We hold these principles to be self-evident (2011)

What is to be taught in an undergraduate course on developmental biology? As in all branches of biology, there is far too much known to be able to teach it all, and any introductory course would sacrifice depth. Inevitably, choices must be made, and one choice is to emphasize important principles and concepts of development across all organisms. Lewis Wolpert and Cheryll Tickle, with a cast of impressive supporting authors, have made excellent selections in Principles of Development. This is the fourth edition of the book and the thoughtful choice of topics that went into the first edition is still evident, although there have also been many useful updates.The book begins with some history and a summary of general concepts. The concepts are important ones, especially when framed by the title of the book, but they may be a little dry out of the context of real organisms. However, one has to start somewhere, and the general concepts are illustrated in later chapters with examples from real animals and plants. The principles and concepts could be re-stated more forcefully throughout the book, though, as they may be missed by the inattentive reader. Along the way, boxes explain the important experimental techniques that provide approaches to questions. The figures are drawn in a consistent style, which helps to give a coherent presentation and lets the student focus on content. Although the images are variants of the kinds of drawings we have seen in original journal articles and other textbooks, they are rendered here with style and clarity. The photographs are usually well chosen, though in some cases they don’t seem to be as clear or as relevant as they should be. For example, it isn’t clear why a well-camouflaged California false hellebore, the source of the teratogen cyclopamine, is shown, rather than the (admittedly grisly) cyclopic consequences of its action.Once the concepts are dealt with, the book goes justifiably into detail on different organisms. It is difficult to grasp general concepts without knowing the descriptive background; although the plethora of terms and structures may be daunting to the student, there seems to be no other way to understand the mechanisms underlying developmental processes than to know, at the descriptive level, how an organism undergoes normal development. Chapter 2 focuses on Drosophila development, appropriately because, as the authors point out, we have more in-depth developmental knowledge about the fly than about any other animal. The treatment is systematic and clearly presented, with beautiful illustrations. Most of the important concepts are presented here; although, not surprisingly in a text of limited size, much of the crucial evidence is not. It is a justifiable way to present material to the student, presumably providing reinforcement for lecture material, but leaving the lecturer something of substance to talk about. Some might argue that the descriptions of concepts don’t matter a whit, and what is more important is that the students can reason and can design or interpret experiments to establish for themselves how a concept is established. The ideal probably lies in between. Perhaps in a book about principles, the authors could have put more emphasis on some of the general principles of developmental biology experiments, such as the inferences that can be drawn from gain- and loss-of-function experiments, approaches which enjoy so many variations. The boxes that include the methods are generally excellent, though they are not necessarily presented in a consistent manner: The Cre-lox system in the mouse and mutagenesis in the fish receive nice pink boxes, but homologous recombination in embryonic stem cells is in line with the text.The latter techniques are described in the chapters on vertebrate development. An introductory chapter to this section describes vertebrate development and common methods, and is followed by a chapter on setting up the body axes. This represents a change from the previous edition, in which these topics were covered in a single chapter, and is one of the good updates. The chapters on vertebrate development, just like the others, are clear and beautifully written and illustrated. Chapters covering the more complex topics, such as somite and nervous system development, follow as epigenesis proceeds. At this juncture, the early embryology of other prominent model animals, the nematode, sea urchin and ascidian, take center stage, and a chapter on plant development rounds out the coverage of organisms.The book then circles back to various developmental processes. Admirable examples from various phyla are used to illustrate the process of morphogenesis, in which underlying cellular processes can be seen to give rise to changes in the shape of the organism or tissue. The selection of examples from diverse organisms is made possible by the earlier descriptive coverage, which no longer need get in the way of the discussion of concepts. Germ cells and sex determination are followed by gene expression, cell differentiation and plasticity, including another excellent update in the current edition – inevitably, on stem cells and the plasticity of gene expression.A book by Wolpert and Tickle would be incomplete without extended treatment of the limb, and a chapter on organogenesis includes vertebrate and insect limbs, eyes and various internal organs. The authors’ expertise in limb development is reflected in the greater citation of experiments in this area, though this may challenge the undergraduate who wants a clear-cut story. One has to admire Wolpert for giving prominent treatment to his progress zone model, which most people might present as a great idea that is no longer tenable. But, perhaps if a book is to emphasize principles, it only matters if they are good possibilities, not whether they are currently supported.The book closes with an excellent summary of developmental neurobiology, a chapter on post-embryonic growth and development, an expanded treatment of regeneration and a good evo-devo summary.If the book is to remain manageable in size, it is inevitable that some favorite topics of the reader might be glossed over. However, despite the admirable emphasis on principles and concepts, I occasionally felt short-changed. With these authors, I might have expected a deeper treatment of what surely must be one of the most important principles: the existence of threshold responses to morphogens – molecules that diffuse from a source and set up a graded concentration. Instead, when it gets to the nitty gritty of boundaries, the activating and repressing activities of one gradient (such as hunchback or dorsal) are mentioned, but how one protein both activates and represses is not explained. Indeed, the text gets pretty dense and lacks figures that would clarify the molecular mechanisms at play in threshold activation. Later in the book, one might have expected a deeper treatment of hedgehog signaling and threshold responses, as that might be the clearest case of a morphogen gradient at work in vertebrates. And what is the best evidence that these molecules really act as morphogens? It certainly isn’t simple, but many students can deal with the evidence. Similarly, an important developmental principle is that internal organs achieve a high surface area by branching morphogenesis. The treatment is again a bit glib; genes and proteins that promote or inhibit branching are enumerated, but I don’t think the authors can have stood up in front of a class and tried to argue that this really explains branching, unless their students are more passive than ours. One hopes that the undergraduates will test the lecturer’s understanding at points like this and demand a deeper explanation, or an admission that the emperor indeed has no clothes.Finally, although the opening chapter addresses what may be some important principles and concepts, these should be more explicitly re-stated throughout the book. I don’t think that students find these concepts obvious, particularly when the details, terms and descriptions come thick and fast. Indeed, one might ask why there should be principles of development – after all, evolution selects whatever process happens to work and developmental biology is full of ad hoc solutions to problems. Nevertheless, there are some core principles that recur in development, suggesting that these must be either ancient and inherited, or arrived at by convergence. Anterior-posterior patterning by deployment of Hox genes might be an example of the former, whereas branching morphogenesis might be an example of the latter, for which it is arguable that each organ might employ a different morphogenetic program.It is inevitable that a reviewer will raise some criticisms; nevertheless, as it stands this is an outstanding text with a well-digested and comprehensive presentation of current views in developmental biology. It is a text I can enthusiastically recommend for adoption in developmental biology courses.For those courses that have previously used this text, is the fourth edition worth the upgrade? The hardback edition is expensive and it would be hard to justify the burden of an expensive new edition on a fresh set of undergraduates when the cheaper previous editions of Principles of Development might be available. The paperback is a better deal and my review copy hasn’t fallen apart yet. Still, as Oliver Wendell Holmes said, “A man is usually more careful of his money than of his principles.”