CHAPTER
27
Reproduction and Embryonic
Development
Lecture Outline
I. Introduction
1. STDs can be caused by pathogens, including
a. bacteria,
b.protists, and
c. fungi.
2. Viral STDs are uniquely problematic because viruses can hide within a cell, making them very difficult to eradicate.
3. Although STDs from other types of pathogens can often be successfully treated, viral STDs cannot be cured and so last a lifetime.
4. Prevention of infection through safe sex practices is the best option for remaining uninfected.
II. Asexual and Sexual Reproduction
A.27.1 Asexual reproduction results in the generation of genetically identical offspring
1. Although every individual animal has a relatively short life span, species transcend this time limit because of reproduction, the creation of new individuals from existing ones.
2. Animals reproduce in a great variety of ways, but there are two modes:
a. asexual and
b.sexual.
3. Asexual reproduction is the creation of genetically identical offspring by one parent.
4. Asexual reproduction can proceed via
a. budding, the outgrowth and eventual splitting off of a new individual from a parent,
b. fission, the separation of a parent into two or more offspring of about equal size, or
c. fragmentation/regeneration, the breaking of the parent body into several pieces, followed by regeneration, the regrowth of lost body parts.
B. 27.2 Sexual reproduction results in the generation of genetically unique offspring
1. Sexual reproduction is the creation of offspring by fertilization, the fusion of two haploid (n) sex cells or gametes to form a diploid (2n) zygote.
2. The male gamete, the sperm,
a. is relatively small and
b.moves by means of a flagellum.
3. The female gamete, the egg,
a. is a much larger cell and
b.is not self-propelled.
4. Some organisms, such as sea anemones, can reproduce both
b.sexually.
5. Some animals exhibit hermaphroditism, in which an individual has both female and male reproductive systems.
6. Hermaphroditism makes it easier to find a mate for animals that are solitary or less mobile.
7. Hermaphrodites may
a. exchange gametes with other individuals or
b.fertilize their own eggs.
8. External fertilization is used by many aquatic invertebrates, fish, and amphibian species.
a. Eggs and sperm are discharged near each other.
b.Fertilization occurs in the water.
9. Internal fertilization is used by some fish and amphibian species and nearly all terrestrial animals.
a. Sperm is deposited in or near the female reproductive tract.
b.Fertilization occurs within the female reproductive tract.
III. Human Reproduction
A.27.3 Reproductive anatomy of the human female
1. Both sexes in humans have
a. a set of gonads, the organs that produce gametes,
b.ducts that store and deliver gametes, and
c. structures that allow mating.
2. Ovaries contain follicles that
a. nurture eggs and
b.produce the female sex hormone estrogen.
3. An immature egg is ejected from the follicle in a process called ovulation about every 28 days.
4. After ovulation, what remains of the follicle grows within the ovary to form a solid mass called the corpus luteum, which secretes additional estrogen as well as progesterone.
5. Oviduct
a. collect eggs after ovulation,
b. are the typical site of fertilization, and
c. convey eggs to the uterus, where a fertilized egg develops.
6. The uterus
a. has a thick muscular wall,
b.is lined by the endometrium, richly supplied with blood vessels,
c. is the actual site of pregnancy, and
d.opens into the vagina through the cervix.
7. The term embryo is used for the stage in development from the first division of the zygote, until body structures begin to appear, about the 9th week in humans.
8. From the 9th week until birth, a developing human is called a fetus.
9. An ectopic pregnancy
a. results when the embryo implants somewhere other than the uterus and
10.The female reproductive anatomy consists of
a. the vulva, the collective term for the external female genitalia,
b.a pair of slender folds, the labia minora, which border the openings, and
c. a pair of thick, fatty ridges, the labia majora, which protect the vaginal opening.
d.Until sexual intercourse or vigorous physical activity ruptures it, a thin piece of tissue called the hymen partly covers the vaginal opening.
e. The clitoris consists of a short shaft supporting a rounded glans, or head, covered by a small hood of skin called the prepuce.
