In this explainer, we will learn how to identify structures in the female genital (reproductive) system, state their functions, and describe the formation of ova in the ovary.
Did you know that, at birth, a female human has approximately 1 million egg cells? By the time she reaches puberty, this number decreases to about 300 000. Over her reproductive lifetime, only 300–400 of these eggs will be released!
Egg cells, sometimes called ova (singular: ovum), are a female’s gametes, or sex cells.
Key Term: Ovum (Egg Cell)
An egg cell is the female reproductive cell, or gamete.
Gametes are an organism’s reproductive cells, which contain half the genetic material of a normal body cell.
Each ovum contains half the genetic material of a normal body cell, and so ova are haploid cells. Sperm cells are also haploid, as you can see in Figure 1 below.
Key Term: Haploid Cell
A haploid cell is a cell that only has a single set of chromosomes n.
This is because in fertilization, when an ovum and a sperm cell fuse, they will form a zygote with a full set of genetic material in its nucleus. The zygote is therefore diploid, as its nucleus contains a full set of 46 chromosomes.
Key Term: Diploid Cell
A diploid cell is a cell that has two complete sets of chromosomes (2n) arranged into homologous pairs.
Let’s look at some of the main structures in the female reproductive system that you can see in Figure 2 below.
The female reproductive system contains two ovaries, which are small oval-shaped organs about the size of an almond. The ovaries are responsible for releasing ova and sex hormones.
Key Term: Ovaries
The ovaries (singular: ovary) are the female reproductive organs from which egg cells and hormones are released.
Each ovary is attached to a tube called a fallopian tube, or oviduct, which has a funnel-shaped opening close to the ovary. When ova are released from the ovary, they are received by finger-like projections on the end of the corresponding fallopian tube and will then travel through the fallopian tube toward the uterus. Fallopian tubes are lined with ciliated epithelial cells, which have structures called cilia on their surface. These cilia help to waft, or move, the ovum along the fallopian tube toward the uterus as you can see in Figure 3. Some cells in the membrane of the fallopian tube will also release mucus to aid this movement. The fallopian tubes are usually where a sperm cell will fuse with an ovum if fertilization is successful.
Key Term: Fallopian Tubes (Oviducts)
Fallopian tubes link each ovary to a female’s uterus. After ovulation, an egg cell travels along a fallopian tube toward the uterus. If an egg is fertilized by a sperm cell, it will likely occur in a fallopian tube.
Example 1: Identifying Structures with Ciliated Cells
A front view of the human female reproductive system is shown in the figure.
In which part are ciliated epithelial cells most abundant?
Ciliated epithelial cells play an important role in moving substances along tracts and tubes in the human body. For example, there are ciliated epithelial cells in the female reproductive system that are responsible for wafting an egg from an ovary along a fallopian tube toward the uterus.
Egg cells are released from the ovary, which is labeled C in the diagram, in a process called ovulation. The egg cell then needs to move toward the uterus as this is where an embryo will implant if the egg is fertilized by a sperm cell. The embryo is an early stage of human development that follows the division of this fertilized egg cell. This majority of this movement is done by ciliated cells in the fallopian tubes. If the egg is fertilized by a sperm cell, it is most likely to occur in one of the fallopian tubes.
The label A in the diagram is the cervix, a region of tissue at the base of the uterus. Label D is the vagina, which usually receives sperm in sexual intercourse for an egg to be fertilized. The fallopian tube is labeled as B.
Therefore, the part of the female reproductive system in which ciliated epithelial cells are most abundant is B, the fallopian tube.
The uterus is a hollow organ in a woman’s pelvic region. It has a muscular wall filled with blood vessels and smooth muscle that allows it to contract.
The main role of the uterus is to provide a place for the implantation of an embryo. When a sperm cell fertilizes an egg cell, it initially forms a cell called a zygote. After this zygote divides, it is called an embryo. The uterus not only provides a place for this embryo to implant, but also a home for the growing embryo to develop in for the 9 months of pregnancy. There are ligaments within the female reproductive system that play a role in supporting the uterus and maintaining its position during pregnancy.
At the base of the uterus is a region of tissue called the cervix. The cervix connects the uterus to the vaginal lumen.
