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CHAPTER
28
Pregnancy and Human
Development
Objectives
From Egg to Zygote
1. Describe the importance of sperm capacitation.
2. Explain the mechanism of the slow block to polyspermy.
3. Define fertilization.
Events of Embryonic Development: Zygote to Blastocyst Implantation
4. Describe the process and product of cleavage.
Events of Embryonic Development: Gastrula to Fetus
7. Name and describe the formation, location, and function of the extraembryonic
membranes.
Events of Fetal Development
11. Indicate the duration of the fetal period, and note the major events of fetal development.
Effects of Pregnancy on the Mother
12. Describe functional changes in maternal reproductive organs and in the cardiovascular,
respiratory, and urinary systems during pregnancy.
13. Indicate the effects of pregnancy on maternal metabolism and posture.
Lactation
17. Explain how the breasts are prepared for lactation.
Assisted Reproductive Technology and Reproductive Cloning
18. Describe some techniques of ART including IVF, ZIFT, and GIFT.
Suggested Lecture Outline
I. From Egg to Zygote (pp. 1065–1067; Figs. 28.1–28.3)
A. Accomplishing Fertilization (pp. 1065–1067; Figs. 28.1–28.3)
1. Fertilization occurs when a sperm fuses with an egg to form a zygote.
2. Millions of sperm ejaculated into the female reproductive tract are lost due to leakage
from the vaginal canal, destruction by the acidic environment of the vagina, inability
to pass the cervical mucus, or destruction by defense cells of the uterus.
5. Polyspermy, or fertilization by more than one sperm cell, leads to a lethal number of
chromosomes and must be prevented.
a. The entry of a sperm cell into the oocyte causes waves of Ca++ to be released into
the oocyte’s cytoplasm, which activates the oocyte to prepare for the second
meiotic division.
b. The cortical reaction, triggered by the Ca++ surge in the cytoplasm results in
destruction of sperm receptors, preventing other sperm from binding to the oocyte.
c. The slow block to polyspermy involves the material spilled from granulocytes just
inside the plasma membrane that form a swollen membrane that removes other
sperm cells from the surface of the oocyte.
II. Events of Embryonic Development: Zygote to Blastocyst Implantation
(pp. 1067, 1070–1074; Figs. 28.4–28.8)
A. Early Embryonic Development (pp. 1067, 1070–1074; Figs. 28.4–28.8)
1. Early embryonic development begins with fertilization and continues with the move-
ment of the embryo to the uterus, where it implants in the uterine wall.
2. The mitotic divisions after fertilization occur without much growth between divisions,
resulting in progressively smaller cells, a process called cleavage.
3. Implantation occurs after 6–7 days; the trophoblast adheres to the endometrium and
produces enzymes that irritate the endometrium.
4. Placentation is the formation of the placenta and is the process of proliferation of the
trophoblast.
a. The placenta is fully functional as a nutritive, respiratory, excretory, and endocrine
organ by the end of the third month of gestation.
III. Events of Embryonic Development: Gastrula to Fetus (pp. 1074–1081;
Figs. 28.9–28.14)
A. Formation and Roles of the Extraembryonic Membranes (pp. 1074–1075)
1. While implantation is occurring, the blastocyst is being converted into a gastrula,
in which three primary germ layers form and embryonic membranes develop.
a. The amnion forms the transparent sac ultimately containing the embryo and
provides a buoyant environment that protects the embryo from physical trauma.
B. Gastrulation: Germ Layer Formation (pp. 1075–1076; Fig. 28.9)
1. Gastrulation is the process of transforming the two-layered embryonic disc to a three-
layered embryo containing three germ layers: ectoderm, mesoderm, and endoderm.
2. Gastrulation begins with the appearance of the primitive streak, which establishes the
long axis of the embryo.
C. Organogenesis: Differentiation of the Germ Layers (pp. 1076–1080; Figs. 28.10–28.14)
1. Organogenesis is the formation of organs and organ systems; by the end of the
embryonic period at 8 weeks, all organ systems are recognizable.
a. Chemical signals from the notochord induce neurulation, the formation of the brain
and spinal cord, in the first event of organogenesis.
b. As the embryo develops from the neural plate, the ectoderm folds into the neural
tube, which gives rise to the brain at the anterior end and the spinal cord from the
rest.
IV. Events of Fetal Development (pp. 1081–1082; Fig. 28.15; Table 28.1)
A. The fetal period extends from weeks 9–38 and is a time of rapid growth of body
structures established in the embryo (p. 1081; Fig. 28.15).
B. During the first half of the fetal period, cells are still differentiating into specific cell
types to form the body’s distinctive tissues (pp. 1081–1082).
V. Effects of Pregnancy on the Mother (pp. 1082–1085; Fig. 28.16)
A. Anatomical Changes (pp. 1082–1083; Fig. 28.16)
1. The female reproductive organs and breasts become increasingly vascular and
engorged with blood.