B. 27.4 Reproductive anatomy of the human male
1. Testes (singular, testis) are each housed outside the abdominal cavity in a sac called the scrotum.
2. The scrotum keeps the sperm-forming cells about 2°C, which allows them to function normally.
3. A testis within a scrotum is called a testicle.
4. The epididymis stores sperm as they develop further.
5. During ejaculation, sperm leave the epididymis through the vas deferens.
6. Several glands contribute to semen, the fluid ejaculated from the penis during male
orgasm.
a. The seminal vesicles secrete a thick fluid that contains mucus and the sugar fructose, which provides most of the energy used by the sperm as they propel themselves through the female reproductive tract.
b.The prostate gland secretes a thin, milky fluid that further nourishes the sperm.
c. The bulbourethral glands secret a clear, alkaline mucus that neutralizes any acidic urine remaining in the urethra.
7. The human penis consists mainly of cylinders of erectile tissue that can fill with blood to cause an erection during sexual arousal.
8. During ejaculation,
a. in multiple glands force secretions into the urethra and propel sperm from the epididymis,
b.a sphincter muscle at the base of the bladder contracts, preventing urine from leaking into the urethra from the bladder, and
c. during the expulsion stage, the sphincter at the base of the penis relaxes, admitting semen into the penis.
9. Sperm production by the testes is controlled by hormones from
a. the hypothalamus and
b.the pituitary.
C.27.5 The formation of sperm and egg cells requires meiosis
1. Both sperm and egg are haploid (n) cells that develop by meiosis from diploid (2n) cells in the gonads.
2. There are significant differences in gametogenesis, the formation of gametes, between human males and females.
3. Spermatogenesis occurs in seminiferoustubules.
a. Primary spermatocytesare formed by mitosis and divide by meiosis I to produce secondary spermatocytes.
b.Secondary spermatocytesdivide by meiosis II to produce round spermatids, and spermatids differentiate into elongate sperm.
5. About every 28 days,
a. follicle-stimulating hormone (FSH) from the pituitary stimulates one of the dormant follicles to develop,
b.one primary oocyte resumes meiosis,
c. a secondary oocyte arrested at metaphase of meiosis II is ovulated, and
d.meiosis of the ovum is completed after fertilization.
6. Oogenesis and spermatogenesis are alike in that both produce haploid gametes but different in that
a. oogenesis produces only one mature egg and polar bodies that degenerate,
b.spermatogenesis produces four mature gametes, and
c. spermatogenesis occurs from puberty until death, while the mitotic divisions of oogenesis are thought to be completed before birth and the production of mature ga-metes ceases at menopause.
D.27.6 Hormones synchronize cyclic changes in the ovary and uterus
1. Oogenesis is one part of a female mammal’s reproductive cycle, a recurring sequence of events that
a. produces gametes,
b.makes them available for fertilization, and
c. prepares the body for pregnancy.
2. The reproductive cycle is actually two closely linked cycles.
a. The ovarian cycle controls the growth and release of an egg.
b.During the menstrual cycle, the uterus is prepared for possible implantation of an embryo.
3. Hormonal messages coordinate the two cycles, synchronizing follicle growth in the ovaries and ovulation with the establishment of a uterine lining that can support a growing embryo.
4. The hormone story involves intricate feedback mechanisms.
E. 27.7 CONNECTION: Sexual activity can transmit disease
1. Sexually transmitted diseases (STDs)
a. are also called sexually transmitted infections (STIs) and
b.are contagious diseases spread by sexual contact.
2. Not all STDs can be treated equally.
a. Those caused by bacteria, protozoans, and fungi can be cured with medications.
b.Viral STDs cannot.
c. For bacterial STDs, treatment must be given early, before any permanent damage is done.
3. Chlamydia
a. is the most common bacterial STD,
b.often produces no symptoms, and
c. can lead to pelvic inflammatory disease and infertility.
4. Viral diseases
a. such as genital herpes, papillomavirus (HPV), and HIV
b.can only be controlled.