Key Term: Uterus (Womb)
The uterus is a hollow organ in a woman’s pelvis in which an embryo can grow and develop.
The vagina is a tube with smooth muscle in its wall and is around 7 cm long, though this will vary from woman to woman. The vagina extends from the cervix to the external genitalia, and it is the structure that is usually responsible for receiving sperm from a man’s penis during sexual intercourse.
The vagina is lined with a mucus membrane, but its moisture is mainly maintained by lubrication from the glands of the cervix and glands located at the posterior part of the vaginal opening that can secrete fluids to moisten the vagina. The vagina also expands during labor, which is often called childbirth. The vagina has other helpful adaptations, such as an acidic pH usually between 3.8 and 4.5, and has its own microclimate of “good” bacteria to protect it against more dangerous microorganisms.
Key Term: Vagina
The vagina is a muscular tube in the female reproductive system that extends from the external genitalia to the cervix.
Once sperm has entered the vagina, most sperm cells will attempt to travel up into the uterus through the cervix. From there, sperm can access one of the two fallopian tubes in an attempt to fertilize an ovum.
Let’s look in more detail at how some of these structures in the female’s reproductive system can change over her lifetime and on a monthly basis.
When puberty starts, a woman’s body begins preparing for the possibility of reproduction by going through physical, emotional, and mental changes. The changes that occur during puberty are controlled by the release of hormones, such as estrogen and progesterone, primarily from the ovaries.
One such change that women tend to experience upon starting puberty is the beginning of the menstrual cycle. The menstrual cycle describes the approximately monthly, rhythmic changes in the secretions of hormones that cause changes in the female reproductive system. One change that occurs during the menstrual cycle is the thickness of the lining of the uterus, which you can see outlined in Figure 4 below.
The menstrual cycle begins with menstruation, sometimes called a period. You can see this occurring from approximately day 0 to 6 in Figure 4. The lining of the uterus, sometimes called the endometrium, has a rich blood supply. When a female menstruates, the uterine lining exits the vagina with blood, initiated by sometimes painful contractions in the smooth muscles in the uterine wall. Though on the graph this is shown at about 6 days, the length of menstruation will vary. The duration of the menstrual cycle can change, and differs between women, but a whole cycle tends to last about 28 days from the start of one period to the start of the next.
Menstruation is a process that occurs in most females approximately once every month from puberty until menopause, except during pregnancy. During menstruation, the uterine lining breaks down, shedding blood and other material out of the vagina.
After menstruation, the uterine lining begins building up again, as you can see from approximately day 8 to 18 in Figure 4. This process is also controlled by sex hormones released mainly from the ovaries. When the lining of the uterus has fully built up, it is ready to receive a fertilized egg cell in a process called implantation. The lining of the uterus remains thick until approximately day 28 before the menstrual cycle repeats again.
In order for a sperm cell to fertilize an ovum, the ovum must first be released from the ovary in a process called ovulation. This is also controlled by sex hormones and occurs approximately monthly when the uterine lining is thick. Usually, only one ovum is released per menstrual cycle, and they tend to be released from alternate ovaries each month. If a fertilized egg does implant in the uterine wall, approximately 9 months later, the smooth muscle present in the uterus will play another role in contractions to push the baby out of the uterus in childbirth.
Ovulation is the part of the menstrual cycle when an egg is released from one of a woman’s ovaries.
The menstrual cycle continues, on approximately a monthly basis, until either pregnancy or menopause. Following pregnancy, the menstrual cycle usually restarts again, but menopause stops it completely. This means that a woman will no longer menstruate or ovulate. Ovulation stops as her ovaries become inactive due to a decrease in hormonal secretions. Many women will be fertile and able to give birth between puberty and menopause, which occurs at around a median age of 51 years.
Menopause is the period in a woman’s life when menstruation stops.
Let’s look at how the ova develop and are released from the ovary.
The female ovary is made up of lots of small fluid-filled sacs called follicles, each of which contains an immature ovum. The process by which a mature ovum is produced and develops is called oogenesis, and it begins before a woman is even born while she is still an embryo!
There are three main phases of oogenesis: multiplication (which is also called proliferation), growth, and maturation. You can see cells in different stages of development in the micrograph image of a transverse section (TS) of an ovary below.
