Lesson 1, Topic 1
In Progress

Ovaries

July 2, 2023

Structure and location

The paired ovaries are the gonads of females. They have a puckered, uneven surface; each weighs about 3 g. The ovaries resemble large almonds in size and shape. They are attached to ligaments in the pelvic cavity on each side of the uterus.

Embedded in a connective tissue matrix just below the outer layer of each ovary in a newborn baby girl are about half a million ovarian follicles. Each follicle contains an oocyte, an immature stage of the female sex cell.

By the time a girl reaches puberty, however, further development has resulted in the formation of a reduced number (about 400,000) of what are then called primary follicles. Each primary follicle has a layer of granulosa cells around the oocyte.

The progression of development from primary follicle to ovulation is shown in Figure 23-10. As the thickness of the granulosa cell layer around the oocyte increases, a hollow chamber called an antrum appears, and a secondary follicle is formed.

FIGURE 23-10​Ovary. ​Cross section of ovary shows successive stages of ovarian follicle and ovum development. Begin with the first stage (primary follicle) and follow around clockwise to the final state (degenerating corpus luteum).

During the reproductive lifetime of most women, only about 350 to 500 of these primary follicles fully develop into 628mature follicles. It is the mature follicle that releases an ovum for potential fertilization—a process called ovulation. Follicles that do not mature degenerate and are reabsorbed into the ovarian tissue.

The sac containing a mature ovum is the mature ovarian follicle—often called a graafian follicle, in honor of the Dutch anatomist Regnier de Graaf who discovered it some 300 years ago.

After ovulation, the ruptured follicle is transformed into a hormone-secreting glandular structure called the corpus luteum, which is described later. Corpus luteum is a Latin phrase meaning “yellow body”—an appropriate name to describe the yellow appearance of this glandular structure.

Ovary functions

Oogenesis

The production of female gametes, or sex cells, is called oogenesis.

The unusual form of cell division that results in sperm formation, meiosis, is also responsible for development of ova. During the developmental phases experienced by the female sex cell from its earliest stage to just after fertilization, two meiotic cell divisions occur.

As a result of meiosis in the female sex cell, the number of chromosomes is reduced equally in each daughter cell to half the number (23) found in other body cells (46).

However, the amount of cytoplasm is divided unequally among the daughter cells, as you can see in Figure 23-11. The result is formation of one large ovum and small daughter cells called polar bodies that degenerate. The ovum, with its large supply of cytoplasm, is one of the body’s largest cells and is uniquely structured to provide nutrients for rapid development of the embryo until implantation in the uterus occurs.

FIGURE 23-11​Oogenesis. ​Production of a mature ovum and subsequent fertilization are shown as a series of cell divisions. Notice that meiosis pauses in meiosis I before birth, then resumes in some primary oocytes beginning at puberty. Meiosis II does not complete until fertilization occurs.

At fertilization, the final phase of meiotic cell division in the ovum completes, and the last polar body is released. 629The sex cells from both parents unite fully and the normal chromosome number (46) is achieved in the zygote that is formed.

Production of estrogen and progesterone

The second major function of the ovary, in addition to oogenesis, is secretion of the sex hormones, estrogen and progesterone. Hormone production in the ovary begins at puberty with the cyclic development and maturation of the ovum. The granulosa cells around the oocyte in the growing and mature follicle secrete estrogen. The corpus luteum, which develops after ovulation, chiefly secretes progesterone but also some estrogen.

Estrogen is the sex hormone that causes the development and maintenance of the female secondary sex characteristics and stimulates growth of the epithelial cells lining the uterus. Some of the actions of estrogen include the following:

  1. Development and maturation of female reproductive organs, including the external genitals
  2. Appearance of pubic hair and breast development
  3. Development of female body contours by deposition of fat below the skin surface and in the breasts and hip region
  4. Initiation of the first menstrual cycle

Progesterone is produced by the corpus luteum, which is a glandular structure that develops from a follicle that has just released an ovum. If stimulated by the appropriate anterior pituitary hormone, the corpus luteum produces progesterone for about 11 days after ovulation. Progesterone stimulates proliferation and vascularization of the epithelial lining of the uterus and acts with estrogen to initiate the menstrual cycle in girls entering puberty.

The surgical term oophorectomy is used to describe removal of the ovaries. If both ovaries are removed, sterility results and menopause follows.

QUICK CHECK

  1. Identify the essential organs of reproduction in women.
  2. Where are the female glands located?
  3. What is oogenesis?
  4. Which female sex hormone causes the development and maintenance of the female secondary sex characteristics?

CLINICAL APPLICATION

HORMONE REPLACEMENT THERAPY

Hormone replacement therapy (HRT) using estrogen alone or in combination with progestin (synthetic progesterone) is sometimes used to reduce moderate to severe symptoms of menopause such as hot flashes. These symptoms result from the drop in estrogen that characterizes menopause. Although HRT may have some benefits in reducing or preventing chronic disorders such as osteoporosis, dementia, or heart disease, there are potentially serious health risks as well. Therefore HRT is used only for the more serious menopause cases and not to prevent chronic disease. To keep the risks low, HRT requires careful analysis of an individual’s situation and determination of the lowest possible effective dose in the shortest possible treatment period. Alternative drug therapies may be just as effective as HRT in some cases.