13.12: Ovaries and Female Gamete Production

The ovaries produce oocytes and reproductive hormones through coordinated processes that change across the female reproductive lifespan.

Master this section and you'll be able to:

• Describe the location and the function of the ovaries.
• Explain the processes of oogenesis and folliculogenesis.
• List the main three hormones produced by the ovaries and outline their roles.

The female reproductive system functions to produce gametes and reproductive hormones, just like the male reproductive system; however, it also has the additional task of supporting the developing fetus and delivering it to the outside world. Unlike its male counterpart, the female reproductive system is located primarily inside the pelvic cavity. Recall that the ovaries are the female gonads. The gamete they produce is called an oocyte. We’ll discuss the production of oocytes in detail shortly. First, let’s look at some of the structures of the female reproductive system.

 

Figure \(\PageIndex{1}\):  Female Reproductive System. The major organs of the female reproductive system are located inside the pelvic cavity.  (Image credit: "Female Reproductive System - Anterior and Midsagittal" by Blausen.com staff (2014), from the Medical gallery of Blausen Medical 2014, WikiJournal of Medicine, is licensed under CC BY 3.0.  Modifications by Jennifer Lange.  Image credit: "Female Reproductive Organs - Anterior View" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by StacyRed-Wiki via Wikimedia Commons. )

 

Ovaries and the Ovarian Cycle

The ovaries are the female gonads. Each ovary is a small, almond-sized organ (about 2 to 3 cm long) located within the pelvic cavity. The pair sits on either side of the uterus, and each ovary is connected to the uterus by the ovarian ligament.

The ovary is covered by a layer of cuboidal cells called the germinal epithelium. Beneath this covering is the cortex, the outer region of the ovary where oocytes develop. Each oocyte is surrounded by supporting cells to form a follicle.  

Deep to the cortex lies the medulla, which contains the ovary’s blood vessels, lymphatic vessels, and nerves.

The ovarian cycle refers to the predictable changes that occur in a female’s oocytes and ovarian follicles. During the reproductive years, this cycle repeats about every 28 days. It relates to the menstrual cycle but is not the same process. The ovarian cycle describes the monthly events in the ovaries that lead to the development and release of an egg, while the menstrual cycle describes the coordinated changes in the uterus that prepare the uterine lining for possible pregnancy or result in menstruation if pregnancy does not occur.

The ovarian cycle includes two connected events:

• Oogenesis, which is the formation of female gametes, and
• Folliculogenesis, which is the growth of ovarian follicles.

Oogenesis

Oogenesis is the process of forming an egg cell, or oocyte. It begins before birth. During fetal development, stem cells in the ovary called oogonia divide to form primary oocytes. These primary oocytes enter the first stage of meiosis and then pause. They remain in this paused state from before birth until puberty.

At puberty, the beginning of ovulation signals the start of reproductive maturity. From this point on, ovulation typically happens about once every 28 days. Just before ovulation, a surge in luteinizing hormone causes a primary oocyte to resume meiosis.

The cell divides unevenly. One large cell becomes the secondary oocyte that is released during ovulation, and one small cell becomes the first polar body. The polar body does not become an egg and eventually breaks down. Even though the division can create up to four cells, only the secondary oocyte survives to continue the cycle.

Figure \(\PageIndex{2}\): Oogenesis and the Formation of Polar Bodies. A primary oocyte (2n) begins meiosis I and divides unevenly, producing a large secondary oocyte (n) and a much smaller first polar body. The secondary oocyte then starts meiosis II and, if fertilization occurs, completes the division to form one mature ovum (n) and additional polar bodies. Polar bodies contain the excess chromosomes but very little cytoplasm, allowing nearly all available cytoplasm to remain in the single functional ovum.
 

This second diagram (see image on the right) relates oogenesis to the stages of a female’s life. To understand what is happening, remember that meiosis is a special type of cell division that produces sex cells with half the usual number of chromosomes. It happens in two steps, meiosis I and meiosis II, and it reduces a diploid cell (2n) with paired chromosomes into haploid cells (n) with single chromosome sets.

Oogenesis begins before birth. Early germ cells called oogonia (2n) divide normally by mitosis to increase in number. Some of these oogonia prepare to become eggs. They start meiosis I and become primary oocytes. These primary oocytes pause in a very early stage of meiosis I and remain paused throughout childhood.