Oogenesis is the process by which an ovum is produced and developed.
Figure 6 shows what is happening in the micrograph image in some more detail.
First, let’s look at the multiplication phase.
When a female is a fetus between 8–20 weeks old, lots of her cells are dividing and multiplying by mitosis. The immature ova at this stage are called primary ova, primary germ cells, or primordial germ cells (PGCs). PGCs are diploid cells that give rise to reproductive cells in both males and females. In males, PGCs will develop into spermatozoa in the testes, and in females PGCs will develop into oocytes in the ovaries. As PGCs are diploid, they have a full set of 46 chromosomes.
This multiplication of cells is the first stage of oogenesis and produces diploid cells called oogonia (singular: oogonium), as you can see in Figure 6. This process continues until a few weeks before birth.
Key Term: Multiplication Phase
The multiplication phase is the first stage of gamete formation, during which primary (primordial) germ cells (2n) are converted into spermatogonia (2n) in spermatogenesis or oogonia (2n) in oogenesis.
Next, let’s look at the growth phase.
The second stage of oogenesis is the growth phase, which also occurs in the ovaries while the female is still an embryo. Each oogonium increases in size to become a primary oocyte in the growth phase, as you can see in Figure 6. The follicle that contains the oocyte also becomes enriched with nutrients such as proteins and hormones. Primary oocytes begin meiosis I, but this is halted during prophase early in the process. As the primary oocytes have not completed meiosis yet, they are also diploid cells as you can see in Figure 7. Primary oocytes remain dormant in their follicles until ovulation begins at puberty.
Key Term: Primary Oocyte
A primary oocyte is a diploid cell formed in oogenesis by the growth of an oogonium.
Key Term: Growth Phase
The growth phase is the second stage of gamete formation, during which spermatogonia or oogonia divide and differentiate into primary spermatocytes and primary oocytes respectively.
Sometimes, the stages of oogenesis are classified according to whether they occur before the birth of a fetus, sometimes called prenatal oogenesis, or after its birth in postnatal oogenesis.
While the first two stages of oogenesis are prenatal, the final stage of oogenesis is postnatal, during which oocytes undergo maturation. This occurs in a female once she has started her menstrual cycle, and it occurs during ovulation triggered by hormones. The maturation phase is how a primary oocyte is converted into a secondary oocyte, as you can see in Figure 6.
Key Term: Maturation Phase
The maturation phase is the third stage of gamete formation. In spermatogenesis, one primary spermatocyte undergoes meiosis I and meiosis II to produce two secondary spermatocytes and then four spermatids. In oogenesis, the primary oocytes undergo meiosis I to become secondary oocytes, and meiosis II is arrested in metaphase, only completing upon successful fertilization by a sperm cell.
Meiosis consists of two stages of division. The primary oocyte completes the first stage of meiosis, meiosis I, which splits its nucleus to produce two haploid daughter cells. Remember that this process began during a prenatal stage of oogenesis, but was arrested, or stopped, during prophase I.
Half of the primary oocyte’s genetic material goes into a cell called a secondary oocyte, and the other half of the genetic material goes into a much smaller nonfunctional cell called the first polar body. As they contain half the genetic material of a normal body cell, both the secondary oocyte and the first polar body are haploid. The polar body is a much smaller cell containing very little cytoplasm, and it is sometimes called nonfunctional as it will not develop into a mature ovum.
Key Term: Secondary Oocyte
A secondary oocyte is a haploid cell formed from a primary oocyte going through meiosis I in the final stage of oogenesis.
Key Term: Polar Body
A polar body is a small nonfunctional cell produced during each stage of meiosis during oogenesis, which does not develop into a mature ovum.
Meiosis II, the second stage of meiosis, will begin after the completion of meiosis I, but it will stop during metaphase before the secondary oocyte and first polar body split. Meiosis II will only be completed to form a mature ovum if the secondary oocyte is successfully fertilized by a sperm cell.
You can see a summary in Figure 7 of how the chromosome number changes through mitotic and meiotic divisions in each of the stages of oogenesis, and then fertilization.
