The Monthly Cycle
Egg production by the ovary is part of the menstrual cycle. The menstrual cycle is a series of changes in the reproductive system of mature females that repeats every month. It includes events that occur in the uterus as well as the ovary.
Menstruation
While the egg and follicle are developing in the ovary, tissues are building up inside the uterus. The uterus develops a thick lining that is rich in tiny blood vessels. This prepares the uterus to receive an egg. If a sperm does not unite with the egg in the Fallopian tube, the lining of the uterus breaks down. Blood and other tissues from the lining are shed from the uterus. They pass through the vagina and out of the body. This is called menstruation. Menstruation is also called a menstrual period. It lasts about 4 days, on average. When the menstrual period ends, the cycle repeats.
Some people think that the average length of a menstrual period is the same as the “normal” length. They assume that shorter or longer menstrual periods are not normal. In fact, menstrual periods can vary from 1 to 8 days in length. Such variation is usually normal. The length of the overall menstrual cycle can also vary. The average length of the cycle is about 28 days, but there is no “normal” cycle length.
Hormones and the Menstrual Cycle
Hormones control the events of the menstrual cycle. The hormones are estrogen, progesterone, LH, and FSH. The ovaries secrete estrogen and progesterone. The pituitary gland secretes LH and FSH (see the Controlling the Body chapter).
The events of the menstrual cycle, including how hormone levels change throughout the cycle, are shown in Figure below. As the figure indicates, a menstrual cycle begins with menstruation. When menstruation ends, the ovaries start secreting more estrogen. Estrogen causes the lining of the uterus to build up, which prepares the uterus to receive an egg. It also causes the pituitary gland to secrete FSH. FSH, in turn, causes an egg and follicle to mature in an ovary. The maturing follicle secretes even more estrogen. When estrogen reaches a certain level, it causes the pituitary gland to release a surge of LH. The LH surge causes ovulation. It also causes the empty follicle to develop into a corpus luteum. The corpus luteum secretes progesterone. This hormone maintains the lining of the uterus so it doesn’t break down.
Figure 22.5
This diagram shows the changes that normally occur in the ovary and uterus during the menstrual cycle. It also shows how hormone levels change during the cycle. The menstrual cycle begins with menstruation. Ovulation occurs about half way through the cycle.
What happens next depends on whether a sperm unites with the egg. If it does, the egg secretes a hormone that prevents the corpus luteum from breaking down. The corpus luteum, in turn, keeps secreting progesterone. This maintains the lining of the uterus. What happens if a sperm does not unite with the egg is shown in Figure above. The corpus luteum breaks down and stops secreting progesterone. As a result, the lining of the uterus is no longer maintained. It breaks down and is shed from the uterus. Thus, menstruation begins and the cycle repeats.
Dr. JoAnne Richards is a world-famous endocrinologist. An endocrinologist is a scientist that studies hormones. Dr. Richards helped discover how FSH and LH control the growth of follicles and ovulation. She has received many awards and honors for her work, including the 2007 Women in Endocrinology Award.
Lesson Summary
The functions of the female reproductive system are to produce eggs, secrete estrogen, and support a baby as it develops before birth.
Female reproductive organs include the vagina, uterus, ovaries, and Fallopian tubes.
Eggs are female gametes that form in the ovaries and are released into the Fallopian tubes.
The menstrual cycle is a monthly cycle of changes in the ovaries and uterus. It includes ovulation and menstruation.
Review Questions
What are eggs?
What is the main sex hormone in females?
List the two major roles of estrogen in females.
What are the functions of the uterus in female reproduction?
Describe ovulation.
Compare and contrast eggs and sperm.
Explain how an egg develops in an ovary of a mature female.
Explain why menstruation occurs if a sperm does not unite with the egg that is released by an ovary.
How do LH and FSH control changes in the ovary during the menstrual cycle?
Explain why the lining of the uterus breaks down if a sperm does not unite with an egg. What role does progesterone play?
Further Reading / Supplemental Links
CK–12.org, High School Biology, Chapter 40, Lesson 2.
Kate Pfeifer. American Medical Association’s Girl’s Guide to Becoming a Teen. Jossey-Bass, 2006.
Sophie Waters. The Female Reproductive System (Girl’s Health). Rosen Publishing Group, 2007.