5. AIDS, caused by HIV, poses one of the greatest health challenges in the world today.
6. In the United States, there are 56,000 new infections each year, one-third of which result from heterosexual contact.
8. Although viral STDs cannot be cured, many other STDs can be effectively treated if addressed early.
9. If left untreated, an STD may lead to long-term problems or even death.
10.Anyone who is sexually active should
a. have regular medical exams,
b.be tested for STDs, and
c. seek immediate help if any suspicious symptoms appear, even if they are mild.
11.STDs are most prevalent among teenagers and young adults.
a. Nearly two-thirds of infections occur among people under 25.
b.The best way to avoid the spread of STDs is, of course, abstinence.
c. Alternatively, latex condoms provide the best protection for “safe sex.”
F. 27.8 CONNECTION: Contraception can prevent unwanted pregnancy
1. Contraception is the deliberate prevention of pregnancy.
2. Several forms of contraception can prevent pregnancy, with varying degrees of success.
IV. Principles of Embryonic Development
A.27.9 Fertilization results in a zygote and triggers embryonic development
1. Embryonic development begins with fertilization, the union of sperm and eggto form a diploid zygote.
2. Sperm are adapted to reach and fertilize an egg.
3. Sperm have
a. a streamlined shape, which moves easily through fluids,
b.many mitochondria, which provide ATP for tail movements, and
c. a head that contains a haploid nucleus and is tipped with an acrosome containing enzymes that help it penetrate the egg.
4. Figure 27.9Csummarizes the timeline for the sequence of events that occur during fertilization.
5. Figure 27.9Dillustrates the sequence of events that occur during fertilization.
6. About 20 minutes after the sperm nucleus enters the egg, the sperm and egg nuclei fuse.
7. Gearing up for the enormous growth and development that will soon follow, DNA synthesis and cellular respiration begin.
B. 27.10 Cleavage produces a blastula from the zygote
1. Cleavage is a rapid series of cell divisions that produces
a. more cells,
b.smaller cells, and
c. a fluid-filled cavity, the blastocoel, in an embryo called a blastula.
C.27.11 Gastrulation produces a three-layered embryo
1. During gastrulation,
a. cells migrate to new locations,
b.a rudimentary digestive cavity forms, and
c. the basic body plan of three layers is established with
i. ectoderm outside—becomes skin and nervous systems,
ii. endoderm inside—becomes digestive tract,
iii. mesoderm in the middle—becomes muscle and bone.
D.27.12 Organs start to form after gastrulation
1. Organs develop from the three embryonic layers.
b.The neural tube develops above the notochord and will become the
i. brain and
ii. spinal cord.
2. As the embryo elongates, paired somites
a. form along the sides of the notochord,
b.hollow out to form the body cavity, or coelom, and
c. eventually contribute to muscles, bone, and other connective tissues.
3. Other systems develop at the same time.
E. 27.13 Multiple processes give form to the developing animal
1. Most developmental processes depend on signals passed between neighboring cells and cell layers, telling embryonic cells precisely what to do and when to do it.
2. The mechanism by which one group of cells influences the development of an adjacent group of cells is called induction.
3. Induction plays a major role in the early development of virtually all tissues and organs.
4. Tissues and organs develop by
a. changes in cell shape,
b.cell migration, and
c. apoptosis, the timely and tidy suicide of cells, a type of programmedcell death. In humans, the timely death of specific cells in developing hands and feet creates the spaces between fingers and toes.
F. 27.14 EVOLUTION CONNECTION: Pattern formation during embryonic development is controlled by ancient genes
1. Pattern formation, the emergence of the parts of a structure in their correct relative positions,
a. involves the response of genes to spatial variations of chemicals in the embryo and
b.results in tissues and organs developing in their proper positions at the correct times.
2. Homeotic genes
a. contain common nucleotide sequences (homeoboxes),
b.guide pattern formation in embryos,
c. occur in diverse groups such as
i. prokaryotes,
ii. yeast,
iii. plants, and
iv. animals, and
v. reveal the shared evolutionary history of life.