At puberty, hormones trigger one primary oocyte each month to continue meiosis I. This division is uneven. Most of the cytoplasm stays in one large cell called the secondary oocyte (n). A tiny cell called the first polar body is also formed. The polar body contains chromosomes but almost none of the cytoplasm needed to support development.

The secondary oocyte then starts meiosis II, but it pauses again, this time in metaphase II. This is the stage in which the oocyte is released during ovulation.

Meiosis II of a secondary oocyte is completed only if a sperm succeeds in fertilizing the secondary oocyte, which must occur within 24 hours of ovulation. If fertilization occurs, meiosis II produces the mature ovum (n) and additional polar bodies.

The purpose of polar bodies is to discard extra chromosomes so the developing oocyte keeps almost all of the cytoplasm needed to support early embryonic growth.

If sperm does not penetrate, then the secondary oocyte will break down after 24 hrs.

The larger amount of cytoplasm contained in the female gamete is used to supply the developing zygote with nutrients during the period between fertilization and implantation into the uterus. Interestingly, sperm contribute only DNA at fertilization — no cytoplasm. Therefore, the cytoplasm and all of the cytoplasmic organelles in the developing embryo are of maternal origin. This includes mitochondria, which contain their own DNA. Scientific research in the 1980s determined that mitochondrial DNA was maternally inherited, meaning that you can trace your mitochondrial DNA directly to your mother, her mother, and so on back through your female ancestors.

The number of primary oocytes decreases over time. A baby girl has one to two million, about four hundred thousand remain at puberty, and none remain by the end of menopause.

To keep the big picture straight, it helps to see oocyte development as a simple timeline:

• Before birth: primary oocytes (diploid = 2n) form and pause

• From puberty, each cycle: one primary oocyte finishes meiosis I and starts meiosis II → secondary oocyte (haploid = n)

• At fertilization: secondary oocyte finishes meiosis II → ovum (n)
   

Folliculogenesis

Folliculogenesis is the process in which ovarian follicles (an oocyte plus its supporting cells) grow and mature. During each roughly 28-day cycle, usually one follicle reaches full maturity and is ovulated, while many others begin to grow but then die. This normal, programmed death of follicles is called atresia.

Females are born with about one to two million ovarian follicles, each containing a primary oocyte. This supply is not renewed. Instead, the number of follicles steadily decreases over time. By the time a woman reaches menopause, all follicles have been used up or are lost, and no functional follicles remain.

As follicles develop, they move through several stages: primordial, primary, secondary, and tertiary (or Graafian) follicles. Even though the follicle grows and changes during these stages, the oocyte inside it remains a primary oocyte until just before ovulation, when it finally completes meiosis I.

Figure \(\PageIndex{3}\): Folliculogenesis. The maturation of a follicle is shown in a clockwise direction proceeding from the smallest, resting primordial follicles to the large, fluid-filled tertiary (Graafian) follicle that releases a secondary oocyte at ovulation. Unlike the previous diagram that focused only on meiotic divisions and polar bodies, this image highlights the growth of the follicle around the oocyte: primordial follicles with a single layer of cells, primary and secondary follicles with expanding layers of follicular cells, and the tertiary follicle with a large antrum. It also illustrates atresia, the natural degeneration of many follicles, and the post-ovulation structures of the corpus luteum and corpus albicans. Once the follicle is mature, it ruptures and releases the oocyte. Cells remaining in the follicle then develop into the corpus luteum.  (Image credit: “Ovarian Follicles" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of "Slagter - Drawing Ovarium with folliculogenesis and ovulation - English labels" by Ron Slagter, Hope Wicks, LUMC, Cindy J.M. Hulsman, MUMC+ et al.)

 

Hormones Produced by the Ovaries

The ovaries make three main hormones: estrogen, progesterone, and small amounts of testosterone.

• Estrogen helps create the secondary sex characteristics that appear at puberty, such as breast development and wider hips. It also supports the growth and ongoing health of the female reproductive organs.

• Progesterone prepares the uterus for a possible pregnancy by thickening the endometrium, and it helps prepare the mammary glands for milk production. Together with estrogen, it drives the monthly changes of the menstrual cycle.

• Testosterone is present in low amounts in women but is still important. It helps support muscle mass, bone strength, energy levels, and contributes to libido.