2.2 Proposed Models for Interface Temperature
2.2.2 Linear Temperature Profile Approximation
Many mammals give birth to more than one offspring at a time or to a litter (multiparous), each member of which has come from a separate egg. There are some mammals, however, that have only one offspring at a time (uniparous), although occa-sionally they may have more than one. The armadillo ( Dasypus ) is almost unique among mammals in giving birth to four off-spring at one time—all of the same sex, either male or female, and all derived from the same zygote.
Human twins may come from one zygote ( identical, or monozygotic twins; Figure 7.19A ) or two zygotes ( nonidentical, dizygotic, or fraternal twins; Figure 7.19B ).
Fraternal twins do not resemble each other any more than other children born separately in the same family, but identical twins are, of course, strikingly alike and always of the same sex. Triplets, quadruplets, and quintuplets may include a pair of identical twins. The other babies in such multiple births usually come from separate zygotes. About 33% of identical twins have separate placentas, indicating that the blastomeres separated at an early, possibly the two-cell, stage ( Figure 7.19A , top ).
All other identical twins share a common placenta, indicating that splitting occurred after formation of the inner cell mass (see Figure 8.25 on p. 180). If splitting were to happen after placenta formation, but before the amnion forms, the twins would have individual amniotic sacs ( Figure 7.19A , middle ), as observed in the great majority of identical twins. Finally, a very small percentage of identical twins share one amniotic sac and a single placenta ( Figure 7.19A , bottom ), indicating that separation occurred after day 9 of pregnancy, by which time the amnion has formed. In these cases, the twins are at risk of becoming conjoined, a condition known as Siamese twinning.
Embryologically, each member of fraternal twins has its own placenta and amnion ( Figure 7.19B ).
The frequency of twin births in comparison to single births is approximately 1 in 86, that of triplets 1 in 86 2 , and that of quadru-plets approximately 1 in 86 3 . Frequency of identical twin births to all births is about the same the world over, whereas frequency of fraternal births varies with race and country. In the United States, three-fourths of all twin births are dizygotic (fraternal), whereas in Japan only about one-fourth are dizygotic. The tendency for frater-nal twinning (but apparently not identical twinning) seems to run in family lines; fraternal twinning (but not identical twinning) also increases in frequency as mothers get older.
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Figure 7.19
Formation of human twins. A, Monozygotic (identical) twin formation. B, Dizygotic (fraternal) twin formation. See text for explanation.
Fertilization of 2 different oocytes
DIZYGOTIC (FRATERNAL) TWINS
Yolk sac Embryo
MONOZYGOTIC (IDENTICAL) TWINS
Fertilization of single oocyte
Inner cell mass Placenta
Splitting at 2-cell stage
Complete split of inner cell mass
Split of inner cell mass late in development
Blastocoel
Two amnions
B A
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S U M M A R Y
Reproduction is the production of new life and provides an opportu-nity for evolution to occur. Asexual reproduction is a rapid and direct process by which a single organism produces genetically identical copies of itself. It may occur by fi ssion, budding, gemmulation, or fragmentation. Sexual reproduction involves production of germ cells (sex cells or gametes), usually by two parents (bisexual reproduc-tion), which combine by fertilization to form a zygote that develops into a new individual. Germ cells are formed by meiosis, reducing the number of chromosomes to haploid, and the diploid chromo-some number is restored at fertilization. Sexual reproduction recom-bines parental characters and thus reshuffl es and amplifi es genetic diversity. Genetic recombination is important for evolution. Two alternatives to typical bisexual reproduction are hermaphroditism, the presence of both male and female organs in the same individual, and parthenogenesis, the development of an unfertilized egg.
Sexual reproduction exacts heavy costs in time and energy, requires cooperative investments in mating, and causes a 50% loss of genetic representation of each parent in the offspring. The classi-cal view of why sex is needed is that it maintains variable offspring within the population, which may help the population to survive environmental change.
In vertebrates the primordial germ cells arise in the yolk-sac endoderm, then migrate to the gonad. In mammals, a gonad becomes a testis in response to masculinizing signals encoded on the Y chromosome of the male, and the reproductive tract masculin-izes in response to circulating male sex steroids. Female reproduc-tive structures (ovary, uterine tubes, uterus, and vagina) develop in the absence of signals encoded on the Y chromosome, although recent data suggests that a female-determining region on the X chro-mosome has a role in differentiation of female reproductive organs.