3. Figure 27.14Bhighlights some striking similarities in the chromosomal locations and the developmental roles of some homeobox-containing homeotic genes in two quite different animals.
4. By their presence in such diverse creatures, homeotic genes illustrate one of the central themes of biology: unity in diversity due to shared evolutionary history.
V. Human Development
A.27.15 The embryo and placenta take shape during the first month of pregnancy
1. Pregnancy, or gestation, is the carrying of developing young within the female reproductive tract.
3. Human development begins with fertilization in the oviduct.
4. Cleavage produces
a. ina blastocyst,whose inner cell mass becomes the embryo, and
b.the trophoblast, the outer cell layer, which
i. attaches to the uterine wall and
ii. forms part of the placenta.
5. Gastrulation occurs and organs develop from the three embryonic layers.
6. Four extraembryonic membranes develop.
a. The amnion
i. surrounds the embryo and
ii. forms a fluid-filled amniotic cavity that protects the embryo.
b.The yolksac,
i. in reptiles, stores yolk, and
ii. in humans, does not store yolk but is a source of the first germ cells and blood cells.
c. The allantois
i. contributes to the umbilical cord,
ii. forms part of the urinary bladder, and
iii. in reptiles, stores embryonic waste.
d.The chorion
i. contributes to the placenta and
ii. secretes human chorionic gonadotropin (hCG), which prevents menstruation
in mammals.
7. The placenta is a composite organ consisting of chorionic villi closely associated with theblood vessels of the mother’s endometrium.
8. The placenta is a vital organ with both embryonic and maternal parts that mediates exchange of nutrients, gases, and the products of excretion between the embryo and the mother.
B. 27.16 VISUALIZING THE CONCEPT: Human development from conception to birth is divided into three trimesters
1. For humans, pregnancy, the period of development from conception to birth, is divided into three trimesters, each lasting about 3 months.
2. The first trimesteris the time of the most radical change for both mother and embryo.
3. During this time, the embryo is particularly susceptible to damage by radiation, drugs, and alcohol, all of which can lead to birth defects or miscarriage.
4. During the second trimester, the fetus continues to grow and has increasingly human features, but the changes are not as dramatic as those changes of the first trimester.
5. The fetus’s eyes can open, its teeth are forming, and its bones have begun to harden.
a. Theplacentabegins to secrete progesterone (and the corpus luteum stops secreting progesterone), which helps maintain the placenta, and at the same time, the placenta stops secreting hCG, and the corpus luteum, no longer needed to maintain pregnancy, degenerates.
6. The third trimester is a time of rapid growth as the fetus gains the strength it will need to survive outside the protective environment of the uterus.
a. Babies born prematurely, as early as 24 weeks, may survive but require special medical care after birth.
C.27.17 Childbirth is induced by hormones and other chemical signals
1. The series of events that expel an infant from the uterus is called labor.
2. Hormonal changes induce birth.
a. Estrogen makes the uterus more sensitive to oxytocin.
b.Oxytocin acts with prostaglandins to initiate labor.
c. The cervix dilates to about 10 cm.
d.The baby is expelled by strong uterine contractions.
e. The placenta dislodges and is expelled after the baby.
3. The induction of labor involves positive feedback, a type of control in which a change triggers mechanisms that amplify that change.
a. Oxytocin and prostaglandins cause uterine contractions that in turn stimulate the release of more oxytocin and prostaglandins.
b.The result is a steady increase in contraction intensity, climaxing in forceful muscle contractions that propel a baby from the uterus.
4. Labor occurs in three stages:
a. dilation of the cervix,
b.expulsion, delivery of the infant, and
c. delivery of the placenta.
D.27.18 CONNECTION: Reproductive technologies increase our reproductive options
1. New techniques can help many infertile couples.
a. About 15% of couples wanting children experience infertility, the inability to conceive.
b.Drug therapies can help address problems of impotence (erectile dysfunction) and induce ovulation.
c. Assisted reproductive technologies (ART) require eggs to be harvested from the ovaries, fertilized, and returned to a woman’s body.
