EssentialDevelopmentalBiology

Thank you for your purchase. Each new copy includes accessto the student website. You will find your unique passwordprinted on the card inserted into this book. If you purchased a used copy of this text, you may purchase a password card togain access to the animations, review questions, and artworkby visiting www.blackwellpublishing.com/slack andclicking on Order Password Card.

EDBA01 1/08/2005 09:29 Page i

EDBA01 1/08/2005 09:29 Page ii

EssentialDevelopmentalBiologyJ.M.W. Slack

Department of Biology and BiochemistryUniversity of BathUnited Kingdom

Edition2nd

EDBA01 1/08/2005 09:29 Page iii

© 2006 by Jonathan SlackFirst edition © 2001 by Blackwell Science Ltda Blackwell Publishing company

BLACKWELL PUBLISHING350 Main Street, Malden, MA 02148-5020, USA9600 Garsington Road, Oxford OX4 2DQ, UK550 Swanston Street, Carlton, Victoria 3053, Australia

The right of Jonathan Slack to be identified as the Author of this Work has been asserted in accordance with the UK Copyright, Designs, and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs, and Patents Act 1988, without the prior permission of the publisher.

First edition published 2001 by Blackwell Science Ltd,Second edition published 2006 by Blackwell Publishing Ltd

1 2006

Library of Congress Cataloging-in-Publication Data

Slack, J.M.W. (Jonathan Michael Wyndham), 1949–Essential developmental biology / J.M.W. Slack.– 2nd ed.

p. cm.Includes bibliographical references and index.ISBN-13: 978-1-4051-2216-0 (pbk. : alk. paper)ISBN-10: 1-4051-2216-1 (pbk. : alk. paper)

1. Developmental biology–Laboratory manuals. I. Title.

QH491.S6 2006571.8′1–dc22

2005004145

A catalogue record for this title is available from the British Library.

Set in 9.5/12pt Minionby Graphicraft Limited, Hong KongPrinted and bound in Italyby Rotolito Lombarda, SPA

The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp processed using acid-free and elementary chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards.

For further information onBlackwell Publishing, visit our website:www.blackwellpublishing.com

EDBA01 1/08/2005 09:29 Page iv

Preface, vii

Section 1: Groundwork, 1

1 The excitement of developmentalbiology, 3

Where the subject came from, 3Central position in biology, 3Impact on society, 4Future impact, 4

2 Common features of development, 6

Genomic equivalence, cloning of animals, 7Gametogenesis, 8Early development, 11Morphogenetic processes, 14Growth and death, 18

3 Developmental genetics, 21Developmental mutants, 21Screening for mutants, 25Cloning of genes, 27Transgenesis, 28Gene duplication, 30

4 Experimental embryology, 33Normal development, 33Developmental commitment, 35Acquisition of commitment, 37Homeotic genes, 40Criteria for proof, 43

5 Techniques for the study ofdevelopment, 45

Microscopy, 45Study of gene expression by biochemical methods, 48

Study of gene expression by in situ methods, 50Reporter genes, 52Microinjection, 53Cell-labeling methods, 54Cell sorting, 55

Section 2: Major model organisms, 59

6 Model organisms, 61The big six, 61Availability and cost, 61Access and micromanipulation, 63Genetics and genome maps, 63Relevance and tempo, 64

7 Xenopus, 67Oogenesis, maturation, fertilization, 67Embryonic development, 68Experimental methods, 73Regional specification, 77Inductive interactions, 79

8 The zebrafish, 89Normal development, 89Mutagenesis, 91Regional specification, 92

9 The chick, 96Normal development, 97Regional specification of the early embryo, 102Description of organogenesis in the chick, 105

10 The mouse, 112Mammalian fertilization, 113Normal embryonic development, 115Technology of mouse development, 122Regional specification in development, 129Other topics in mouse development, 133

Contents

EDBA01 1/08/2005 09:29 Page v

11 Drosophila, 139Insects, 139Normal development, 140Drosophila developmental genetics, 143Overview of the developmental program, 146The dorsoventral pattern, 148The anteroposterior system, 150

12 Caenorhabditis elegans, 162Normal development, 163Regional specification in the embryo, 164Analysis of postembryonic development, 170Programmed cell death, 173

Section 3: Organogenesis, 177

13 Tissue organization and stemcells, 179

Types of tissue, 179Tissue renewal, 182Skin, 186Intestine, 189Hematopoietic system, 194

