Leon W. Browder, Carol A. Erickson and William R. Jeffery
Developmental Biology, Third Edition
1991
Saunders College Publishing, Philadelphia
ISBN 0-03-013514-1

• Contents
• Preface

PART I Introduction

PART II Gametogenesis

PART III From Sperm and Egg to Embryo

PART IV Organizing the Multicellular Embryo

PART V Genetic Regulation of Development

PART VI The Organized Generation of Cell Diversity

We are privileged to be living in a period of intense and highly productive research activity in the field of developmental biology, with exciting discoveries occurring almost daily. Because change itself is the essence of the phenomena that we study, developmental biologists by temperament revel in new discoveries. We have presented developmental biology here as ongoing and evolving, rather than as a mature discipline with established dogma. This book not only details our current understanding of development but aims to give students the tools to assist them in understanding the process of discovery itself and-most importantly (because discovery is the life blood of any science)-to help them pose questions for further investigation.

The scope of this edition of Developmental Biology has been expanded substantially to provide students with a comprehensive appreciation of the developmental process in animals, presenting more detailed descriptions of development along with a conceptual framework for understanding how development occurs. As active researchers ourselves, we believe that the key to full comprehension of any experimental science is an understanding of the experimental process itself. We discuss in depth many of the crucial experiments upon which our current level of knowledge is based. This approach gives the student an appreciation for the role of experimentation in science and a better understanding of the experimental results. Familiarity with these experiments should also facilitate the understanding of new research results introduced by instructors, help students to read and understand research papers in the literature, and, we hope, stimulate them to pose questions and design experiments about the many unanswered aspects of development.

INSTRUCTIONAL FEATURES
Essays and Research Appendix To help students comprehend the experiments that we have analyzed, this book contains 14 essays and an extensive appendix that describe the most common methodology used in contemporary developmental biology research. The appendix has been designed as a ready reference source that summarizes the powerful techniques of molecular biology, including a discussion of nucleotide complementarity, which is the key to understanding the organization of the genome and is the basis for nucleic acid technology. Other essays present supplementary material on topics such as Gene Organization and Regulation of Transcription, Growth Cone Guidance in Insects, Eukaryotic Transcription Regulatory Proteins, and DNA Methylation.

Visuals and Use of Color We believe that visual material is an important aid to the learning process and have, therefore, included numerous illustrations in the text. The entire graphic art program has been redesigned with the judicious use of color to emphasize important points and to assist in identifying key components of the drawings. In addition to the graphic art, we have included a large number of high quality photographs in both black and white and color. The extensive 24-page color insert serves two purposes: The graphic art utilizes color as a pedagogical tool to enable the reader to follow the germ layers during complex morphogenetic processes such as gastrulation. Color is also used to present research results that utilize color for analytical purposes, such as the use of fluorescent dyes to trace cells and tissues and the use of genes that encode enzymes that produce a colored product to detect the activity of that gene. The inclusion of these color figures should illustrate the power of these techniques as analytical tools. We are grateful to the many investigators and publishers who have granted permission to reproduce these figures. Complete citations for the sources of previously published figures can be found in the reference list at the end of each chapter.

Glossary This new edition also contains a comprehensive glossary. It has been designed to provide a ready reference source for the technical terms and names of organisms that are used in this book.

Biographies of Researchers We owe a debt of gratitude to those individuals who have defined the field of developmental biology, posed the questions that led to early experimentation, and designed experimental approaches to answer those questions. We salute some of these investigators with a photograph and a short biographical sketch at the beginning of each chapter. Because we believe that the literature is the foundation upon which any discipline is based, each chapter ends with an extensive reference list, which provides the student with a guide to the key sources for the material discussed in the text.

Two additional authors have increased the breadth of material that is covered in this edition and have added their unique perspectives, especially concerning embryogenesis, morphogenesis, patterning, and gene control in the early embryo, to enable us to present a comprehensive analysis of developmental biology. The order of presentation of the material has been revised in this edition to enable students to acquire their understanding of the developmental process in a logical sequence. In Chapter 1, we describe the origins of developmental biology and build upon this base as the story unfolds. We begin our analysis of development with gametogenesis. In addition to describing the formation of the gametes and the regulation of gametogenesis, we focus on the developmental legacy of the gametes.

