Review Questions
List three functions of the female reproductive system.
State two ways that puberty differs in girls and boys.
Describe the uterus and its functions in reproduction.
What is ovulation and when does it occur?
Tara is 13 and worried that she may not be developing normally. She began developing breasts about six months ago but still has not had her first menstrual period. Should she be concerned? Explain your answer.
Explain how blockage of both Fallopian tubes would affect a woman’s ability to reproduce naturally.
Create a timeline showing the steps in which an oogonium develops into a mature egg.
Explain the roles of estrogen, LH, and FSH in the menstrual cycle.
Further Reading / Supplemental Links
Stanley, Deborah, Sexual Health Information for Teens. Omnigraphics, 2003.
Walker, Pam and Wood, Elaine, Understanding the Human Body: The Reproductive System. Lucent Books, 2002.
http://en.wikibooks.org/wiki/Human_Physiology/The_female_reproductive_system
http://www.kidshealth.org/parent/general/body_basics/female_reproductive_system.html
http://www.kidshealth.org/teen/sexual_health/changing_body/female_repro.html
http://www.medicalook.com/human_anatomy/systems/Female_reproductive_system.html
http://www.merck.com/mmhe/sec22/ch241/ch241a.html
http://en.wikipedia.org
Vocabulary
adolescent growth spurt
Rapid growth in height seen during puberty.
corpus luteum
Formed in the ovary from the ruptured follicle after ovulation; if the egg is not fertilized by a sperm, the corpus luteum degenerates and virtually disappears from the ovary; produces progesterone.
egg (ova)
Female gamete, or sex cell, which is necessary for reproduction; haploid.
estrogen
Major female sex hormone.
Fallopian tube
Tube which accepts oocyte after ovulation; site of fertilization; attached to uterus.
female reproductive system
System with several major functions: producing eggs, secreting female sex hormones, receiving sperm during sexual intercourse, supporting the development of a fetus, delivering a baby during birth, and breastfeeding a baby after birth.
follicle
Structure in which each egg is located; consists of the egg surrounded by special cells that protect the egg until puberty and then help the egg mature.
follicle-stimulating hormone (FSH)
Hormone that stimulates the ovary to produce estrogen.
luteinizing hormone (LH)
The main pituitary hormone responsible for puberty in females; stimulates the ovary to produce estrogen.
menarche
The beginning of menstruation, or monthly periods.
menopause
When a woman has gone through 12 consecutive months without a menstrual period; she can no longer reproduce because her ovaries no longer produce eggs.
menstruation
The process in which blood and other tissues are shed from the uterus and leave the body through the vagina; also called a menstrual period, or menses.
oogenesis
The process of producing eggs in the ovary.
ovary
Small, oval-shaped organs that lie on either side of the uterus; the egg-producing organs of the female reproductive system; contain hundreds of thousands of immature eggs.
ovulation
The release of a secondary oocyte by the ovary; occurs every 28 days, on average.
progesterone
A hormone that promotes gestation, or the carrying of a fetus; also maintains the endometrium of the uterus.
uterus
A muscular organ where a fetus grows and develops until birth; has a thick lining of tissues known as the endometrium; the lower, narrower end of the uterus is called the cervix.
vulva
The external female reproductive structures; includes the labia and mons pubis.
Points to Consider
If an egg is fertilized by a sperm and implants in the uterus, the endometrium helps support and nourish it. However, the new organism soon needs more nutrients than the endometrium can provide. It needs to obtain nutrients from the mother’s blood. How does this happen?
What structures are involved with pregnancy? When do they develop?
Lesson 25.3: Fertilization, Gestation, and Development
Lesson Objectives
Explain how fertilization, cleavage, and implantation lead to the formation of an embryo.
Describe how the embryo forms specialized cells and organs through the processes of gastrulation, differentiation, and organogenesis.
Identify major events in the growth and development of the fetus.
Explain how the placenta provides the fetus with oxygen and nutrients and eliminates fetal wastes.
Describe how an expectant mother can help her fetus grow and develop normally, and summarize the events of childbirth.
Sequence milestones in growth and development from infancy through adolescence.
Describe the life stages of early and middle adulthood and old age, and explain why aging occurs.
Introduction
Sexual reproduction begins when an egg is fertilized by a sperm and implants in the uterus. Following these events, the remainder of growth and development before birth is divided into two main stages. The first stage is the embryonic stage, which lasts about two months. This is followed by the fetal stage, which lasts for another seven months until birth.
Fertilization, Cleavage, and Implantation
A day or two after an ovary releases an egg, the egg may unite with a sperm. However, before it becomes an embryo, it must go through other processes. These processes include cleavage and implantation.