To summarize the process of oogenesis as we can see in Figure 7, first, the oogonium (2n) divides by mitosis to develop into the primary oocyte (2n). The primary oocyte undergoes meiosis I to form a secondary oocyte n and the first polar body n. Both of these haploid cells then undergo meiosis II, which is completed upon successful fertilization of the secondary oocyte by a sperm cell, to form 3 secondary polar bodies and one mature ovum.
The secondary oocyte is contained within a follicle called the Graafian follicle. When ovulation occurs and the secondary oocyte has been produced, the Graafian follicle bursts open, as you can see in stage 4 in Figure 6, and the secondary oocyte is released into the fallopian tube.
Once the secondary oocyte has left the ovary, the Graafian follicle transform into a structure called the corpus luteum, shown in stage 5 of Figure 6. If the secondary oocyte is fertilized, the corpus luteum is useful in releasing hormones to control early pregnancy. If the secondary oocyte is not fertilized, the corpus luteum breaks down causing the release of hormones to decrease and triggering menstruation to begin.
Key Term: Graafian Follicle
A Graafian follicle is a fluid-filled structure in the ovary within which an immature ovum develops prior to ovulation.
Key Term: Corpus Luteum
The corpus luteum is a hormone-secreting structure that develops in an ovary after an ovum has been discharged in ovulation, but degenerates after a few days unless pregnancy has begun.
The image in Figure 8 summarizes the main processes that occur to transform the oogonium into a fertilized egg.
Example 2: Identifying the Structure That Forms in the Ovary after Ovulation
What structure forms in the ovary every month after ovulation has occurred?
Ovulation is the process by which an egg cell, or ovum, is released from a female ovary.
When in the ovary, the immature egg cell is contained within a structure called the Graafian follicle. When ovulation is triggered by hormones, usually one immature egg cell bursts out of the Graafian follicle and exits the ovary to travel along the fallopian tube and potentially be fertilized by a sperm cell. When fertilization occurs, the immature ovum is now a mature fertilized ovum that can form a zygote and possibly an embryo.
After the egg has left the ovary, the Graafian follicle transforms into corpus luteum. If fertilization occurs, the corpus luteum is useful in producing hormones to influence early pregnancy. If fertilization does not occur, the corpus luteum is inactive and breaks down. This decreases the secretion of hormones, and triggers menstruation to occur.
Therefore, the structure that forms in the ovary each month following ovulation is the corpus luteum.
Example 3: Identifying the Cells Present in an Ovary at Birth
In which of the following stages can cells be found in a healthy infant ovary?
- Secondary oocytes
- Polar bodies
- Primary germ cells
- Primary oocytes
The process by which a mature ovum is produced and develops is called oogenesis and it begins before a woman is even born while she is still an embryo! There are three main phases of oogenesis: multiplication, growth, and maturation.
In multiplication, primary germ cells, sometimes called primordial germ cells, are converted into many oogonia in a process called mitosis. In the growth stage, these oogonia grow into primary oocytes. Both of these processes occur while the female is still in her embryonic development.
When a female has been born and years later begins puberty during her adolescence, she starts ovulating approximately once per month. At this point, the maturation phase occurs, and the primary oocyte is converted into a large secondary oocyte and a smaller polar body. This secondary oocyte will probably be released from the ovary by ovulation each month and might be fertilized by a sperm cell.
The answer is not secondary oocytes nor is it polar bodies, as these are not produced until puberty when ovulation begins and the maturation phase can occur.
Primary germ cells are the first cells that exist in the ovary, and during embryonic development of the female ovary, these primary germ cells will be converted into oogonia and eventually primary oocytes. As both the multiplication phase that produces oogonia and the growth phase that produces primary oocytes occur in a female before she is born, the final form of cells that will be present in her ovaries when she is an infant are the primary oocytes.
Therefore, the cells that can be found in a healthy infant ovary from birth are primary oocytes.
As mentioned, meiosis II will only be completed if a sperm cell fertilizes the egg cell. When this occurs, the secondary oocyte will divide a second time, forming an ovum and a second polar body. The first polar body will also divide a second time as you can see in Figure 7, producing two more second polar bodies. Therefore, the final products of meiosis are three polar bodies that will usually degrade, and one mature ovum that has been fertilized by a sperm cell. Sometimes, the polar bodies remain and play a role in the future life cycle events of an organism.
