`There is an intangible something about the process of gastrulation that invariably generates enormous awe and curiosity', writes Claudio Stern in the Preface to an excellent and timely book that he has edited, titled Gastrulation: From Cells to Embryo, and that has been recently published by Cold Spring Harbor Laboratory Press. Indeed, it is the process of gastrulation that transforms an unimpressive, amorphous heap of cells that makes up each animal at the blastula stage of development, into a gastrula in which three germ layers, the endoderm, mesoderm and ectoderm, are shaped into a body plan that is characteristic of each systematic group. It is also during gastrulation that largely pluripotent embryonic cells become initially biased,then specified and finally committed to form specialized cell types such as muscles, blood and neurons. Thus, a two-dimensional map of tissues and organs in the blastula is translated through dramatic gastrulation movements into a three-dimensional configuration. In this triploblastic structure, the future skin and neural tissues reside in the most superficial ectoderm, the prospective alimentary structures form from the most internal endoderm layer,and sandwiched between these two is the mesodermal layer, which will give rise to the cardiovascular organs, musculature and bones.
Gastrulation is accomplished by the concurrent and concerted actions of four evolutionarily conserved morphogenetic movements. Epibolic movements result in the expansion and thinning of tissues; internalization movements(emboly) bring future mesodermal and endodermal germ layers beneath the ectoderm; and, lastly, the nascent germ layers narrow mediolaterally through convergence movements, while extension movements elongate them from head to tail. This dynamic and complex combination of cell movements, rearrangements and inductive events during gastrulation is so intriguing, almost magical,that many students, upon encountering it, become hooked forever.
The same features of gastrulation that make it an object of fascination also make it a challenging subject of study. Whereas in the past two decades embryologists, geneticists and molecular biologists made large strides in our understanding of the inductive processes that specify embryonic polarity, germ layers and their patterning, the morphogenetic aspects of gastrulation proved to be less tractable. Consequently, only a few books that focus on gastrulation have been published in recent decades(Keller et al., 1991; Stern and Ingham, 1992).
In recent years, however, new imaging and cell tracing technologies,fluorescent fusion proteins, as well as new molecular and genetic approaches in many model systems, have begun to unveil the mysteries of gastrulation. Many molecular pathways have been implicated in this process, and in some cases have been linked to individual gastrulation movements, or even to specific cell behaviors. This fast rate of discovery in gastrulation research is likely to continue. Although several reviews have been published that focus on individual model systems, or on the roles of specific pathways in gastrulation, a comprehensive and current view of gastrulation has been missing. For the gastrulation enthusiast, hungry for a good and modern source of information, the new book edited by Claudio Stern is simply a feast. It is the most extensive treatment of gastrulation yet attempted in book form. The editor has collected together an impressive constellation of experts in the field to write for the book, who have contributed 54 chapters discussing individual aspects of gastrulation. It is carefully crafted with the first chapter placing the study of gastrulation into a historical context. A short glossary at the beginning of the book will also help uninitiated readers to appreciate the intricacies of gastrulation. Chapters are written in an accessible style and include effective illustrations.
Gastrulation is a truly organismal phenomenon, whereby the entire embryo changes its architecture and shape through the concerted movements of individual cells, cell populations and cellular sheets. These morphogenetic cell behaviors are precisely regulated in time and space, and are coordinated with cell fate specification events. The specification of both cell movement behavior and fate likely reflect the current position of a cell in the embryo,as well as its developmental history. Therefore, the complete set of instructions on how to construct an embryo during gastrulation from its cellular constituents must be encoded in the genome. The book takes this reductionistic approach to gastrulation, guiding the reader from the embryonic through cellular level, and finally to the molecular genetic mechanisms of gastrulation. In the first part, `The Embryology of Gastrulation', we learn about gastrulation movements in different animals, from simple sponges to humans. A real strength of this book is that it is not limited to the most popular model systems; it also features chapters that describe gastrulation movements in less well-studied organisms, such as crustaceans, ctenophores,mollusks, dogfish, reptiles and the rabbit. On the one hand, these chapters illustrate the wonderful diversity of gastrulation movements among metazoans,and on the other, they reveal that the basic types of morphogenetic movements of epiboly, internalization, convergence and extension are universally, albeit somewhat differently, employed by these distinct organisms.
