Vertebrate patterning: how to cross-stitch an embryo

I doubt the dancer Margot Fonteyn was ever a student of developmental biology; however, if she was, she would have appreciated that vertebrate embryonic development is a truly mesmerizing and beautiful process.

In Patterning in Vertebrate Development, Cheryl Tickle has compiled a comprehensive account, contributed by well-known developmental biologists, that provides a fascinating insight into the complexity of vertebrate development and the consequences of patterning gone awry. Each chapter focuses on several species and emphasizes the recent genetic discoveries that underpin the molecular basis of vertebrate patterning. Patterning is the mechanism that coordinates cell position, proliferation and differentiation during embryogenesis, and is mediated by cell signaling interactions, of which morphogen gradients are of key importance. This complex mechanism ensures that characteristic features such as the head, limbs,nervous system, organs, and even individual cartilage and bone elements, such as the vertebrae, develop in the correct location with the appropriate size and shape.FIG1 

One of the major questions in developmental biology is how cells acquire positional information. It is therefore very fitting that the opening chapter of the book introduces us to morphogen gradients and the concept that cells acquire positional information based on their abilities to respond to differential levels or thresholds of a signaling gradient. This provides the perfect introduction to the chapters that follow, which can be broadly categorized into three themes: (1) body axis and mesoderm patterning; (2)central and peripheral nervous system development; and (3) limb and evolutionary development.

What is clear in each section is the underlying importance of morphogen gradients and the reiteration of similar signaling pathways in quite distinct developmental processes. For example, the second chapter describes establishing the vertebrate body plan and highlights the fact that despite the significantly distinct geometry and morphology that exists between frogs,fish, mice and birds, the determination of polarity and the process of gastrulation are remarkably similar. It was Lewis Wolpert who famously and appropriately said, `it is not birth, marriage, or death, but gastrulation,which is truly the most important time of your life'. To any embryo, the failure to gastrulate properly is terminal. Following on from gastrulation,the mesoderm theme continues with the exploration of somite and axial development, and in particular discusses how a genetic oscillator patterns unsegmented mesoderm into somite blocks. We also learn how somites differentiate to give rise to a reiterated pattern of vertebrae and muscles and how these are specified in an anteroposterior fashion primarily by the Hox genes.

By far the largest section of the book describes the patterning of the nervous system. The importance of a coordinated integration of positional information is no better illustrated than in the complex development of the adult central and peripheral nervous systems. Beginning with a chapter on neural competence and neural plate induction, we then discover how the neural plate becomes regionalized both anterioposteriorly and dorsoventrally in response to local signaling gradients. This is augmented by a discussion of the mechanisms that control axon guidance, which ultimately provides the essential scaffolding upon which the elaborate cytoarchitecture of the brain and spinal cord are superimposed.

The remaining one-third of the book, which I particularly enjoyed, covers the migration and patterning of neural crest cells, as well as limb patterning, and is rounded off by a final chapter that describes the importance of these tissues in vertebrate evolution. Neural crest cells give rise to an enormous number of cell types, tissues and organs during vertebrate development, and are synonymous with craniofacial evolution and the transition from a sessile to a predatory lifestyle. Similarly, the formation of paired fins and tetrapod limbs helped to usher the vertebrate invasion of land, and we are beginning to understand the genetic basis of the distinction between forelimbs and hindlimbs. Fittingly, the final chapter on the evolution of vertebrate patterning describes these major evolutionary transitions and reinforces many of the ideas that arise in the preceding chapters on neural crest cell and limb development. This chapter also touches on distant extant vertebrate relatives, such as amphioxus, lampreys and ascidians, all of which have contributed enormously to the understanding of vertebrate evolution through studies of comparative genetics and anatomy.

The trouble with any book these days is that the field of developmental biology is moving so rapidly that many ideas become outdated or surpassed very quickly. For example, there appears to be increasing evidence that a link might exist between the oscillator or somite-clock mechanism and Hox gene patterning, which might facilitate the coordination of body plan segmentation with anteroposterior patterning. This recently emerged topic is discussed only briefly in this book. Similarly, the field of neural crest cell and craniofacial development has hotly debated whether cranial neural crest cells act cell autonomously or are plastic and responsive to the environment; the current consensus is that both mechanisms play important roles. This debate is also only briefly touched upon in this book, as the neural crest chapter mainly covers the development of the trunk neural crest. Overall, however,this book provides a concise yet comprehensive insight into the major events that occur during vertebrate embryonic patterning and should prove to be a valuable resource for researchers and the more advanced students of developmental biology.