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25.2: Female Reproductive System

Created by: CK-12

Lesson Objectives

  • Identify female reproductive structures and their functions.
  • Explain how the female reproductive system develops.
  • Describe how eggs are produced.
  • Outline the phases of the menstrual cycle.

Vocabulary

estrogen
female sex hormone secreted by the ovaries
Fallopian tube
one of two female reproductive organs that carry eggs from the ovary to the uterus and provide the site where fertilization usually takes place
follicle-stimulating hormone (FSH)
pituitary gland hormone that stimulates the ovaries to secrete estrogen and follicles in the ovaries to mature
menarche
beginning of menstruation; first monthly period in females
menopause
period during which menstrual cycles slow down and eventually stop in middle adulthood\
menstrual cycle
monthly cycle of processes and events in the ovaries and uterus of a sexually mature human female
menstruation
process in which the endometrium of the uterus is shed from the body during the first several days of the menstrual cycle; also called monthly period
oogenesis
process of producing eggs in the ovary
ovulation
release of a secondary oocyte from the uterus about half way through the menstrual cycle
vulva
external female reproductive structures, including the labia and vaginal opening

Introduction

The female reproductive system consists of structures that produce female gametes called eggs and secrete the female sex hormone estrogen. The female reproductive system has several other functions as well:

  1. It receives sperm during sexual intercourse.
  2. It supports the development of a fetus.
  3. It delivers a baby during birth.
  4. It breast-feeds a baby after birth.

Female Reproductive Structures

The main structures of the female reproductive system are shown in Figure below. Most of the structures are inside the pelvic region of the body. Locate the structures in the figure as you read about them below. To watch an animation of the female reproductive system, go to this link: http://www.medindia.net/animation/female_reproductive_system.asp.

CC-BY-SA 3.0

Female Reproductive Structures. Organs of the female reproductive system include the vagina, uterus, ovaries, and fallopian tubes.

External Structures

The external female reproductive structures are referred to collectively as the vulva. They include the labia (singular, labium), which are the “lips” of the vulva. The labia protect the vagina and urethra, both of which have openings in the vulva.

Vagina

The vagina is a tube-like structure about 9 centimeters (3.5 inches) long. It begins at the vulva and extends upward to the uterus. It has muscular walls lined with mucous membranes. The vagina has two major reproductive functions. It receives sperm during sexual intercourse, and it provides a passageway for a baby to leave the mother’s body during birth.

Uterus

The uterus is a muscular organ shaped like an upside-down pear. It has a thick lining of tissues called the endometrium. The lower, narrower end of the uterus is known as the cervix. The uterus is where a fetus grows and develops until birth. During pregnancy, the uterus can expand greatly to make room for the baby as it grows. During birth, contractions of the muscular walls of the uterus push the baby through the cervix and out of the body.

Ovaries

The two ovaries are small, egg-shaped organs that lie on either side of the uterus. They produce eggs and secrete estrogen. Each egg is located inside a structure called a follicle. Cells in the follicle protect the egg and help it mature.

Fallopian Tubes

Extending from the upper corners of the uterus are the two fallopian tubes. Each tube reaches (but is not attached to) one of the ovaries. The ovary end of the tube has a fringelike structure that moves in waves. The motion sweeps eggs from the ovary into the tube.

Breasts

The breasts are not directly involved in reproduction, but they nourish a baby after birth. Each breast contains mammary glands, which secrete milk. The milk drains into ducts leading to the nipple. A suckling baby squeezes the milk out of the ducts and through the nipple.

Sexual Development in Females

Female reproductive organs form before birth. However, as in males, the organs do not mature until puberty.

Development Before Birth

Unlike males, females are not influenced by the male sex hormone testosterone during embryonic development. This is because they lack a Y chromosome. As a result, females do not develop male reproductive organs. By the third month of fetal development, most of the internal female organs have formed. Immature eggs also form in the ovary before birth. Whereas a mature male produces sperm throughout his life, a female produces all the eggs she will ever make before birth.