14 Development of the nervoussystem, 199

Overall structure and cell types, 199Anteroposterior patterning of the neural plate, 202Dorsoventral patterning of the neural tube, 205Neurogenesis and gliogenesis, 206The neural crest, 210Development of neuronal connectivity, 213

15 Development of mesodermalorgans, 221

Somitogenesis and myogenesis, 221The kidney, 227Germ cell and gonadal development, 229Limb development, 233Heart and blood vessels, 242

16 Development of endodermalorgans, 249

Normal development, 249

vi u Contents

Determination of the endoderm, 253The pancreas, 258

17 Drosophila imaginal discs, 264Metamorphosis, 264Genetic study of larval development, 264Mitotic recombination, 266Disc development, 268Regional patterning of the wing disc, 272

Section 4: Growth, regeneration, andevolution, 279

18 Growth, aging, and cancer, 281Size and proportion, 281Growth in stature, 285Aging, 287Postnatal disorders of growth and differentiation, 288

19 Regeneration of missing parts, 293Distribution of regenerative capacity, 293Planarian regeneration, 294Vertebrate limb regeneration, 297

20 Evolution and development, 307Macroevolution, 307The primordial animal, 311What really happened in evolution?, 318

Appendix: Key molecular components, 324

Genes, 324Transcription factor families, 326Signaling systems, 327Inducing factor families, 329Cytoskeleton, 333Cell adhesion molecules, 334Extracellular matrix components, 335

Glossary, 337

Index, 349

EDBA01 1/08/2005 09:29 Page vi

This book presents the basic ideas and facts of modern develop-mental biology of animals. Special attention has been given tokeeping it compact and concise. It should be suitable as a coretext for undergraduate courses from the second to the fourthyear, and for beginning graduate courses. The first edition hasbeen “road tested” by myself and by many other instructors, and it has been found suitable for both biologically based andmedically oriented courses. A basic knowledge of cell andmolecular biology is assumed, but no prior knowledge of devel-opment, animal structure, or histology should be necessary.

Organization

The book is arranged in four sections and the order of topics isintended to represent a logical progression. The first sectionintroduces the basic concepts and techniques. The second sec-tion covers the six main “model organisms,” Xenopus, zebrafish,chick, mouse, Drosophila, and Caenorhabditis elegans, describ-ing their early development to the stage of the general body plan.The third section deals with stem cells and organ development,mostly of vertebrates but including also Drosophila imaginaldiscs. The fourth section deals with growth, regeneration, andevolution. To assist readers unfamiliar with the the families ofgenes and molecules that are important in development, theyare listed in the Appendix in the context of a short revision guideto basic molecular and cell biology.

Distinctive approach

This book differs from its main competitors in four importantrespects, all of which I feel are essential for effective education.Firstly, it keeps the model organisms separate when early development is discussed. This avoids the muddle that arises alltoo often when students think that knockouts can be made inXenopus, or that bindin is essential for mammalian fertiliza-tion. Secondly, I have avoided all considerations of history and

experimental priority because students do not care who didsomething first if it all happened twenty years ago. Thirdly, onthe other hand, I have been careful to stress at all stages why webelieve what we do. Understanding does not come from simplymemorizing long lists of gene names, so I have insisted that stu-dents understand how to investigate developmental phenomenaand what sorts of evidence are needed to prove a particular typeof result. Finally, the work is highly focused. In order to keep thetext short and concise I have not wandered off into areas such as the development of plants or lower eukaryotes that may beinteresting but are really separate branches of biology.

Changes to this edition

The first edition was very well received by both users and reviewers and I hope that the second edition will make EssentialDevelopmental Biology an even more popular choice for under-graduate teaching around the world. The changes made for this edition reflect both the requests of users and the changes inthe subject matter over the last few years. Users overwhelminglywanted color in the illustrations, so this has been provided.There is now a glossary at the end which defines all the key termsshown in bold in the text, and each chapter also contains a set of summary bullet points. The web-based materials have beenexpanded and now include animations in addition to the full set of illustrations used in the book. The users we consulted alsotended to want more material on their own favorite topic. Thiswas more difficult to provide as everyone’s favorite topic is different, and to please everyone would have led to an explosionin length that would have ended up pleasing nobody. Howeversome modest additions have been made. There is more on mammalian fertilization, which is always of interest to students.There is more on the heart and the gut, as these are such centraltopics in human embryology, and there is much recent researchprogress. There is more on stem cells, growth, and aging, all hotresearch topics with obvious practical significance. Finally, anew chapter on evolution and development now gives this area

Preface

EDBA01 1/08/2005 09:29 Page vii

a higher profile than in the first edition. Otherwise, the text has all been rewritten and updated, the grouping of topics hasbeen reorganized to some extent, the references have been rationalized, and errors have been removed.

