OVARIES: CHARACTERISTICS, HORMONES, FUNCTIONS, DISEASES - BIOLOGY - 2025

The ovaries are two gonads, or pelvic nodular organs, that are part of the female reproductive system. These organs produce hormones that allow the development of secondary sexual characteristics and pregnancy.

The basic functional unit of the ovaries is the follicle, or Graff's follicle, from which an egg is expelled in the middle of each sexual cycle. If the egg is fertilized by a sperm, it implants in the uterus, where it will develop into a fetus and a placenta, which will later develop into a child.

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At birth, girls have between 150,000 and 2 million primordial follicles. When they reach adolescence, the number of follicles decreases. During reproductive age, approximately 400 follicles grow to form eggs, while the rest degenerate.

As age advances, the number of follicles decreases, and reproductive capacity declines until it ceases, which is known as menopause.

Anatomy

At birth, the ovaries are 1.5 to 2 cm long; 0.5 cm wide, and from 1 to 3.5 mm thick, weighing approximately 0.35 g. In adult women, the ovaries are 2.5 to 5 cm long; from 1.5 to 3 cm wide; and from 0.6 to 1.5 cm thick, weighing between 5.0 and 8.0 g.

In adolescence, the ovaries resemble smooth-surfaced structures and lack the scars produced by ovulation. As you approach your 40s, your ovaries display multiple follicle scars and cysts. After age 50, they are cerebriform in appearance due to scarring.

The ovaries are attached to the uterus and fallopian tubes by various ligaments, namely:

- The broad ligament, which extends laterally from the uterus towards the wall of the pelvic cavity. Its posterior surface is attached to the anterior margin of the ovary (hilus), by a double fold of the peritoneum called the mesovarium.

- The utero-ovarian (or ovarian) ligament joins the middle pole of the ovary to the ipsilateral uterine horn.

- The suspensory ligament (infundibulum-pelvic) joins the superior pole of the ovary to the wall of the fallopian tube, adjacent to the end of the fimbriae.

Histology of the ovary

The ovary has a superficial layer of cube-shaped epithelium, called the germinal epithelium. Beneath this epithelium is the cortex, an outer layer, and the medulla, an inner layer.

The cortex is a layer of connective tissue called the tunica albuginea, where elongated cells and fibroblasts build the matrix of the superficial cortex. While the medulla is mainly made up of blood vessels, lymphatic channels and nerves. These last elements also make up another region of the ovaries: the hilus.

In relation to the arteries, some branches of the ovarian artery enter the mesovarium, and divide into the hilum and the medulla forming curls. While the veins start from the hilus as a pampiniform plexus.

In the cortex and medulla, cystic follicles, and the corpora lutea and albicans are observed. The follicles contain an ovum inside, surrounded by granulosa cells and an outer layer of theca cells.

The follicles present different stages (primordial, primary and secondary) before reaching the antral or mature state, during which the ovum will be expelled. The maturation of the follicles involves growth and development of the granulosa cells, among other changes.

Hormones produced by the ovaries

In reproductive age, between 13 and 46 years, there are monthly rhythmic variations of female hormones, which cause physical changes in the ovaries and other sexual organs.

The hormones produced by the ovaries are estrogen and progesterone. These hormones act in concert with the hormones produced by the anterior pituitary, such as follicle stimulating hormone (FSH) and luteinizing hormone (LH).

The hormones FSH and LH produce follicle-related disturbances of the ovaries, including endowment and maintenance, initial recruitment, maturation, cyclical atresia or recruitment, ovulation, and depletion.

The monthly cycle, which lasts on average 28 days, begins with the recruitment phase. In this phase, there is an increase in blood FSH that induces the growth of 6 to 12 primordial follicles. These follicles are characterized by having a single layer of granulosa cells, and the prophase of meiotic division is arrested.

