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Advances in the development and application of electron microscopic techniques have occurred recently such that the electron microscope has evolved to become an essential tool in both basic and clinical research. Use of this instrument has contributed significantly to the formation of new perspectives and concepts concerning cell fine structure. These structural perspectives are now being integrated with specific functional, biochemical and pathophysiological events and processes of cells and tissues. Most recently, utilization of innovative electron microscopic techniques such as freeze-fracture, freeze­ etching, and scanning and high-voltage electron microscopy offers both the basic and clinical scientist potentially fundamental insights into many morphodynamic processes related to the activities of cells and tissues. Such an approach has been especially rewarding when applied to the dynamic events of gametogenesis and early embryonic development. The chapters comprising this book have been selected and edited with the aim of providing an up-to-date and comprehensive account of the most important aspects of vertebrate gamets and embryos as revealed by the integration of several different submicroscopic methods. The organization of the chapters is designed to indicate present gaps in our knowledge of the developmental and reproductive biology of gametes and the developing embryo and possible Iines of research which may lead to a lessening of these gaps.

Externally the vertebrate body plan presents a bilateral symmetry in relation to the midline. However, inside the body the distribution of the visceral organs follows a very particular pattern that is not symmetrical in relation to the midline. The last 10 years have seen remarkable advances in our understanding of how the internal asymmetries typical of the vertebrate body are established and controlled. The use of different development models has permitted to uncover fascinating ways of creating asymmetry, like the activity of the nodal cilia. A host of studies has also unravelled the involvement of many genes in the left right patterning pathway. Based on this knowledge the genetic basis of human laterality defects are beginning to be revealed. It is a major challenge now to understand how all these genes control left right development as well as the complex set of interactions established between them.

Epithelial phenotype is a dynamic stage of differentiation that can be modulated during several physiological or pathological events. The rapid conversion to a mesenchymal-like phenotype is called an epithelial-mesenchymal transition (EMT). The Rise and Fall of Epithelial Phenotype is the first book to comprehensively introduce the concept of EMT. The first part of this volume describes main examples and models and explains their physiological relevance. These examples include hydra morphogenesis, gastrulation in mouse, drosophila and sea urchin, as well as neural crest cell migration and heart morphogenesis in vertebrates. Part two reviews in detail, specific EMT molecular pathways covering extracellular induction, transduction and transcription response and modulation of cell-cell adhesion structures. It emphasizes new specific pathways with potential medical applications. EMTs can also be linked to pathological events such as wound healing and cancer progression, as detailed in this section of the book.