Germ cells mature in the gonads by a process called game-togenesis (spermagame-togenesis in males and oogenesis in females), involving both mitosis and meiosis. In spermatogenesis, each pri-mary spermatocyte gives rise by meiosis and growth to four motile sperm, each bearing the haploid number of chromosomes. In oogen-esis, each primary oocyte gives rise to only one mature, nonmotile, haploid ovum. The remaining nuclear material is discarded in polar bodies. During oogenesis an egg accumulates large food reserves within its cytoplasm.
Sexual reproductive systems vary enormously in complexity, ranging from some invertebrates, such as polychaete worms that lack any permanent reproductive structures to the complex sys-tems of vertebrates and many invertebrates consisting of permanent gonads and various accessory structures for transferring, packaging, and nourishing gametes and embryos.
The male reproductive system of humans includes testes, com-posed of seminiferous tubules in which millions of sperm develop,
and a duct system (vasa efferentia and vas deferens) that joins the urethra, glands (seminal vesicles, prostate, bulbourethral), and penis.
The human female system includes ovaries, containing thousands of eggs within follicles; oviducts; uterus; and vagina.
The seasonal or cyclic nature of reproduction in vertebrates has required evolution of precise hormonal mechanisms that control production of germ cells, signal readiness for mating, and prepare ducts and glands for successful fertilization of eggs. Neurosecretory centers within the hypothalamus of the brain secrete gonadotropin-releasing hormone (GnRH), which stimulates endocrine cells of the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which in turn stimulate the gonads.
Estrogens and progesterone in females, and testosterone and dihydrotestosterone (DHT) in males, control the growth of accessory sex structures and secondary sex characteristics, in addition to feeding back to the hypothalamus and anterior pituitary to regulate GnRH, FSH, and LH secretion.
In the human menstrual cycle, estrogen induces the initial pro-liferation of uterine endometrium. A surge in GnRH and LH, induced by rising estrogen levels from the developing follicle(s), midway in the cycle causes ovulation and the corpus luteum to secrete proges-terone (and estrogen in humans), which completes preparation of the uterus for implantation. If an egg is fertilized, pregnancy is main-tained by hormones produced by the placenta and mother. Human chorionic gonadotropin (hCG) maintains secretion of progesterone and estrogen from the corpus luteum, while the placenta grows and eventually secretes estrogen, progesterone, hCG, human placental lactogen (hPL), human placental growth hormone (hPGH), prolactin (PRL), endogenous opioids, placental corticotropin-releasing hormone (CRH), and relaxin. Estrogen, progesterone, PRL, and hPL, as well as maternal prolactin, induce development of the mammary glands in preparation for lactation. hPL, hPGH, and maternal growth hormone also increase nutrient availability for the developing embryo.
Birth or parturition (at least in most mammals) appears to be initiated by release of placental CRH. In addition, a decrease in progesterone and an increase in estrogen levels occur so that the uterine muscle begins to contract. Oxytocin (from the posterior pituitary) and uterine prostaglandins continue this process until the fetus (followed by the placenta) is expelled. Placental relaxin makes the birth process easier by enabling expansion of the pelvis and dilation of the cervix.
Multiple births in mammals may result from division of one zygote, producing identical, monozygotic twins, or from separate zygotes, producing fraternal, dizygotic twins. Identical twins in humans may have separate placentas, or (most commonly) they may share a common placenta but have individual amniotic sacs.
R E V I E W Q U E S T I O N S
Defi ne asexual reproduction, and describe four forms of asexual reproduction in invertebrates.
Defi ne sexual reproduction and explain why meiosis contributes to one of its great strengths.
1.
2.
Explain why genetic mutations in asexual organisms lead to much more rapid evolutionary change than do genetic mutations in sexual forms. Why might harmful mutations be more deleterious to asexual organisms compared with sexual organisms?
3.