Then the follicles grow and more layers of granulosa cells develop, forming the primary follicles. Due to the action of FSH, teak is formed. Then the follicles produce estrogen, and the vesicular follicle is formed. A single follicle reaches the antral phase. The rest degenerate.

Role of estrogens and progesterone

Estrogens exert their effect on the uterus and vagina. When the girl reaches puberty, estrogens cause an increase in the size of the uterus and vagina.

Under the influence of estrogens, proliferation of endometrial cells occurs, which is important for the nutrition of the fertilized egg implanted in the uterus. In addition, it increases the number of ciliated epithelial cells that cover the fallopian tubes, and help transport the fertilized egg to the uterus.

Other functions of estrogens are: development of breast tissue, increased osteoblastic activity in bones, increased body metabolism rate, and hair growth, among others.

Progesterone prepares the uterus for implantation of the fertilized egg by disrupting secretory cells in the endometrium, and decreases uterine contractions, which helps maintain pregnancy.

Progesterone causes increased secretion of the mucous lining in the tubes, which is important for nourishing the fertilized egg.

Additionally, progesterone produces an increase in the size of the breasts and a development of breast tissue during pregnancy, which will later allow lactation.

Major diseases

Polycystic ovaries (PCOS)

It is an endocrine disorder that affects approximately 7% of women of reproductive age. Symptoms include oligomenorrhea, hirsutism, and acne. This causes anovulation, insulin resistance, and a high androgen concentration. PCOS has been associated with cancers of the breast, endometrium, and ovaries.

Endometriosis

It consists of the development of endometrial tissue in abnormal places, where it grows and menstruates. The most common place is the ovaries, and it causes infertility because it causes fibrosis that prevents the release of the egg. Treatment includes suppression of ovulation, or surgery to preserve the ability to conceive.

Ovarian tumors

It consists of the abnormal growth of ovarian tissue. Several genes responsible for ovarian cancer have been identified. Treatment consists of chemotherapy, radiation therapy, and surgery. Diagnosis is made by determining tumor markers in the blood, and high-resolution ultrasonography, among other methods.

Failed ovulation

It consists of the appearance of menstrual cycles that lack ovulation. Causes include hyposecretion of gonadotropic hormones and abnormality of the ovaries. Ovulation can be verified during the second half of the cycle by measuring a product of progesterone metabolism, pregnandiol, in the urine.

Hyperovulation

Hyperovulation is a widely used technique in in vitro fertilization. It consists of the application of gonadotropins that hyperstimulate the ovaries for the production of follicles. Consequently, a much greater number of follicles are produced than normal that would occur each month. The goal is to get more than one mature egg.

In vitro fertilization consists of the extraction of the ovules, before they are released by the follicles, by laparoscopy. The ovules must be in metaphase of meiosis II. The eggs are then placed in a culture medium and mixed with the sperm.

The conditions of the culture medium must allow the fertilization of the ovules. Two haploid sets of chromosomes form in each fertilized egg, one that is attached to the haploid set of sperm chromosomes, and another that is removed, called the polar body.

Then the fertilized egg, called a zygote, begins to divide. When the zygote reaches eight cells, in two or three days, it is transferred to the uterus, where an embryo is expected to implant and develop. Generally, a maximum of two fertilized eggs are transferred, which usually prevents multiple pregnancies.

Contraceptive methods related to the ovaries

It consists of the use of methods that prevent pregnancy. There are several contraceptive methods that are related to ovarian function. One of the most popular is the use of the hormones estrogen and progestin, which can be administered orally, transdermally, or transvaginally.

The probable association between the use of contraceptive hormones and the risk of developing breast cancer has been investigated. Studies indicate that there is a risk, but it is very small. Longer use of hormonal contraceptives increases the potential for breast cancer.

On the other hand, hormone replacement therapy, using progestins, can increase the risk of breast cancer in postmenopausal women. However, genetic predisposition plays an important role in the risk of developing this type of cancer.

References

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