Embryos at the core of life

The structure of the book is logical, much like the acute mind of its main author, as those who are lucky enough to know Lewis are well aware. It moves effortlessly from the general to the specific and back to the general,reminiscent of the harmonious deployment of a Mahler symphony. Principles of Development is divided into 15 chapters. Opening with a short history on the beginnings of the field, it then delves into the vertebrate,invertebrate and plant model systems that are used in everyday research, and provides a short introduction into gene identification protocols. Next come chapters dedicated to early pattern formation events in vertebrates(particularly frogs, chicks and mice); Chapter 3 takes us through body axes and the formation of the three metazoan embryonic layers, endoderm, mesoderm and ectoderm, and Chapter 4 digs a bit deeper into mesoderm patterning events and the formation of the neural system. Chapters 5 and 6 are devoted to invertebrate embryogenesis, and review the most important model systems: the almighty Drosophila melanogaster (which has been given, and no doubt deserves, the whole of Chapter 5 as a result of its fundamental role in helping to attain our present knowledge of developmental processes) and those other heroes of experimental lore, nematode worms, sea urchins, ascidian tunicates and slime molds. Chapter 7 outlines the exciting world of Arabidopsis thaliana and plant development in a concise, albeit very pedagogic, manner.FIG1 

An excellent entry point into the generic processes of development is provided by Chapters 8 and 9, which discuss the morphogenetic processes and cell dynamics that generate form and cell differentiation, and the molecular mechanisms that underpin cell specialization in the embryo. In Chapters 10 and 11, the book moves on to the specifics of structure formation in chick limbs,the nematode vulva, insect wings and legs, and internal vertebrate organs,such as the heart, lungs, kidneys and nervous system. Chapter 12 describes the formation of germ cells and the mechanisms of sex determination, as well as the details of fertilization events (one wonders why this chapter is almost at the end of the book!). Chapters 13 and 14 deal with the important issues of regeneration and growth, issues that, today, touch upon the very hot topic of stem cell research. The closing chapter attempts to bring together two often-separated issues in modern biology: development and evolution. However,this last topic could have been covered more extensively to explain to students how development can help in explaining the origination of novel features in evolution, which is the last bone of contention used by creationists to criticize evolutionary theory.

In summary, this excellent book covers most of the recent findings and general knowledge of current developmental biology. Perhaps a whole chapter dedicated to theoretical concepts and mathematical modeling could have been added, as this would have provided an entry point to those rare, but much needed, students who want to venture into cross-disciplinary roads. Subjects such as bioinformatics, tissue engineering, and mathematical modeling of regulatory gene networks and cellular dynamics would also have very much enriched the contents of the book.

An interesting paradox lies at the heart of the discipline of developmental biology. On the one hand, there is an emphasis on genes as the controlling operators of development. On the other hand, there is an explicit recognition that cells are the actual `units of development' and that cell behavior provides the crucial link between genes and development. Wolpert introduces the concept of `luxury' proteins as those that are specific for cell types,but how does a knowledge of these proteins further our understanding of embryogenesis? By focusing our attention on these genes and proteins we reduce the whole of developmental biology to `differentiation', a narrow path indeed. Instead, the truly important question we must ask in development is about generic cell behavior. How do genes and proteins affect cell proliferation,cell growth, cell migration, cell adhesion and cell death? From here, the next bridge to cross is that of the generic properties of embryonic tissues. How does cell behavior determine the making of epithelia, of mesenchyme, of sheets, of tubes or of vesicles? Only then can we begin to understand how organisms develop.

Wolpert, along with his high-profile editors, certainly recognizes these issues. Principles of Development tries hard to provide an array of specific answers to all of these issues, but sometimes from questions that are too genocentric. Development is not only about how the genotype controls the phenotype, but also about how information deploys and increases from rather meager information units, such as genes, into the rich complexity of a developing embryo. Despite an explicit definition of developmental biology as a discipline that “deals with the process by which the genes in the fertilized egg control cell behavior in the embryo and so determine its pattern, its form, and much of its behavior”, Wolpert's scientific career has been built upon the recognition that many interacting forces are at play during development and that the emergence of new levels of complexity act together to shape the organism.

For example, Wolpert's positional information and gradient formation model can be found several times throughout the book as it applies to different scenarios (e.g. the response of ectodermal cells to activin, the development of anterior structures in Drosophila as a response to bicoid, or the formation of compartment boundaries, to mention just a few). The establishment of these gradients depends upon many interacting factors involving the cell and its environment. It is the appearance of these and many other complex interactions that prevents development from being treated in a genotype-specifies-phenotype fashion.

Thus, the introductory chapter warns against taking genes as the ultimate cause of development, and on page 15 we find the following illuminating comment:

Indeed this is the case: much must happen from the moment a gene is expressed to the actual formation of an organ. Inside the cell cytoplasm,molecular interactions exhibit complex dynamics, including positive and negative feedback loops, oscillatory phenomena and cooperative enzymatic actions. Most importantly, cells act together and in coordination to exhibit emergent behaviors that cannot be deduced from the molecular interactions of the individual cells. Again, Wolpert is not taken off-guard, a great deal of these fascinating issues are delivered in Chapters 8 and 9, with an emphasis in linking morphogenetic processes to intracellular molecular interactions. The old search for how the linear information encoded in DNA specifies a three-dimensional organism needs to be transformed into a search for how the linear information of DNA is translated, reprocessed and spatio-temporally deployed across network levels (molecules, cells, tissues, organs, the whole embryo) into a three-dimensional organism. Textbooks about developmental biology need to emphasize this if we want to educate a new generation of developmental biologists that are able to bring more integrative views to our science.

If writing a textbook these days is a dangerous task, then reading it, and poring over its sections and subsections, headings and definitions is an act of love. It strikes us as something from a past, romantic era – hardly 5 years ago – when RAM memory was still expensive, when bringing home a 1 Ghz computer was possible only for rich students, and when high-speed Internet access was only a dream. If there is a class of people that knows how to use computers and browse the Internet, it is undoubtedly college students. The future of academia lies within the Internet. The possibilities are endless:simulations, virtual rooms with virtual three-dimensional reconstructions,pages and pages of illustrations, and almost infinite cross-hyperlinks between different sources. We are sure that publishers have the same vision of gathering academic data in places where everyone, regardless of geographic situation, can visit and use it at leisure.

Indeed, Principles of Development can be visited at the Oxford University Press website, where some of the text and all of the book's pictures have been made available to the public. We welcome this initiative as well as Lewis Wolpert's effort in bringing this unique updated version of the core of development to the biology classroom.