B. Metabolic Changes (p. 1083)
1. As the placenta enlarges, it produces human placental lactogen (hPL), which works
with estrogen and progesterone to promote maturation of the breasts for lactation.
3. Plasma levels of parathyroid hormone and activated vitamin D rise, ensuring a positive
maternal calcium balance throughout pregnancy.
C. Physiological Changes (pp. 1083–1085)
1. Many women suffer morning sickness during the first few months of pregnancy, until
their systems adapt to elevated levels of hCG, estrogens, and progesterone.
VI. Parturition (Birth) (pp. 1085–1087; Figs. 28.17–28.18)
A. Parturition is the process of giving birth and usually occurs within 15 days of the
calculated due date, which is 280 days from the last menstrual period (p. 1085).
B. Initiation of Labor (p. 1085; Fig. 28.17)
1. Estrogen levels peak, possibly due to rising levels of fetal adrenal cortical hormones
(cortisol), stimulating myometrial cells of the uterus to form abundant oxytocin
receptors and antagonizing the quieting effect of progesterone on uterine muscle.
C. Stages of Labor (pp. 1085–1087; Fig. 28.18)
1. The dilation stage of labor extends from onset of labor to the time when the cervix is
fully dilated by the baby’s head, at about 10 cm in diameter.
a. At first, only the superior uterine muscle is active, but as labor progresses, contrac-
2. The expulsion stage extends from full dilation until the time the infant is delivered.
a. Crowning occurs when the baby’s head distends the vulva, and once the head has
been delivered, the rest of the baby follows much more easily.
b. When the baby is in the vertex, or head-first, presentation, the skull acts as a wedge
to dilate the cervix.
c. After birth, the umbilical cord is clamped and cut.
VII. Adjustments of the Infant to Extrauterine Life (p. 1087)
A. The Apgar score is an assessment of the infant’s physiological status based on heart rate,
respiration, color, muscle tone, and reflexes; a score from 8–10 indicates a healthy baby.
B. Taking the First Breath and Transition
1. Once the placenta is no longer removing carbon dioxide from the blood, it builds up in
the infant’s blood, resulting in acidosis that signals the respiratory control centers.
C. Occlusion of Special Fetal Blood Vessels and Vascular Shunts
1. After birth, the umbilical arteries and veins constrict and become fibrosed, becoming
the medial umbilical ligaments, superior vesical arteries of the bladder, and the round
ligament of the liver, or ligamentum teres.
VIII. Lactation (pp. 1087–1089; Fig. 28.19)
A. Lactation is the production of milk by the hormone-prepared mammary glands
(pp. 1078–1089; Fig. 28.19).
1. Rising levels of placental estrogens, progesterone, and lactogen stimulate the
hypothalamus to produce prolactin-releasing factors (PRFs), which promote secretion
of prolactin by the anterior pituitary.
5. Advantages of breast milk are: better absorption and more efficient metabolism of
many components; antibodies and other chemicals that protect the infant; a natural
laxative effect that helps to prevent physiological jaundice; and encouragement of the
natural intestinal fauna.
IX. Assisted Reproductive Technology and Reproductive Cloning (pp. 1089–1091)
A. Hormones can be used to increase sperm or egg production and surgery can be used to
open blocked tubes (p. 1089).
B. Assisted reproductive technology involves surgically removing oocytes from a woman’s
ovaries, fertilizing the eggs, and returning them to the woman’s body (p. 1089).
1. In vitro fertilization (IVF) combines oocytes and sperm in culture dishes for several
days, to allow fertilization to occur; the two-cell or blastocyst stage embryo is
transferred to the woman’s uterus.
Cross References
Additional information on topics covered in Chapter 28 can be found in the chapters listed below.
1. Chapter 2: Enzymes
2. Chapter 16: Hormones and hormone function; parathyroid hormone and calcium balance;
oxytocin; prolactin
Lecture Hints
1. Emphasize the difference between the terms conceptus, embryo, and fetus (and the
associated periods).
2. Stress that the early cell divisions of the conceptus increase total cell number but do not
result in cell size increase. Cells become increasingly smaller until the zona pellucida
ruptures.
6. Emphasize that from the mother’s point of view, the placenta is just another organ
drawing resources from the mother’s blood supply. This idea is helpful to establish the
placenta as an exchange organ.
10. As a point of interest, reveal that “eating for two” is a popular belief that has no
physiological basis.
11. Review hypothalamic and pituitary control of the ovarian cycle.
12. Point out the logic behind the various modifications of fetal circulation and how those
shunts must be redirected when the umbilical cord is cut.
Activities/Demonstrations
1. Audiovisual materials are listed in the Multimedia in the Classroom and Lab section of
this Instructor Guide (p. 387).
2. Have the students bring in a recent article that deals with the effects of maternal
drug-taking or disease (such as AIDS, herpes, etc.) on the well-being of the fetus.