Developmental biology has become a very detailed and com-plex subject and this means that inevitably most of the referencesin an elementary text have to be to reviews. A consequence ofthis could be that students would never read any original sci-entific papers, which would be very undesirable. So I have nowincluded boxes with primary references to some key major discoveries. The choice of these references is of course personaland subjective but I hope they will communicate the excitementinvolved in research to those who look them up. I have probablybeen even more subjective in my choices of future priorities in the boxes on “new directions for research,” but the object ofthese is to indicate that the subject is still moving and these boxesmay be useful as the starting points for some discussions.

Students sometimes consider developmental biology to be adifficult subject, but this need not be the case as long as certainobstacles to understanding are identified at an early stage. Thenames and relationships of embryonic body parts are generallynew to students, so in this book the number of different partsmentioned is kept to the minimum required for understandingthe experiments, and a consistent nomenclature is adopted (e.g. “anterior” is used throughout rather than “rostral” or “cra-nial”). The competitor texts all mix up species and, for example,would typically consider sea urchin gastrulation, Xenopus meso-derm induction, and chick somitogenesis in quick succession.This leaves the student unsure about which processes occur in which organisms. In order to avoid confusion, I have keptseparate the animal species in section 2, and for sections 3 and 4it is made clear to which organisms particular findings apply.Although most students do understand genetics in its simpleMendelian form, they do not necessarily appreciate certain keyfeatures prominent in developmental genetics. Among these arethe fact that one gene can have several mutant alleles (e.g. loss offunction, constitutive, or dominant negative), or that the nameof a gene often corresponds to its loss of function phenotyperather than its normal function (e.g. the normal function of thedorsal gene in Drosophila is to promote ventral development!).Furthermore, pathways with repressive steps, such as the Wntpathway, cause considerable trouble because of a failure tounderstand that the lack of something may be just as importantas the presence of something. Here, these issues are fullyexplained in the early chapters, with appropriate reinforcementlater on. Finally, I have tried to keep the overall level of detail, in terms of the number of genes, signaling systems and othermolecular components, to the bare minimum required toexplain the workings of a particular process. This sometimesmeans that various parallel or redundant components are notmentioned, and the latest detail published in Cell is omitted.

Summary of key new features:• Instructor CD with artwork in downloadable format

viii u Preface

• Website including 25 animations, interactive exercises, all text, artwork, and also simple schematic art. Animations areindicated in the margin with the icon. Access is free with purchase of new book (access may also be purchased by visiting www.blackwellpublishing.com and searching forISBN 1-4051-4646-X)• New chapters on Tissue Organization and Stem Cells(Chapter 13), Development of Endodermal Organs (Chapter16), and Evolution and Development (Chapter 20)• Expanded coverage of mammalian fertilization, the heart,growth control, and aging• “Classic Experiment” boxes with primary references• “New Directions for Research” boxes• End-of-book glossary• End-of-chapter summaries for quick review• Numerous new figures, including model organism compar-ison chart (Chapter 6)• Four-color used throughout

When students have completed a course corresponding to the content of this book they should be able to understand themain principles and methods of the subject. If they wish to entergraduate school, they should be well prepared to enter a gradu-ate program in developmental biology. If they go to work in the pharmaceutical industry, they should be able to evaluateassays based on developmental systems where these are used forthe purposes of drug screening or drug development. If theybecome high school teachers, they should be able to interpret the increasing flow of stories in the media dealing with devel-opmental topics, which are sometimes inaccurate and often sensationalized. Whether the story deals with human cloning,four-legged chickens, or headless frogs, the teacher should beable to understand and explain the true nature of the results andthe real motivation behind the work. It is in all our interests toensure that the results of scientific research are disseminatedwidely, but also that they are a source of enlightenment and notof sensation.

Acknowledgments

Finally, I should like to thank some people who have beeninvolved with the work: Nancy Whilton who enthusiasticallycommissioned and guided this edition; Elizabeth Wald who very capably managed the day-to-day details in developing thisedition; Debbie Maizels of Zoobotanica for the excellent illus-trations; Rosie Hayden, Sarah Edwards, and Brian Johnson who have skillfully handled the complex production; and the numerous reviewers, listed below, who have made manyhelpful comments on sections of the manuscript. The respons-ibility for any residual errors is mine and I shall be pleased tohear from readers who discover them.