Changes of Puberty

Like baby boys, baby girls are born with all their reproductive organs present but immature and unable to function. Female reproductive organs also grow very little until puberty. Girls begin puberty a year or two earlier than boys, at an average age of 10 years. Girls also complete puberty sooner than boys, in about 4 years instead of 6. Puberty in girls starts when the hypothalamus “tells” the pituitary gland to secrete hormones that target the ovaries. Two pituitary hormones are involved: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones stimulate the ovary to produce estrogen. Estrogen, in turn, promotes growth and other physical changes of puberty. It stimulates growth and development of the internal reproductive organs, breasts, and pubic hair (see Figure below). You can watch an animation of these and other changes that girls experience during puberty at this link: http://www.bbc.co.uk/science/humanbody/body/interactives/lifecycle/teenagers/.

Changes in Females During Puberty. Two obvious changes of puberty in girls are growth and development of the breasts and pubic hair. The stages begin around age 10 and are completed by about age 14.

Adolescent Growth Spurt

Like boys, girls also go through an adolescent growth spurt. However, girls typically start their growth spurt a year or two earlier than boys (and therefore a couple of centimeters shorter, on average). Girls also have a shorter growth spurt. For example, they typically reach their adult height by about age 15. In addition, girls generally do not grow as fast as boys do during the growth spurt, even at their peak rate of growth. As a result, females are about 10 centimeters (about 4 inches) shorter, on average, than males by the time they reach their final height.

Menarche

One of the most significant changes in females during puberty is menarche. Menarche is the beginning of menstruation, or monthly periods. In U.S. girls, the average age of menarche is 12.5 years, although there is a lot of variation in this age. The variation may be due to a combination of genetic factors and environmental factors, such as diet.

Egg Production

At birth, a female’s ovaries contain all the eggs she will ever produce. However, the eggs do not start to mature until she enters puberty. After menarche, one egg typically matures each month until a woman reaches middle adulthood.

Oogenesis

The process of producing eggs in the ovary is called oogenesis. Eggs, like sperm, are haploid cells, and their production occurs in several steps that involve different types of cells, as shown in Figure below. You can follow the process of oogenesis in the figure as you read about it below.

Oogenesis. Oogenesis begins before birth but is not finished until after puberty. A mature egg forms only if a secondary oocyte is fertilized by a sperm.

Oogenesis begins long before birth when an oogonium with the diploid number of chromosomes undergoes mitosis. It produces a diploid daughter cell called a primary oocyte. The primary oocyte, in turn, starts to go through the first cell division of meiosis (meiosis I). However, it does not complete meiosis until much later. The primary oocyte remains in a resting state, nestled in a tiny, immature follicle until puberty.

Maturation of a Follicle

Beginning in puberty, each month one of the follicles and its primary oocyte starts to mature (also see Figure below). The primary oocyte resumes meiosis and divides to form a secondary oocyte and a smaller cell, called a polar body. Both the secondary oocyte and polar body are haploid cells. The secondary oocyte has most of the cytoplasm from the original cell and is much larger than the polar body.

Maturation of a Follicle and Ovulation. A follicle matures and its primary oocyte (follicle) resumes meiosis to form a secondary oocyte in the secondary follicle. The follicle ruptures and the oocyte leaves the ovary during ovulation. What happens to the ruptured follicle then?

Ovulation and Fertilization

After 12–14 days, when the follicle is mature, it bursts open, releasing the secondary oocyte from the ovary. This event is called ovulation (see Figure above). The follicle, now called a corpus luteum, starts to degenerate, or break down. After the secondary oocyte leaves the ovary, it is swept into the nearby fallopian tube by the waving, fringelike end (see Figure below).

Egg Entering Fallopian Tube. After ovulation, the fringelike end of the fallopian tube sweeps the oocyte inside of the tube, where it begins its journey to the uterus.

