11 Cytoskeleton in Teleost Eggs and Early Embryos: Contributions to Cytoarchitecture and Motile Events

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Successful interaction between gametes initiates a cascade of pathways that lead to cleavage, cellular differentiation, morphogenesis, and eventual development of an adult organism. Shortly after fertilization of the teleost egg, the egg cortex undergoes a predictable sequence of structural changes in response to intracellular signals. Early transformation of the cortex includes formation of the fertilization cone and second polar body, sperm entry into the egg, cortical granule exocytosis, and surface membrane retrieval by endocytosis. Subsequently, cytoplasm moves toward the animal pole where it accumulates in preparation for cell division and cellular rearrangement. By analogy to other cell systems that show secretion, mitosis, intracellular cytoplasmic flow, and cell migration, it is predicted that similar events in eggs are mediated by action of the cytoskeleton. Most of the knowledge about the egg cytoskeleton has been obtained from analyses of sea urchin, amphibian, and mammalian eggs. By comparison, the composition, organization, and function of the egg cytoskeleton in teleost fishes have received considerably less attention. The eggs of teleost fish have long provided favorable material for the study of vertebrate development. Unlike mammalian and amphibian eggs, teleost eggs can be obtained in large numbers, are optically transparent, and are easily accessible for experimental manipulation. Furthermore, teleosts now also offer the possibility of employing genetic and molecular approaches to investigate the role of the cytoskeleton in early development. This chapter surveys the teleost egg cytoskeleton and evaluates its proposed role(s) in events of fertilization, ooplasmic segregation, and gastrulation.