Getting down to the cellular level, Part II considers the morphogenetic cell behaviors that execute gastrulation, including how they are patterned in time and space. The reader will enjoy exploring various mechanisms of mesendoderm internalization, such as invagination, involution and ingression,or their intermediates such as `synchronized ingression', which is proposed to occur in teleost gastrulae. Gastrulation aficionados will appreciate delving into more complex issues. For example, how local morphogenetic cell behaviors,such as intercalations that are uniform across a cellular field, might result in different rates of movement in distinct regions of the field. A few chapters consider the role of chemotaxis and the extracellular matrix in orchestrating gastrulation cell behaviors. In this part of the book, one can also learn about the inductive and patterning processes through which embryos gear up for the morphogenetic movements of gastrulation. After all,gastrulation movements make embryonic asymmetries morphologically apparent,but these asymmetries are established before the morphogenetic movements are initiated. Several chapters provide overviews of the early symmetry breaking events in distinct embryos, such as frog and mammals. The next series of chapters guide the reader through processes that specify and then pattern the mesodermal, endodermal and ectodermal germ layers. The concepts of morphogens and their specific deployment in patterning the nascent mesodermal and neuro-ectodermal tissues are discussed. I especially enjoyed reading these chapters, as they do not focus on any particular animal model, but rather take a comparative approach. Of note here are the chapters discussing endoderm development and the specification of the left/right axis, two areas that have experienced particularly fast progress in recent years.
But it is Part III, concerning `The Molecular Biology of Gastrulation',that makes this book qualitatively different from its predecessors published a decade ago. Whereas only a handful of pathways and molecules were linked to inductive or morphogenetic events of gastrulation at that time, now almost 20 chapters of this book are devoted to either the different signaling pathways(all the usual suspects from Nodal, through FGFs and the canonical and noncanonical WNTs, to BMPs and Notch), the array of transcription factors, or the extracellular matrix components involved in gastrulation. This part of the book illustrates the rapid progress in our understanding of gastrulation, but it also articulates many outstanding questions and future experimental priorities. For example, we learn that among the genes implicated in gastrulation, those that impact primarily on embryonic pattern and cell fates predominate over those regulating only morphogenetic cellular behaviors. Therefore, the question remains as to the relationship between genes regulating cellular fates and those regulating movements during gastrulation. However, few answers can be offered at the moment. Another conclusion emerging from this part of the book is that with increasing knowledge of the molecular players in gastrulation comes somewhat daunting complexity. It becomes clear that a gastrulating cell needs to integrate its positional information,developmental history and a multitude of movement cues from several sources. The chapter `System-level Properties Revealed by a Gene Regulatory Network Analysis of Pre-gastrular Specification in Sea Urchins' offers a view of how such complex molecular information can be harnessed.
Part IV looks at gastrulation from the evolutionary perspective, exploring fossil embryos, as well as taking a comparative approach to cellular and molecular aspects of gastrulation in modern animals. Chapters in this part of the book illustrate well the symbiotic relationship between the studies of evolution and gastrulation; we learn about the evolution of animals by studying their development and vice versa. The final chapter of the book is contributed by Lewis Wolpert, a long-standing proponent of the importance of gastrulation over the less exciting events in life such as birth, death or even marriage. He considers the progress achieved in the field, as well as the challenges ahead, concluding that many of the mysteries of gastrulation are still to be uncovered.
The limitations of the book are few and largely unavoidable. The rate of progress in the area of gastrulation research is so rapid that, even in a book still hot off the press, important new regulators of gastrulation such as STAT3 and its downstream effector LIV1 are not discussed. Whereas there was an effort to maintain a uniform color scheme for germ layers and specific tissues in the book's illustrations, this could be more consistent. Moreover, some colors must have been misprinted during the printing process, with yellow looking rather greenish in some chapters. These are, however, only minor issues in this otherwise outstanding book.
Gastrulation is a dynamic process and nothing illustrates it so well as movies. The book is associated with a modest but useful web site(www.gastrulation.org)where time-lapse movies of live embryos, or schematic visualizations provide an excellent addition to the individual chapters. This website will likely grow with new editions of the book, or possibly take on a life of its own,with new materials being added as they become available.
In conclusion, I enthusiastically recommend this book, and expect it will be highly valuable to students, teachers and researchers. The book provides a comprehensive, authoritative and modern view of our knowledge of gastrulation,and captures the excitement in this rapidly developing field. I imagine that many young readers will become intrigued by gastrulation and at least a few will become hooked by it forever after reading this book.