Susan Meredith. What’s Happening to Me?: Girls Edition. Usborne Books, 2006.
http://www.bcm.edu/mcb/?PMID=7691
http://www.kidshealth.org/parent/general/body_basics/female_reproductive_system.html
http://www.merck.com/mmhe/sec22/ch241/ch241e.html
http://www.nytimes.com/2007/06/12/science/12angi.html
http://www.women-in-endo.org/awards/#richards
http://en.wikipedia.org/wiki
Vocabulary
cervix
Narrow part of the uterus where it connects with the vagina.
corpus luteum
Structure that develops from a follicle after the egg bursts out of the follicle and through the ovary wall during ovulation.
eggs
Female gametes or sex cells.
estrogen
Main sex hormone in females.
fallopian tubes
Female reproductive organs through which eggs pass to reach the uterus and where an egg may unite with a sperm.
follicle
Nest of cells in an ovary that enclose an egg; protects egg during maturation prior to ovulation.
menstrual cycle
Monthly cycle of changes that occur in the uterus and ovaries.
menstruation
Monthly shedding of the lining of the uterus through the vagina; also called a menstrual period.
ovaries
Female reproductive organs that produce eggs and secrete estrogen.
ovulation
Release of an egg by an ovary.
uterus
Female reproductive organ where a baby develops until birth.
vagina
Female reproductive organ where sperm are deposited and through which a baby passes to leave the mother’s body during birth.
Points to Consider
The next lesson discussion reproduction.
After an egg is released, what must occur in order for reproduction to proceed?
What is the name of the first cell of the new organism?
Do you know where it fertilization occurs?
Lesson 22.3: Reproduction and Life Stages
Lesson Objectives
Explain how fertilization occurs.
Identify major events of pregnancy and childbirth.
List important developments of infancy and childhood.
Outline changes that occur during adolescence.
Describe the stages of adulthood.
Check Your Understanding
What are sperm and eggs?
How many chromosomes do sperm and eggs have?
What is the role of sex hormones during the teen years?
Introduction
The sperm and egg pictured in Figure 1 below don’t look anything like a human baby. After these two gametes unite, however, they will develop into a human being. How does a single cell become a complex organism made up of billions of cells? Keep reading to find out.
Fertilization and Implantation
Sexual reproduction occurs when an egg unites with a sperm. This is called fertilization. Sperm are deposited in the vagina during sexual intercourse. They “swim”
through the uterus and enter a Fallopian tube. This is where fertilization normally takes place. A sperm that is about to enter an egg is shown in Figure below. If the sperm breaks through the egg’s membrane, it will cause changes in the egg that keep other sperm out. It will also trigger the egg to complete meiosis. Recall that meiosis begins long before an egg is released from an ovary.
Figure 22.6
This sperm is ready to penetrate the membrane of this egg. Notice the difference in size of the sperm and egg. What will happen if the sperm manages to break through the eggs membrane?
The sperm and egg each have only half the number of chromosomes as other cells in the body. Therefore, when they unite, they form a cell with the full number of chromosomes. The cell they form is called a zygote. The zygote slowly travels down the Fallopian tube to the uterus. As it travels, it divides by mitosis many times. It forms a hollow ball of cells. After the ball of cells reaches the uterus, it embeds in the lining of the uterus. This is called implantation. It usually occurs about a week after fertilization.
Pregnancy and Childbirth
Once the ball of cells implants in the uterus, it is called an embryo. The embryo stage lasts until the end of the 8th week after fertilization. After that point until birth, the developing baby is called a fetus. To see how an embryo and fetus grow and develop, go to http://www.pennhealth.com/health_info/animationplayer/fetal_dvlp_tool.html
Growth and Development of the Embryo
During the embryo stage, the baby grows in size. It also develops different types of cells and organs. Cells of different types start to develop by the 3rd week after fertilization. They form structures that suit them for different roles in the body. Cells that will form muscles and skin, for example, start to develop at this time.
In 2004, French scientist Nicole Le Douarin won the first Pearl Meister Greengard Prize in Science. This award has been called the “American Nobel Prize for women scientists.” Dr. Le Douarin received the prize for developing a method to follow the path of individual cells during the development process. This, in turn, has helped scientists discover how different organs develop.
During the 4th week after fertilization, body organs begin to form. By the end of the 8th week, all the major organs have started to develop. Figure 2 shows some of the changes that take place during the 4th and 8th weeks. Look closely at the two embryos in the figure. Do you think that the older embryo looks more human? Notice that it has arms and legs and lacks a tail. Its facial features have also started to form. The older embryo is much bigger, as well.
The pictures in the Table (below) show how a developing baby looks, beginning at 4 weeks after fertilization and ending at 38 weeks after fertilization. The pictures show the baby at the same size at each stage. However, the actual size increases greatly during development. The length of the baby at each stage is given in the figure. Read the information in the boxes to learn what organs and other features have developed by each stage.
Human Embryo Image Developmental characteristics
Facial features are just starting to form.
Tail is present.
Legs have formed, and arm buds have appeared.
Heart is partly formed and begins to beat.
Spinal cord and brain have started to develop.
Most other organs have started to form, including the liver, pancreas, gall bladder, spleen, and lungs.
Facial features are starting to look human, and external ears and eyes are beginning to form.