If the secondary oocyte is fertilized by a sperm as it is passing through the fallopian tube, it completes meiosis and forms a mature egg and another polar body. (The polar bodies break down and disappear.) If the secondary oocyte is not fertilized, it passes into the uterus as an immature egg and soon disintegrates. You can watch an animation of all these events and the hormones that control them at the link below. http://health.howstuffworks.com/adam-200017.htm

Menstrual Cycle

Ovulation is part of the menstrual cycle, which typically occurs each month in a sexually mature female unless she is pregnant. Another part of the cycle is the monthly period, or menstruation. Menstruation is the process in which the endometrium of the uterus is shed from the body. The menstrual cycle is controlled by hormones from the hypothalamus, pituitary gland, and ovaries. For an interactive animation of the menstrual cycle, you can go this link: http://health.howstuffworks.com/adam-200132.htm

Phases of the Menstrual Cycle

As shown in Figure below, the menstrual cycle occurs in several phases. The cycle begins with menstruation. During menstruation, arteries that supply the endometrium of the uterus constrict. As a result, the endometrium breaks down and detaches from the uterus. It passes out of the body through the vagina over a period of several days.

Phases of the Menstrual Cycle. The menstrual cycle occurs in the phases shown here.

After menstruation, the endometrium begins to build up again. At the same time, a follicle starts maturing in an ovary. Ovulation occurs around day 14 of the cycle. After it occurs, the endometrium continues to build up in preparation for a fertilized egg. What happens next depends on whether the egg is fertilized.

If the egg is fertilized, the endometrium will be maintained and help nourish the egg. The ruptured follicle, now called the corpus luteum, will secrete the hormone progesterone. This hormone keeps the endometrium from breaking down. If the egg is not fertilized, the corpus luteum will break down and disappear. Without progesterone, the endometrium will also break down and be shed. A new menstrual cycle thus begins.

Menopause

For most women, menstrual cycles continue until their mid- to late forties. Then women go through menopause, a period during which their menstrual cycles slow down and eventually stop, generally by their early fifties. After menopause, women can no longer reproduce naturally because their ovaries no longer produce eggs.

Lesson Summary

  • The female reproductive system consists of structures that produce eggs and secrete female sex hormones. They also provide a site for fertilization and enable the development and birth of a fetus. They include the vagina, uterus, ovaries, and fallopian tubes.
  • Female reproductive organs form before birth. However, they do not mature until puberty.
  • Immature eggs form in the ovaries before birth. Each month, starting in puberty, one egg matures and is released from the ovary. Release of an egg is called ovulation.
  • The menstrual cycle includes events that take place in the ovary, such as ovulation. It also includes changes in the uterus, including menstruation. Menopause occurs when menstruation stops occurring, usually in middle adulthood.

Lesson Review Questions

Recall

1. List three general functions of the female reproductive system.

2. Describe the uterus, and state its role in reproduction.

3. State two ways that puberty differs in girls and boys.

4. Describe ovulation.

5. Define menstruation. What is the first menstrual period called?

6. What is menopause? When does it occur?

Apply Concepts

7. Create a flow chart showing the steps in which an oogonium develops into a mature egg.

8. Make a cycle diagram to represent the main events of the menstrual cycle in both the ovaries and the uterus, including the days when they occur.

Think Critically

9. Predict how blockage of both fallopian tubes would affect a woman’s ability to reproduce naturally. Explain your answer.

10. Males and females are quite similar in height when they begin the adolescent growth spurt. Why are females about 10 centimeters shorter than males by adulthood?

11. Compare and contrast what happens in the menstrual cycle when the egg is fertilized with what happens when the egg is not fertilized.

Points to Consider

If an egg is fertilized by a sperm and implants in the uterus, the endometrium helps nourish it. However, as the new organism grows, it soon needs more nutrients than the endometrium can provide. These nutrients are provided by the mother’s bloodstream.

  • How do you think the fetus is able to obtain nutrients from the mother’s blood? What structures and processes might be involved?
  • The fetus also produces wastes. How do you think the wastes are excreted?

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