Reviewers:Judith E. Heady, University of Michigan-Dearborn

EDBA01 1/08/2005 09:29 Page viii

David Heathcote, University of Wisconsin-MilwaukeeMargaret Saha, College of William and MaryHan Wang, University of OklahomaGrant N. Wheeler, University of East AngliaW.B. Wood, University of ColoradoLauren Yaich, University of Pittsburgh

Preface u ix

Most importantly, I should like to thank my lab memberswho have put up with a lot of unavailability on my part, and my family whose patience and support during this long writingprocess was also invaluable.

Jonathan SlackBath, 2005

EDBA01 1/08/2005 09:29 Page ix

EDBA01 1/08/2005 09:29 Page x

SectionGroundwork

1

EDBC01 1/08/2005 09:07 Page 1

EDBC01 1/08/2005 09:07 Page 2

Where the subject came from

One of the most amazing conclusions of modern biologicalresearch is that the mechanisms of development are very sim-ilar for all animals, including humans. This fact has only beenknown since it has become possible to examine the molecularbasis of developmental processes. As recently as 1980 we knewnothing of these mechanisms, but 25 years later we know a lot and it is possible to write undergraduate textbooks on thesubject. Over this period, developmental biology has been one ofthe most exciting areas of biological research. These dramaticadvances came from three main traditions that became fusedtogether into a single world-view: experimental embryology,developmental genetics, and molecular biology.

Experimental embryology had been in existence since thebeginning of the twentieth century, consisting mainly of micro-surgical experiments on embryos of frogs and sea urchins. Thesehad demonstrated the existence of embryonic induction: chem-ical signals that controlled the pathways of development of regionsof cells within the embryo. The experiments showed where andwhen these signals operated, but they could not identify the signals, nor the molecular nature of the responses to them.

Developmental genetics has also existed for a long time, but it really flowered in the late 1970s when mass genetic screenswere carried out on the fruit fly Drosophila, in which thou-sands of mutations affecting development were examined. Thesemutagenesis screens resulted in the identification of a high proportion of the genes that control development, not just inDrosophila, but in all animals.

Molecular biology had started with the discovery of the three-dimensional structure of DNA in 1953, and became a practicalscience of gene manipulation in the 1970s. The key technicalinnovations were methods for molecular cloning to enable single genes to be amplified to a chemically useful quantity,methods for nucleic acid hybridization to enable the identifica-tion of DNA or RNA samples, and methods for DNA sequenc-ing to determine the primary structures of genes and theirprotein products. Once this toolkit had been assembled it could

be applied to a whole range of biological problems, includingthose of development. It was used initially to clone the develop-mental genes of Drosophila. This turned out to be of enormousimportance because most of the key Drosophila genes werefound to exist also in other animals, and frequently to be con-trolling similar developmental processes. Molecular biologicalmethods were also applied directly to vertebrate embryos andused to identify the previously mysterious inducing factors andthe genes regulated by them.

The application of molecular biology meant that the mechan-isms of development could for the first time be worked out inmolecular detail. It also meant that the path of developmentcould be experimentally altered by the introduction of newgenes, or the selective removal of genes, or by an alteration of theregulatory relationships between genes. It has turned out that allanimals use very similar mechanisms to control their develop-ment. This is particularly exciting because it means that we reallycan learn about human development by understanding how ithappens in the fruit fly, zebrafish, frog, or mouse.

Central position in biology

Developmental biology occupies a pivotal position in modernbiology. This is because it unites the disciplines of molecularbiology, cell biology, genetics, and morphology. Molecular andcell biology tell us about how the individual components work:the inducing factors, their receptors, the signal transductionpathways, the transcription factors. Genetics tells us directlyabout the function of an individual gene and how it relates to theactivities of other genes. Morphology, or anatomical structure, isboth a consequence and a cause of the molecular events. Thefirst processes of development create a certain simple morpho-logy which then serves as the basis on which further rounds ofsignaling and responses can occur, eventually to create a morecomplex morphology.

So developmental biology is a synthetic discipline in which anunderstanding of molecular biology, genetics, and morphology

Chapter

The excitement ofdevelopmental biology

1

EDBC01 1/08/2005 09:07 Page 3

is necessary. When thinking about developmental problems it is necessary to be able to use concepts from these three areassimultaneously because they are all necessary to achieve a com-plete picture.