Tail has disappeared.
Arms have developed; fingers and toes are starting to form.
Heart is well developed.
Digestive system is developing rapidly but does not yet function.
Cartilage and bones have started to form, and muscles are developing.
Internal ears and eyes are developing.
Nails have appeared on fingers and toes.
Reproductive organs have developed into either male or female organs.
Brain is developing rapidly.
Lungs are developing, but breathing is not yet possible.
Fetus is active, and mother may start to feel fetus moving.
Eyes are fully formed.
Eyelashes and eyebrows are present.
Hair has started to grow on the head.
Lungs are almost completely developed but unable to breathe on their own.
Muscles and bones are developing rapidly.
Muscle tone is increasing.
All organs are completely formed and functioning.
Fat is accumulating quickly.
Weight is increasingly rapidly.
Fetus is fully developed and ready to be born.
Growth and Development of the Fetus
Table (above) also shows changes that take place after the embryo becomes a fetus. Compare the 18th-week fetus with the 8th-week embryo. Some of the differences between them are obvious. For example, the fetus has ears and eyelids. Its fingers and toes are also fully formed. The fetus even has fingernails and toenails. In addition, the reproductive organs have developed along male or female lines. The brain and lungs are also developing quickly. The fetus has started to move around inside the uterus. This is usually when the mother first feels the fetus moving.
By the 28th week, the fetus is starting to look much more like a baby. Eyelashes and eyebrows are present. Hair has started to grow on the head. The body of the fetus is also starting to fill out, as muscles and bones develop. Babies born after the 28th week are usually able to survive. However, they need help breathing because their lungs are not yet fully mature.
During the last several weeks of the fetal period, all of the organs become mature. The most obvious change, however, is an increase in body size. The fetus rapidly puts on body fat and gains weight during the last couple of months. Compare the pictures in Table (above) of the fetus at 28 weeks and 38 weeks. Do you see how much chubbier the older fetus looks? By the end of the 38th week, all the organs are functioning, and the fetus is ready to be born. This is when birth normally occurs.
The Amniotic Sac and Placenta
During pregnancy, other structures also develop inside the mother’s uterus. They are the amniotic sac, placenta, and umbilical cord. They are shown in Figure below.
Figure 22.7
This fetus is 38 weeks old and ready to be born. Surrounding the fetus is the fluid-filled amniotic sac. The placenta and umbilical cord are also shown here. They provide a connection between the mothers and fetuss blood for the transfer of nutrients and gases.
The amniotic sac is a membrane that surrounds the fetus. It is filled with water and dissolved substances. Imagine placing a small plastic toy inside a balloon and then filling the balloon with water. The toy would be cushioned and protected by the water. It would also be able to move freely inside the balloon. The amniotic sac and its fluid are like a water-filled balloon. They cushion and protect the fetus. They also let the fetus move freely inside the uterus.
The placenta is a spongy mass of blood vessels. Some of the vessels come from the mother. Some come from the fetus. The placenta is attached to the inside of the mother’s uterus. The fetus is connected to the placenta by a tube called the umbilical cord. The cord contains two arteries and a vein. Substances pass back and forth between the mother’s and fetus’s blood through the placenta and cord. Oxygen and nutrients pass from the mother to the fetus. Carbon dioxide passes from the fetus to the mother.
It is important for the mother to eat plenty of nutritious food during pregnancy. She must take in enough nutrients for the fetus as well as for herself. She needs extra calories, proteins, and lipids. She also needs more vitamins and minerals. In addition to eating well, the mother must avoid substances that could harm the embryo or fetus. These include alcohol, illegal drugs, and some medicines. It is especially important for her to avoid these substances during the first eight weeks after fertilization. This is when all the major organs are forming. Exposure to harmful substances during this time co
uld have major effects on the developing body systems.
Childbirth
During childbirth, a baby passes from the uterus, through the vagina, and out of the mother’s body. Childbirth usually starts with the amniotic sac breaking. Then the muscles of the uterus start contracting. The contractions get stronger and closer together. They may go on for hours. Eventually, the contractions squeeze the baby out of the uterus. Once the baby enters the vagina, the mother starts pushing. She soon pushes the baby through the vagina and out of her body.
As soon as the baby is born, the umbilical cord is cut. After the cord is cut, the baby can no longer get rid of carbon dioxide through the cord and placenta. As a result, carbon dioxide builds up in the baby’s blood. This triggers the baby to start breathing. The amniotic sac and placenta pass through the vagina and out of the body shortly after the birth of the baby.
Infancy and Childhood
The first year after birth is called infancy. Infancy is a period of very fast growth. During infancy, the baby doubles in length and triples in weight. Other important changes also occur during infancy.
The baby teeth start to come in, usually at about six months of age (Figure below).
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