But how does the sperm cell stimulate the completion of this second meiotic division?
The Graafian follicle which surrounded the secondary oocyte while it was still in the ovary contained many tiny follicle cells. When it is released from the ovary at ovulation, some of these follicle cells remain attached to the surface of the secondary oocyte, forming a layered structure called the corona radiata that surrounds the ovum.
The corona radiata is many cell-layers thick. The oocyte is believed to secrete an intracellular glycoprotein matrix outside the oocyte cell membrane that contains carbohydrates, proteins, and a substance called hyaluronic acid. You can see this matrix layer in Figure 9, which is often simply referred to as a jelly coat, or more accurately the zona pellucida in mammals such as humans. This jelly-like layer must be penetrated by a sperm cell for fertilization to occur.
When a sperm cell, or spermatozoon, penetrates the corona radiata of the secondary oocyte, it releases enzymes from a section of the sperm’s head called the acrosome. These enzymes assist with the penetration through the outer layers of the egg cell. Though many sperms’ enzymes may be digesting the egg’s outer layers, only one sperm cell can usually enter the secondary oocyte’s cytoplasm. This activates the completion of meiosis II in the secondary oocyte, forming a mature ovum, and the ovum and sperm can fuse together in fertilization.
A similar process to oogenesis occurs in the male reproductive system to form sperm cells. This process is called spermatogenesis. Figure 10 shows the two processes side by side so we can have a look at the differences between the two processes.
In both spermatogenesis and oogenesis, there are the same initial stages of multiplication by mitosis and growth. At this point, the two processes diverge. During maturation in oogenesis, a large secondary oocyte and a small polar body are produced in meiosis I. The small polar body will eventually split into two and then both will usually degrade. During maturation in spermatogenesis, however, two secondary spermatocytes are produced from one primary spermatocyte in meiosis I.
Oogenesis requires ovulation and subsequently fertilization by a sperm cell to complete the process of meiosis II, which produces one large ovum and another polar body that usually eventually degrades. Spermatogenesis is completed independently of fertilization, and meiosis II occurs in the two secondary spermatocytes forming four haploid spermatids. Spermatogenesis also includes a final stage where the spermatids develop into haploid spermatozoa that are capable of fertilizing an ovum.
Therefore, while oogenesis will eventually produce one large egg cell from a primary germ cell, spermatogenesis will eventually produce four much smaller sperm cells from a primary germ cell.
Example 4: Determining the Number of Eggs That Can Be Produced From Each Cell That Begins Meiosis
How many eggs (ova) are produced from each cell that begins meiosis?
Meiosis is a process in which gametes, or sex cells, are formed. As meiosis includes two divisions, it results in haploid cells being produced, which have half the genetic material of a normal body cell, represented by an “n.” Diploid cells, which have the full genetic material of a normal body cell, are represented by “2n.”
Meiosis occurs toward the end of a process called oogenesis, through which an ovum, or egg cell, is produced. You can see the stages of this process that involve meiosis occurring in the figure below.
Meiosis I resumes in a female when she starts ovulation. Meiosis I converts a primary oocyte into a secondary oocyte and a much smaller cell called the first polar body. Meiosis II will start but will only be completed when an ovum is fertilized by a sperm cell. The secondary oocyte splits in meiosis II, producing a large haploid ovum and another small polar body. The first polar body also splits in meiosis II, so meiosis II overall produces a large egg cell and three small polar bodies. The polar bodies will usually eventually degrade.
Therefore, the number of eggs that are produced from each cell that begins meiosis is 1 egg.
Let’s recap some of the key points we have covered in this explainer.
- The main structures in the female genital system are the ovaries, fallopian tubes, uterus, cervix, and vagina.
- The ovaries release hormones and ova (egg cells) during ovulation.
- Ova travel along the fallopian tube, aided by ciliated epithelial cells in its lining, toward the uterus where any ovum that is fertilized by a sperm cell can develop into an embryo.
- Oogenesis is the process by which a primary germ cell develops into one mature ovum and three small polar bodies that usually degrade.
- The three stages of oogenesis involve mitosis and meiosis. They are multiplication, growth, and maturation that can be classified as prenatal or postnatal.
- Following maturation, meiosis II will be completed when an ovum is fertilized by a sperm cell.