Critical Thinking/Discussion Topics
1. Discuss the importance of folic acid to the prevention of neural tube defects.
2. Explore the drastic changes the fetus must undergo at birth and how those changes
might be minimized. What adaptations does a fetus have that makes these changes
surmountable?
Library Research Topics
1. Research the pros, cons, and contraindications of exercise during pregnancy.
2. Research the types of birth presentations and note the symptoms, prognosis, and
difficulties encountered in each type.
3. Study several types of birth defects by symptom category (that is, skeletal system,
circulatory system, etc.).
List of Figures and Tables
All of the figures in the main text are available in JPEG format, PPT, and labeled & unlabeled
format on the Instructor Resource DVD. All of the figures and tables will also be available in
Transparency Acetate format. For more information, go to www.pearsonhighered.com/educator.
Figure 28.1 Diagrams showing the approximate size of a human conceptus from
fertilization to the early fetal stage.
extraembryonic membrane formation.
Figure 28.8 Detailed anatomy of the vascular relationships in the mature
decidua basalis.
Figure 28.19 Milk production and the positive feedback mechanism of the milk
let-down reflex.
Table 28.1 Developmental Events of the Fetal Period
A Closer Look Contraception: To Be or Not To Be
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix H of the main text.
Short Answer Essay Questions
15. Human chorionic gonadotropin (hCG) is secreted by the trophoblast cells to encourage
16. a. The process of fertilization involves numerous steps. First, sperm deposited in the
vagina must be capacitated; that is, their membranes must become fragile so that the
hydrolytic enzymes in their acrosomes can be released. The acrosomal reaction is the
release of acrosomal enzymes (hyaluronidase, acrosin, proteases, and others) that
occurs in the immediate vicinity of the oocyte. Hundreds of acrosomes must rupture
to break down the intercellular cement that holds the granulosa cells together and to
digest holes in the zona pellucida. Once a path has been cleared and a single sperm
makes contact with the oocyte membrane receptors, its nucleus is pulled into the
oocyte cytoplasm.
b. The effect of fertilization is the formation of a single cell (zygote) with chromosomes
from the egg and sperm, and determination of the offspring’s sex. (pp. 1065–1066)
17. In cleavage, daughter cells become smaller and smaller, resulting in cells with a high
surface-to-volume ratio and providing a larger number of cells to serve as building blocks
for constructing the embryo. (p. 1067)
18. a. Viability of the corpus luteum is due to human chorionic gonadotropin secreted by
trophoblast cells of the blastocyst, which bypasses the pituitary-ovarian controls,
19. The placenta is formed from embryonic (trophoblastic) and maternal (endometrial)
tissues. When the trophoblast acquires a layer of mesoderm it becomes the chorion. The
20. As soon as the plasma membrane of one sperm makes contact with the oocyte membrane,
sodium channels open and ionic sodium moves into the oocyte from the extracellular
21. The gastrulation process gives rise to the three primary germ layers, the ectoderm,
mesoderm, and endoderm from which all tissues are formed. (p. 1075)
22. Breech presentation is buttock-first presentation. Two problems of breech presentation
include a more difficult delivery and the baby’s difficulty in breathing. (p. 1086)
23. The factors that bring about uterine contractions include high levels of estrogen, possibly
due to high levels of fetal adrenal corticoids, which has the effect of promoting formation
of uterine oxytocin receptors, gap junctions between cells, and antagonizing the quieting
Critical Thinking and Clinical Application Questions
1. (c) There could be defects in the fetus, so she should stop using drugs and visit a doctor
as soon as possible. Most major developmental events occur during the first three months
2. An episiotomy is a midline incision from the vaginal orifice laterally or posteriorly
toward the rectum. It is performed to reduce tissue tearing as the baby’s head exits from
the perineum. (p. 1086)
3. a. The woman was correct; she was in labor, the expulsion stage.
4. Mary’s fetus might have respiratory problems or even congenital defects due to her
smoking, because smoking causes vasoconstriction, which would hinder blood (hence
oxygen) delivery to the placenta. (p. 1074)
Suggested Readings
Barinaga, Marsha. “Cells Exchanged During Pregnancy Live On.” Science 296 (5576)
(June 2002): 2169–2172.
Couzin, Jennifer. “Quirks of Fetal Environment Felt Decades Later.” Science 296 (5576)
(June 2002): 2167–2169.
Duesbery, Nicholas S., and George F. Vande Woude. “An Arresting Activity.” Nature
416 (April 2002): 804–805.
Edwards, Robert G. “Human Implantation: The Last Barrier in Assisted Reproduction
Technologies?” Reproductive BioMedicine Online 13 (6) (Sept. 2006): 887–904.
Ivell, Richard. “This Hormone Has Been Relaxin’ Too Long!” Science 295 (5555)
(Jan. 2002): 637–638.
Krone, N., N. A. Hanley, and W. Arlt. “Age-Specific Changes in Sex Steroid Biosynthesis
and Sex Development.” Best Practice and Research. Clinical Endocrinology and
Metabolism 21 (3) (Sept. 2007): 393–401.
