hormonal in nature, and many cases of infertility can be overcome by hormone treatments.

Another ovarian disorder is ovarian cancer, which comes in a variety of forms and is a major cause of death in females. The lifetime risk is about 1.6 percent, but it increases with age and in women with relatives who have had reproductive cancers, due to the genetic factors of the disease. In women with an altered BRCA1 or BRCA2 gene, the risk is 25 to 60 percent, depending on the specific mutation. Pregnancy decreases the overall risk, as do oral contraceptives. Surgery is the treatment of choice, often followed by chemotherapy. Prognosis is poor overall, mostly due to the lack of clear symptoms that would lead to early diagnosis.

Perspective and Prospects

The human ovary was first described by Renier De Graaf in 1672. In 1701, the first surgery to remove an ovarian tumor was performed by Robert Houston, and in 1809 the first known ovary removal (ovariectomy or oophorectomy) was performed by Ephraim McDowell. Throughout the nineteenth and early twentieth centuries, study of the ovaries centered on their anatomy and physiology, while during the latter part of the twentieth century studies turned to their endocrine functions.

Many postulated ovarian hormones are still being searched and studied for their effects. They include transforming growth factor beta (TGF-β), anti-Mullerian hormone (AMH), folliculostatins, and many other peptides involved in the control of growth and differentiation. Gonadotropin surge attenuating factor (GnSAF) has also been hypothesized to be physiologically involved in the control of LH secretion during the menstrual cycle.

Bibliography

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Futterweit, Walter, and George Ryan. A Patient's Guide to PCOS: Understanding-and Reversing-Polycystic Ovary Syndrome. New York: Holt, 2006.

Johnson, J., J. Canning, T. Kaneko, J.K. Pru, and J.L. Tilly. "Germline Stem Cells and Follicular Renewal in the Postnatal Mammalian Ovary." Nature 428, no. 6979 (2004): 145–150.

Kaipia, Antti, and Aaron Hsueh. "Regulation of Ovarian Follicle Atresia." Annual Review of Physiology 59 (1997): 349–363.

Moore, K.L., T.V.N. Persaud, M.G. Torchia, and T.V.N. Persaud. The Developing Human: Clinically Oriented Embryology. Philadelphia: Saunders/Elsevier, 2008.

"Ovaries." You and Your Hormones, Society for Endocrinology, Dec. 2014, www.yourhormones.info/glands/ovaries/. Accessed 26 Oct. 2017.

Plourde, Elizabeth. Your Guide to Hysterectomy, Ovary Removal, and Hormone Replacement: What All Women Need to Know. Ashland, OH: New Voice/Atlas Books, 2002.