Impact on society

Certain areas of developmental biology have had a significantimpact on society in recent decades. In vitro fertilization (IVF)is now a routine procedure and has enabled many previouslyinfertile couples to have a baby. Its variants include artificialinsemination by donor (AID), egg donation, and storage of fertilized eggs by freezing. It is perhaps less widely appreciatedthat AID, IVF, embryo freezing, and embryo transfer betweenmothers is also very important for farm animals. It has been usedfor many years in cattle to increase the reproductive potential ofthe best animals.

Developmental biology also led to the understanding thathuman embryos are particularly sensitive to damage during theperiod of organogenesis (i.e. after the general body plan isformed, and while individual organs are being laid down). Thescience of teratology studies the effects of environmental agentssuch as chemicals, viral infection, or radiation on embryos. Thishas led to an awareness of the need to protect pregnant womenfrom the effects of these agents.

Developmental biology is responsible for an understanding of the chromosomal basis of some human birth defects. In particular Down’s syndrome is due to the presence of an extrachromosome, and there are a number of relatively commonabnormalities of the sex chromosomes. These can be detected incells taken from the amniotic fluid and form the basis of theamniocentesis tests taken by millions of expectant mothersevery year. Many more birth defects are due to mutations ingenes that control development. It is now possible to screen forsome of these, either in the DNA of the parents or in the embryoitself, using molecular biology techniques.

Future impact

Although the past impact of developmental biology is signific-ant, the future impact will be much greater. Some of the benefitsare indirect and not immediately apparent. Some, particularlythose involving human genetic manipulation or cloning, willcause some serious ethical and legal problems. These problemswill have to be resolved by society as a whole and not just the sci-entists who are the current practitioners of the subject. For thisreason it is important that an understanding of developmentalbiology becomes as widespread as possible, because only with anappreciation of the science will people be able to make informedchoices.

The human genome is now fully cataloged and sequenced,and so are the genomes of most of the animals used as experi-

4 u Chapter 1

mental organisms for studying development. Furthermore tech-niques are now well advanced for separating and identifying allthe proteins in a particular tissue sample (proteomics). Thismeans that it has become much easier to identify genes or geneproducts associated with particular developmental mutations ordiseases, and has led to an increased emphasis on understandingtheir functions. Developmental biology is a central componentof these new disciplines of functional genomics and functionalproteomics.

The first main area of practical significance is that an under-standing of developmental mechanisms will assist the pharma-ceutical industry in designing new drugs effective against canceror against degenerative diseases such as diabetes, arthritis, andneurodegeneration. As is well known, these conditions causeenormous suffering and premature death. The processes that fail in degenerative diseases are those established in the course of embryonic development, particularly its later stages. Under-standing which genes and signaling molecules are involved willprovide a large number of potential new therapeutic targets forpossible intervention. Once the targets have been identified bydevelopmental biology, the new powerful techniques of com-binatorial chemistry can be applied by pharmaceutical chem-ists to create drugs that can specifically augment or inhibit theiraction.

Secondly, and as a quite separate contribution to the work of the pharmaceutical industry, various developmental modelsystems are important as assays. The in vivo function of manysignal transduction pathways can be visualized in Xenopusor zebrafish or Drosophila or Caenorhabditis elegans, and can beused to assay substances that interfere with them using simpledissecting microscope tests. Genetically manipulated mouseembryos are increasingly being used as animal models ofhuman diseases, enabling more detailed study of pathologicalmechanisms and the testing of new experimental therapies.These are by no means limited to models for human genetic dis-ease as often a targeted mutation in the mouse can mimic ahuman disease that arises by other means.

Thirdly, there is the possibility of using our understanding ofgrowth and regeneration processes for therapy. This has alreadybeen done to some extent. For example the hematopoietic growthfactors erythropoietin and granulocyte–macrophage colony-stimulating factor (GM-CSF) have both been used in clinicalpractice for some years to treat patients whose blood cells aredepleted by cancer chemotherapy, or for other reasons. In futureother factors may also be developed. For example, somethingthat could make pancreatic β-cells grow would be very useful for the treatment of diabetes, or something that could promoteneuronal regeneration would be useful in treating a variety ofneurodegenerative disorders.

Fourthly, there is the extension of the existing prenatalscreening to encompass the whole variety of single-gene dis-orders. Although this is welcome as a further step in the elimina-tion of human congenital defects, it also presents a problem. Themore tests are performed on an individual’s genetic makeup, the

EDBC01 1/08/2005 09:07 Page 4

more likely that individual is to be denied insurance or par-ticular career opportunities because of a susceptibility to somedisease or other. It also risks the creation of an underclass ofgenetically “suspect” persons, contrasted with the screened and supposedly “clean” ones. This is a problem that society as a whole will have to resolve.