The earliest events of development follow: fertilization, cleavage, embryonic polarization, and gastrulation. Fertilization is not only the physical union of gametes, but it sets into motion the entire developmental process. Consequently, we discuss both of these aspects of fertilization, beginning with the events and specializations that facilitate gamete fusion, followed by the cellular and biochemical responses of the egg to sperm entry. Fertilization endows the egg with the potential for rapid cell division (cleavage), which produces the cells that will later be molded into the embryo. The patterns of cell division during cleavage are tightly regulated and may have profound effects on later developmental processes; hence, we describe the different cleavage patterns in detail. The embryos of most species develop along axes of bilateral symmetry from eggs that are radially symmetrical. The events that establish those axes of symmetry are important developmental events and are discussed using two model systems: Xenopus laevis and Drosophila melanogaster. The formation of the embryonic body plan along the axes of symmetry is dependent upon a reorganization of the cells produced during cleavage by the process of gastrulation, which produces three embryonic germ layers, which form the tissue and organ rudiments.

A major distinction between this edition and its two predecessors is the two chapters (Chapters 7 and 8) detailing the formation of the embryonic rudiments, using both descriptive and experimental approaches. The elaboration by the embryo of diverse tissue and organ rudiments after gastrulation is a remarkable phenomenon that provides much of the fascination of the developmental process and enough unsolved questions to engage the interest of investigators for many years to come. We hope that these descriptions and discussions of morphogenesis will engender an understanding of the importance of these processes. Morphogenesis is a complex process that relies upon the concerted activities of the individual cells of the embryo, which utilize a variety of mechanisms to migrate over great distances, to adhere to one another, and to change shape. Our understanding of the nature of these mechanisms is advancing rapidly and is discussed in Chapter 9.

The role of the genome in development is one of the great challenges of contemporary molecular biology. Part V of this book provides the philosophical basis for understanding gene expression during development. We first describe the evidence that (with few exceptions) the genome remains intact during development of specialized cell types, without losing the information necessary for development of alternative cell types. The integrity of the genome implies that diversity is achieved not by loss of alternative genetic information but by differential use of the genetic material during development. There are two primary means for regulating differential gene expression to achieve cellular diversity during development: (1) the influence of distinct cytoplasms on nuclei in different regions of the early embryo and (2) extrinsic factors, such as inductive signals from neighboring cells. Our understanding of embryonic induction is increasing substantially with the discoveries that growth factors are important embryonic inducers. We describe the most recent findings in this exciting search for inducers, which began several decades ago and dominated embryological research for many years.

One of the most intriguing aspects of the onset of development is the orchestration of genetic control. Control is initially mediated by RNA that is synthesized during oogenesis and stored for delayed utilization during development. The zygote nucleus then assumes belated control over development. We describe this transition in Chapter 13. A hallmark of gene regulation in development is that it is subject to both chronological and spatial control, with different genes being expressed at distinct times during development and in certain regions of the embryo. Our understanding of the spatial regulation of gene expression has progressed rapidly in recent years owing to the judicious use of recombinant DNA technology and genetics. This fascinating story is presented in Chapter 14.

The ultimate goal of embryonic development is the production of diverse functional organs and tissues. The discussion of organogenesis in this edition has been expanded substantially. We have chosen three systems for detailed analysis: limb development and regeneration, gonad development and sex differentiation, and eye development. Among them, they provide a sampling of the complexities involved in coordinating the differentiation of multiple cell types to produce functional entities during development. Cell differentiation in differentiating organs and tissues is dependent upon the selective expression of cell-specific genes. Our understanding of the control over selective gene expression during cell differentiation has progressed rapidly in recent years with the application of the techniques of contemporary molecular biology. The exciting discoveries of factors that interact with gene regulatory elements have added new dimensions to the study of cell-specific gene expression: We are in a position not only to study the interactions of regulatory factors with their target genes but to study the regulation of the regulators themselves, which is a critical phase in cell differentiation. These topics are covered in Chapter 18.

Developmental biology is taught in a variety of formats. We have attempted to write a versatile book that should be appropriate for courses that emphasize cellular and molecular aspects of development as well as those that focus on descriptive embryology and morphogenesis. However, the book will be particularly useful for those courses that cover both the molecular and descriptive aspects of development. The approach that we have taken should be compatible with courses at various levels of instruction, from undergraduate to graduate.

Our objective while preparing this textbook has been to capture the spirit and flavor of this dynamic period in developmental biology while giving our readers an appreciation for the intellectual base embodied in the classical literature. We will have succeeded in this project if we have engendered in our readers a fascination and enthusiasm for studying the processes of change that typify development and have stimulated them to assist in unravelling the secrets of developmental biology.