Fred H. Wilt & Sarah C. Hake
Developmental biology is in the midst of an extraordinary flowering of discovery. For the authors of a new textbook, this was both a blessing and a challenge. The process of choosing from among this cornucopia of material was exhilarating, but the vastness of the field meant we had to make hard choices about what material to include and where to place the critical emphases. In this preface, we shall explain the choices we made in writing Principles of Developmental Biology.
The impetus for writing this book was the need we have felt as teachers for an introduction to developmental biology that presents the essentials while not overwhelming students in terms of length and level of difficulty. Our aim was to write a book that can be taught in a one-semester course and that does not presume the student reader will become a professional developmental biologist. Since the backgrounds of students studying developmental biology are so diverse, the only prerequisite we assume is an introductory college-level course in biology that includes the basics of molecular and cell biology as well as standard organismal biology.
Attaining our goals was a tall order. The subject matter is vast and complex, and factual knowledge in this field is increasing at breakneck speed. Hence, we have put the emphasis on the principles, including some descriptive embryology of selected plants and animals, and on providing some historical context. A relatively small number of examples have been chosen. We are painfully aware of many beautiful experiments and developmental systems that have been omitted-the price for presenting an introduction rather than an exhaustive treatment. Nonetheless, it is our experience that most students can tackle and understand more complex and sophisticated experiments once they have truly mastered the basics through a thorough exploration of a limited number of apt examples.
In order to attain some reasonable brevity and to cut through a thicket of vocabulary and examples, we have chosen to focus on the embryos of fruit fly, frog, chick, and mouse and on the development of maize and Arabidopsis for most of our examples. Other important developing systems-such as those of Caenorhabditis elegans, sea urchins, ascidians, and zebrafish-are briefly introduced where we have judged appropriate. Of course, this results in some loss of breadth, since important experiments have been and are being done using organisms other than those featured here. On the other hand, it has been our experience as teachers that an extensive comparative approach leads most students to acquire more facts but less understanding.
We have followed an order of presentation in which we first focus on describing the actual development of animals and plants, and then in later chapters we emphasize an analysis of the mechanisms of this development. We begin Part One with a brief introduction to the core intellectual issue of developmental biology, namely, how can two cells with the same genetic information become functionally different? This is followed by a description of gametogenesis and fertilization in animals. Part Two describes the early development of Drosophila, amphibians, and amniotes. Our teaching experience has shown that the sequence of describing the development of one organism, followed by the development of a second, when coupled with appropriate comments contrasting the two organisms, is easier for beginning students than a stage-by-stage comparative approach. Part Three is a more detailed examination of organogenesis in vertebrates. This topic was formerly the bedrock of beginning embryology courses, and still constitutes an important subject. While the emphasis here, as in Part Two, is on description, these chapters also introduce some analysis of what drives development, emphasizing mechanisms, especially intercellular communication by way of ligand-receptor interactions. Part Four describes plant development in both meristems and embryos. This is an area in which cellular and molecular studies are beginning to show clearly the similarities and differences between plant and animal development, and so in our judgment is now appropriate for inclusion in an introductory course. Part Five presents some necessary cell biology for subsequent chapters. Part Six provides a deeper exploration of morphogenesis by analyzing the molecular basis of differentiation and pattern formation. Included are discussions of the molecular basis of the regulation of transcription, and posttranscriptional events, with special emphasis on intercellular signaling. The last chapter in the book is a brief treatment of the connections between embryo development and evolution, a classical subject area in developmental biology that is being newly revitalized.
Certain important, complex topics are reconsidered throughout the book. For instance, gastrulation is described in Chapters 3, 4, and 5. But details of the cellular and molecular basis of gastrulation are not taken up until Chapters 13 and 16, after the student has acquired sufficient background understanding. Likewise, vertebrate limb development is briefly described in Chapter 7, and the importance of epithelialmesenchymal interactions in limb development is presented in Chapter 13. Moreover, limb development is revisited in Chapters 16 and 17 because at this point the student can follow complexities in the molecular basis of limb formation. Each of the several discussions of a particular subject area is given either in a different context or at a different level of sophistication. There are substantial pedagogical advantages to revisiting the same topic with a change of focus: the repetition allows the student to learn first about the developmental anatomy, and subsequently about the cellular interactions and changing gene expression underpinning the development.
Textbooks commonly have introductory chapters that cover elementary cell and molecular biology. We have instead chosen to distribute crucial bits of background knowledge and experimental technique throughout the book, where the topic at hand calls for them. Hence, the first chapter contains boxes on nucleic acid hybridization and cloning. Most chapters include various kinds of special information, also in boxes. These may emphasize techniques, as mentioned earlier; or different developmental systems, such as those of the zebrafish or the ascidians; or special topics of interest, such as sex determination, genomics, and stem cells. For instructors who prefer to present introductory and background information at the beginning of the course, two chapters in the middle of our book can be considered at the outset. Chapter 12 is an introduction to cell behavior and the molecules of the extracellular matrix. Chapter 14 is a consideration of the molecular biology of information flow in the cell. (We placed them where they now occur because they contain review information essential for understanding the material immediately following Chapter 13 and Chapters 15 through 17). We hope that teachers using this text will find the CD containing most of the illustrations from the book useful for preparing lectures. The questions posed at the end of each chapter-many without a simple right or wrong answerare intended to stimulate students to think about the material. The references at each chapter end include some important classics, as well as works providing entree into what is often a dense literature.
There are many other useful websites, and new ones keep appearing. Use a search engine, such as Google or Yahoo, to search on some key words to find them.
We have many people to thank. We are grateful to those who reviewed various portions of the manuscript; errors of fact or opinion that remain are, of course, strictly those of the authors. The reviewers were Anonymous, Steve Benson, Graeme Berlyn, David Epel, John Gerhart, Paul Lasko, Judy Lengyel, Mike Levine, Randy Moon, Lisa Nagy, Steve Oppenheimer, Rodolfo Rivas, Chris Rose, Mark Servetnick, Ian Sussex, and Vic Vacquier.
We want to acknowledge our debt to our own teachers and colleagues, among them the late James Ebert, John Gerhart, Don Kaplan, Ray Keller, the late Dan Mazia, and Ian Sussex. We are grateful to several generations of students, who informed and inspired us. We also wish to thank many colleagues, friends, and family who helped us, with special mention for Jim Fristrom, Rob Grainger, Pat Hamilton, Kristen Shepard, and Diane Wilt. The work of Connie Balek and her colleagues at Precision Graphics saved us. Susan Middleton has been a wonderful copy editor from whom we have learned much, and we are indebted to the team at W. W. Norton, among them Kate Barry, John Byram, Sarah Chamberlin, Tom Gagliano, Chris Granvile, Jack Repcheck, Chris Swart, and Joe Wisnovsky.
We are eager to have your comments. Please let us know about errors, idiosyncrasies-in fact, any opinions at all. You can reach us at email@example.com.
Published by Elsevier Science under Auspices of Society for Developmental Biology
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