Fertilization
Fertilization is the union of a sperm and an egg. Recall that a sperm is a male gamete and an egg is a female gamete. Each gamete is a haploid cell. When the two cells unite during fertilization, they form a diploid cell, called a zygote.
Fertilization generally occurs in a Fallopian tube. After sperm are deposited in the vagina during sexual intercourse, they “swim” through the cervix and uterus and into a Fallopian tube. Although millions of sperm are deposited, only a few hundred are likely to reach the egg. A sperm about to penetrate an egg is shown in Figure below. When a sperm finally breaks through the egg’s cell membrane, it sets off a reaction that prevents other sperm from entering. The entry of the sperm also triggers the egg to complete the second meiotic division that began before ovulation.
Figure 25.11
Human sperm and egg.
After the sperm penetrates the egg, its tail falls off, and its nucleus fuses with the nucleus of the egg. The resulting zygote contains all the chromosomes needed for a new individual. Half the chromosomes are from the egg, and half are from the sperm.
Cleavage
The zygote spends the next few days traveling down the Fallopian tube. As it travels, it divides by mitosis several times to form a ball of cells called a morula. The cell divisions, which are called cleavage, increase the number of cells but not their overall size. More cell divisions occur, and soon a fluid-filled cavity forms inside the ball of cells. At this stage, the ball of cells is called a blastocyst. The process of blastocyst formation is shown in Figure below.
Figure 25.12
The morula (1) continues to undergo cell divisions. As it does, cells start to migrate into separate layers, and a cavity starts to develop inside the ball of cells. When cells have migrated into distinct layers, the organism is called a blastocyst (2).
The cells of the blastocyst form an inner and an outer cell layer. This is apparent in Figure below. The inner layer of cells is called the embryoblast. This layer of cells will soon develop into an embryo. The outer layer of cells is called the trophoblast. This layer
will develop into other structures, including the placenta, which you will read more about below.
Figure 25.13
The blastocyst consists of an outer layer of cells called the trophoblast, a fluid-filled cavity, and an inner cell mass called the embryoblast.
Implantation
The blastocyst continues the trip down the Fallopian tube and reaches the uterus about four or five days after fertilization. When the outer cells of the blastocyst contact cells lining the uterus, the blastocyst embeds in the lining. The process of embedding is called implantation. It generally occurs about a week after fertilization. Once implantation occurs, the blastocyst is called an embryo.
Growth and Development of the Embryo
An embryo is a developing human being from the time of implantation through the first eight weeks after fertilization. During this time, the embryo grows in size and undergoes three processes: gastrulation, differentiation, and organogenesis.
Gastrulation
Gastrulation is the development of different layers of cells in the embryo. It generally occurs during the second week after fertilization. During gastrulation, cells of the embryo migrate to form three distinct cell layers: the ectoderm, mesoderm, and endoderm. These layers are shown in Figure below. Each layer will eventually develop into certain types of tissues and cells in the body.
Ectoderm—Forms tissues that cover the outer body; develops into cells such as nerves, skin, hair, and nails.
Mesoderm—Forms tissues that provide movement and support; develops into cells such as muscles, bones, teeth, and blood.
Endoderm—Forms tissues involved in digestion and breathing; develops into cells such as lungs, liver, pancreas, and gall bladder.
Figure 25.14
The three cell layers of the embryo develop into different types of cells. For example, the ectoderm develops into skin cells, the mesoderm into muscle cells, and the endoderm into lung cells.
Differentiation and Organogenesis
During the third week after fertilization, the embryo begins to undergo cellular differentiation. Differentiation is the process by which unspecialized cells become specialized into one of the many different types of cells that make up the body. During differentiation, certain genes are turned on, or activated, while other genes are switched off, or inactivated. As a result of this process, cells develop specific structures and abilities that suit them for their specialized roles in the body. Several examples of specialized cells are shown in Figure above, along with the cell layers from which they develop.
Differentiation of cells leads to the development of specific organs within the three cell layers. This is called organogenesis. All the major organs begin to form during the remaining weeks of embryonic development. A few of the developments that occur in weeks 4 through 8 are listed below.
Embryonic Development During Weeks 4-8
Pictured above is a 4-week-old embryo.
At Week 4
Heart begins to beat.
Arm buds appear.
Liver, pancreas, and gall bladder start to form.
Spleen appears.
At Week 5
Eyes start to form.
Leg buds appear.
Hands appear as paddles.
Blood begins to circulate.
Facial features start to develop.
At Week 6
Lungs start to form.
Fingers and toes form.
At Week 7
Hair follicles start to form.
Elbows and toes are visible.