Fifthly, and even more controversial, there is the possibleapplication of developmental biology to the production ofhuman tissues or organs for transplantation. At present trans-plantation is seriously limited by the availability of donororgans. There are two conceivable routes to this end. The tissueengineering route involves the growth of the tissue or organin vitro either from stem cells or from combinations of maturecells that can be cultivated outside the body. This involves theproduction of novel types of three-dimensional extracellularmatrix, or scaffold, on which the cells grow and with which theyinteract. Tissue engineering will need more input from develop-mental biology in order to be able to create tissues containingseveral interacting cell types, or tissues with appropriate vascularand nerve supplies.

The second route to replacement tissues and organs envisagestheir growth from human embryonic stem cells (ES cells). Thismay be possible by improvement of culture conditions or it may turn out also to require genetic modification of the cells. In either case there are potential ethical problems connectedwith genetic modification of human tissues and with the use ofhuman eggs for a purpose other than conventional reproduc-tion. This issue also intersects with the debate about humancloning. Although there is virtually universal agreement thathuman beings should not be “copied” by cloning methods (theprocedure called reproductive cloning), the majority of scient-ists do favor the potential use of cloned embryos as a source fortissue grafts. This is called therapeutic cloning and involvesgrowing the ES cells from an egg in which the nucleus has beenreplaced by one from the individual needing the graft. Thepotential advantage is that this could be a method for creating a limitless supply of grafts with perfect immunological com-patibility. The continuing ethical debate on this matter arisesbecause the procedure technically involves the creation of anembryo for a purpose other than reproduction.

Finally, we should not overlook the likely applications ofdevelopmental biology to agriculture. With farm animals thepossibilities are likely to be limited by a public wish to retain a “traditional” appearance for cows, pigs, sheep, and poultry,but already technologies have been developed to produce phar-maceuticals in the milk of sheep or vaccines in eggs, and otheropportunities will doubtless present themselves in the future.

The excitement of developmental biology u 5

Further reading

Useful web sitesZygote: http://zygote.swarthmore.edu/The virtual embryo: http://www.ucalgary.ca/UofC/eduweb/virtualembryo/Bill Wasserman’s developmental biology page: http://www.luc.edu/depts/biology/dev.htm

Textbooks, mainly descriptiveGilbert, S.F. & Raunio, A.M. (1997) Embryology: constructing the organ-ism. Sunderland, MA: Sinauer Associates.Larsen, W.J. (1997) Human Embryology, 2nd edn. New York: ChurchillLivingstone.Hildebrand, M. & Goslow, G.E. (2001) Analysis of Vertebrate Structure,5th edn. New York: John Wiley.Carlson, B.M. (2004) Human Embryology and Developmental Biology. St Louis, MO: Mosby.

Textbooks, mainly analyticalTwyman, R. & Gatherer, D. (2001) Instant Notes in DevelopmentalBiology. Oxford: Bios Scientific Publishers.Wolpert, L. (2002) Principles of Development, 2nd edn. Oxford: OxfordUniversity Press.Gilbert, S.F. (2003) Developmental Biology, 7th edn. Sunderland, MA:Sinauer Associates.Wilt, F.H. & Hake, S.E. (2003) Principles of Developmental Biology. NewYork: W.W. Norton.

MonographMartinez-Arias, A. & Stewart, A. (2002) Molecular Principles of AnimalDevelopment. Oxford: Oxford University Press.

Reproductive technology and ethicsEdwards, R.G. (1997) Recent scientific and medical advances in assistedhuman conception. International Journal of Developmental Biology 41,255–262.Austin, C.R. (1997) Legal, ethical and historical aspects of assistedhuman reproduction. International Journal of Developmental Biology41, 263–265.Braude, P. (2001) Preimplantation genetic diagnosis and embryoresearch – human developmental biology in clinical practice. Inter-national Journal of Developmental Biology 45, 607–611.Committee on Biological and Biomedical Application (2002) Stem Cellsand the Future of Regenerative Medicine. Washington DC: NationalAcademy Press.Maienschein, J. (2003) Whose View of Life ? Embryos, cloning and stemcells. Cambridge, MA: Harvard University Press.Hwang, W.S., Ryu, Y.J., Park, J.H. et al. (2004) Evidence of a pluripotenthuman embryonic stem cell line derived from a cloned blastocyst.Science 303, 1669–1674.

EDBC01 1/08/2005 09:07 Page 5