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S E L E C T E D R E F E R E N C E S
Cole, C. J. 1984. Unisexual lizards. Sci. Am. 250: 94–100 (Jan.). Some populations of whiptail lizards from the American southwest consist only of females that reproduce by virgin birth.
Crews, D. 1994. Animal sexuality. Sci. Am. 270: 108–114 (Jan.). Sex is determined genetically in mammals and most other vertebrates, but not in many reptiles and fi shes, which lack sex chromosomes. The author describes nongenetic sex determination and suggests a new framework for understanding the origin of sexuality.
Crow, J. F. 1994. Advantages of sexual reproduction. Developmental Genetics 15: 205–213. An excellent discussion of the advantages and disadvantages of sexual reproduction with a critique of the various hypotheses presented on this issue. Very readable.
Forsyth, A. 1986. A natural history of sex: the ecology and evolution of sexual behavior. New York, Charles Scribner’s Sons. Engagingly written, factually accurate account of the sex lives of animals from unicellular organisms to humans, abounding in imagery and analogy.
Highly recommended.
Johnson, J., J. Cannling, T. Kaneko, J. P. Pru, and J. L. Tilly. 2004. Germline stem cells and follicular renewal in the postnatal mammalian ovary.
Nature 428: 145–150. Exciting new evidence that female mammals possess a renewable germ cell line, refuting an age-old hypothesis of reproductive biology.
Johnson, M. H., and B. J. Everitt. 2000. Essential reproduction, ed. 5.
Oxford, U.K., Blackwell Sciences Ltd. Excellent coverage of reproductive physiology with emphasis on humans.
Jones, R. E. 2006. Human reproductive biology, ed. 3. San Diego, Academic Press. Thorough treatment of human reproductive physiology.
Kinsley, C. H., and K. G. Lambert. 2006. The maternal brain. Sci. Am.
294: 72–79. This excellent review discusses how the hormones secreted
during pregnancy, and lactation in mammals appears to confer long-lasting benefi ts to the brain that alter skills and behavior associated with better parental care.
Kriegsfeld, L. J., D. F. Mei, G. E. Bentley, Y. Ubuka, A. O. Mason, K. Inoue, K. Ukena, K. Tsutsui, and R. Silver. 2006. Identifi cation and characterization of a gonadotropin-inhibitory system in the brains of mammals. Proceedings of the National Academy of Science 103: 2410–2415. An original research paper that presents evidence of a gonadotropin-inhibiting hormone that suppresses the reproductive axis.
Lee, D. M., R. R. Yeoman, D. E. Battaglia, R. L. Stouffer, M. B. Zelinski-Wooten, J. W. Fanton, and D. P. Wolf. 2004. Live birth after ovarian tissue transplant. Nature 428: 137–138. New hope in the future for cancer patients that are made prematurely sterile is provided by the recent news of successful ovarian tissue transplants in monkeys.
Lombardi, J. 1998. Comparative vertebrate reproduction. Boston, Kluwer Academic Publishers. Comprehensive coverage of vertebrate reproductive physiology.
Maxwell, K. 1994. The sex imperative: an evolutionary tale of sexual survival.
New York, Plenum Press. Witty survey of sex in the animal kingdom.
Michod, R. E. 1995. Eros and evolution: a natural philosophy of sex.
Reading, Massachusetts, Addison-Wesley Publishing Company. In this engaging book, the author argues that sex evolved as a way of coping with genetic errors and avoiding homozygosity.
Piñón, R. 2002. Biology of human reproduction. Sausalito, University Science Books. An updated examination of human reproductive physiology.
Ridley, M. 2001. The advantages of sex. www.pbs.org/wgbh/evolution/sex/
advantage/ An essay adapted from a New Scientist publication (4 Dec, 1993) summarizing hypotheses proposed for the evolution of sex.
Defi ne two alternatives to bisexual reproduction—
hermaphroditism and parthenogenesis—and offer a specifi c example of each from the animal kingdom. What is the difference between ameiotic and meiotic parthenogenesis?
Defi ne the terms dioecious and monoecious. Can either of these terms be used to describe a hermaphrodite?
A paradox of sexual reproduction is that despite being widespread in nature, the question of why it exists at all is still unresolved. What are some disadvantages of sex? What are some consequences of sex that make it so important?