At Week 8
Facial features look more human.
External ear begins to take shape.
As the embryo develops, it also grows in size. By the eighth week of development, the embryo is about 30 millimeters long. It may also have begun to move.
Growth and Development of the Fetus
From week 8 until birth, the developing individual is referred to as a fetus. In humans, birth typically occurs 38 weeks after fertilization, so the fetal period lasts about 30 weeks. During this time, the organs that formed during the embryonic period go through further development. The fetus also grows in overall body size. For a detailed animation of the growth and development of the fetus see http://www.youtube.com/watch?v=aR-Qa_LD2m4&feature=related.
Weeks 8 to 15
During the fetus’s early weeks, reproductive organs develop along either male or female lines. The liver starts producing red blood cells, and tooth buds appear. The fetus becomes more human in appearance, with well-formed facial features. The eyelids form but remain closed until later in fetal development. The muscles and bones develop, and the fetus is very active. It can make a fist and move its arms and legs. It also hiccups, stretches, and yawns. The first measurable brain activity occurs around the 12th week. By the end of the 15th week, the fetus is about 15 centimeters long.
Weeks 16 to 26
A fetus at 18-weeks after fertilization is shown in Figure below. At this stage, the brain is developing rapidly, and it starts to take control of some body functions. The alveoli (air sacs) in the lungs also develop, making gas exchange possible, although the lungs are still immature. Most of the internal components of the eyes and ears form and develop at this time. There is more muscle development, as well, and the fetus is more active than ever. The mother usually starts to feel fetal movement during this stage.
Fine hair called lanugo grows and covers the fetus’s body by the end of this stage. Eyebrows, eyelashes, and nails also appear, and the eyelids begin to open and close. By the end of week 26, the fetus is about 38 centimeters long and weighs about 1.2 kilograms.
Weeks 27 to 38
During the final weeks of growth and development, the amount of body fat rapidly increases. Bones develop fully, although they are still soft and pliable. Most of the lanugo disappears, and head hair becomes coarser and thicker. Fingernails grow beyond the end of the fingertips. In the brain, connections form that allow the input of sensations. Starting around week 30, the brain is continuously active. By the 38th week, the fetus is fully developed and ready to be born. A 38-week fetus normally ranges from 36 to 51 centimeters in length and weighs between 2.7 and 4.6 kilograms. A 38-week-old fetus is shown in Figure below.
Sometimes fetuses are born earlier than 38 weeks. After 35 weeks, the fetus is considered “full-term,” which means that it is developed enough for life outside the mother. Fetuses born before 35 weeks are likely to have health problems due to their immaturity, although many are able to survive with medical help. The less time a fetus spends developing in the uterus before it is born, the less likely it is to survive after birth. Fetuses born before 25 weeks rarely survive.
Placenta and Related Structures
The placenta is a temporary organ in which nutrients and wastes are exchanged between the mother and the embryo or fetus. The placenta begins to form in the second week after fertilization. It continues to develop and grow to meet the needs of the growing fetus. A fully developed placenta, like the one in Figure above, is made up of a large mass of blood vessels from both the mother and fetus. The maternal and fetal vessels are close together but separated by empty space. This allows the mother’s and fetus’s blood to exchange substances without actually mixing.
How the Placenta Works
Blood from the mother enters the maternal blood vessels of the placenta under pressure, forcing the blood into the empty spaces. When the mother’s blood contacts the fetal blood vessels, gases are exchanged. Oxygen from the mother’s blood is exchanged with carbon dioxide from the fetus’s blood. A release of pressure brings the mother’s blood back from the placenta and into her veins.
The fetus is connected to the placenta through the umbilical cord, a tube that contains two arteries and a vein. Blood from the fetus enters the placenta through the umbilical arteries, exchanges gases with the mother’s blood, and travels back to the fetus through the umbilical vein.
In addition to gas exchange, the placenta transfers nutrients,
hormones, and other needed substances from the mother’s blood to the fetus’s blood. The placenta also filters many harmful substances out of the mother’s blood so they are not transferred to the fetus. In addition, the placenta secretes hormones that maintain the corpus luteum in the mother’s ovary. Recall that the corpus luteum secretes progesterone, which is needed to keep the endometrium of the uterus from breaking down.
Amniotic Sac and Fluid
Attached to the placenta is the amniotic sac, which surrounds and protects the embryo or fetus. It begins to form in the second week after fertilization. It soon fills with water and dissolved substances to form amniotic fluid. The fluid allows the fetus to move freely until the fetus grows to fill most of the available space. The fluid also cushions the fetus and helps protect it from injury.
CK-12 Biology I - Honors Page 124