What is a germ cell line? How do germ cells pass from one generation to the next?
Explain how a spermatogonium, containing a diploid number of chromosomes, develops into four functional sperm, each containing a haploid number of chromosomes. In what signifi cant way(s) does oogenesis differ from spermatogenesis?
Defi ne, and distinguish among, the terms oviparous, ovoviviparous, and viviparous.
Name the general location and give the function of the following reproductive structures: seminiferous tubules, 4.
5.
6.
7.
8.
9.
10.
vas deferens, urethra, seminal vesicles, prostate gland, bulbourethral glands, mature follicle, oviducts, uterus, vagina, endometrium.
How do the two kinds of mammalian reproductive cycles—
estrous and menstrual—differ from each other?
What are the male sex hormones and what are their functions?
Explain how the female hormones GnRH, FSH, LH, and estrogen interact during the menstrual cycle to induce ovulation and, subsequently, formation of the corpus luteum.
Explain the function of the corpus luteum in the menstrual cycle. If fertilization of the ovulated egg happens, what endocrine events occur to support pregnancy?
Describe the role of pregnancy hormones during human pregnancy. What hormones prepare the mammary glands for lactation and what hormones continue to be important during this process?
If identical human twins develop from separate placentas, when must the embryo have separated? When must separation have occurred if the twins share a common placenta but develop within separate amnions?
11.
12.
13.
14.
15.
16.
O N L I N E L E A R N I N G C E N T E R
Visit www.mhhe.com/hickmanipz14e for chapter quizzing, key term fl ash cards, web links and more!
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8
Principles
of Development
The Primary Organizer
During the fi rst half of the twentieth century, experiments by the German embryologist Hans Spemann (1869 to 1941) and his stu-dent, Hilde Pröscholdt Mangold (1898 to 1924), ushered in the fi rst of two golden ages of embryology. Working with salaman-ders, they found that tissue transplanted from one embryo into another could induce development of a complete organ, such as an eyeball, at the site of the transplant. This phenomenon is called embryonic induction. Mangold later discovered that one particular tissue, the dorsal lip from an embryonic stage called the gastrula, could induce the development of an entirely new salamander joined to the host salamander at the site of the transplant. (This work earned Spemann the Nobel Prize in Physiology or Medicine in 1935, but Hilde Mangold had died in a household accident only a few weeks after her research was published.) Spemann desig-nated this dorsal lip tissue the primary organizer, now often called the Spemann organizer. Recent advances in molecular biology have inaugurated the second golden age of embryology,
still in progress. During this current golden age we are beginning to understand that induction is due to secretion of certain mol-ecules that trigger or repress the activity of combinations of genes in nearby cells. For example, cells of the Spemann organizer migrate over the dorsal midline, secreting proteins with names like noggin, chordin, and follistatin. These proteins allow nearby cells to develop into the nervous system and other tissues along the middle of the back, and those tissues in turn release other proteins that induce development of other parts of the body. Such organizer proteins do not occur only in salamanders; remarkably similar proteins function in development of other vertebrates and even invertebrates. Because all animals appear to share similar molecular mechanisms for development, it may now be possible to understand how changes in such developmental controls led to the evolution of the great variety of animals. Research in this area has given rise to the exciting new fi eld called evolutionary developmental biology.
Spemann organizer cells (color) migrating from the dorsal lip (arrow) of a gastrula.
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H
ow is it possible that a tiny, spherical fertilized human egg, scarcely visible to the naked eye, can develop into a fully formed, unique person, consisting of thousands of billions of cells, each cell performing a predestined functional or structural role? How is this marvelous unfolding controlled?Clearly all information needed must originate from the nucleus and in the surrounding cytoplasm. But knowing where the control system lies is very different from understanding how it guides the conversion of a fertilized egg into a fully differentiated animal. Despite intense scrutiny by thousands of scientists over many decades, it seemed until very recently that developmental biology, almost alone among the biological sciences, lacked a satisfactory explanatory theory. This now has changed. During the last two decades the combination of genetics and evolution with modern techniques of cellular and molecular biology has provided the long-sought explanation of animal development.
Causal relationships between development and evolution have also become the focus of research. We do at last appear to have a conceptual framework to account for development.