rav65819_ch53_1087-1114

rav65819_ch53_1087-1114 - ; chapter 53 i ntroduction Animal...

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chapter 53 introduction Animal Development
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SEXUAL REPRODUCTION in all but a few animals unites two haploid gametes to form a single diploid cell called a zygote. The zygote develops by a process of cell division and differentiation into a complex multicellular organism, composed of many different tissues and organs, as the picture illustrates. At the same time, a group of cells that constitute the germ line are set aside to enable the developing organism to engage in sexual reproduction as an adult. In this chapter, we focus on the stages that all coelomate animals pass through during embryogenesis: fertilization, cleavage, gastrulation, and organogenesis (table 53.1). Development is a dynamic process, and so the boundaries between these stages are somewhat artificial. Although differences can be found in the details, developmental genes and cellular pathways have been greatly conserved, and they create similar structures in different organisms. concept outline 53.1 Fertilization A sperm must penetrate to the plasma membrane of the egg for membrane fusion to occur Membrane fusion activates the egg The fusion of nuclei restores the diploid state 53.2 Cleavage and the Blastula Stage The blastula is a hollow mass of cells Cleavage patterns are highly diverse and distinctive Blastomeres may or may not be committed to developmental paths 53.3 Gastrulation Gastrulation produces the three germ layers Gastrulation patterns also vary according to the amount of yolk Extraembryonic membranes are an adaptation to life on dry land 53.4 Organogenesis Changes in gene expression lead to cell determination Development of selected systems in Drosophila melanogaster illustrates organogenesis In vertebrates, organogenesis begins with neurulation and somitogenesis Migratory neural crest cells differentiate into many cell types Neural crest derivatives are important in vertebrate evolution
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53.5 Vertebrate Axis Formation The Spemann organizer determines dorsal–ventral axis Maternally encoded dorsal determinants activate Wnt signaling Signaling molecules from the Spemann organizer inhibit ventral development Evidence indicates that organizers are present in all vertebrates Induction can be primary or secondary 53.6 Human Development During the rst trimester, the zygote undergoes rapid development and differentiation f During the second trimester, the basic body plan develops further During the third trimester, organs mature to the point at which the baby can survive outside the womb Critical changes in hormones bring on birth Nursing of young is a distinguishing feature of mammals Postnatal development in humans continues for years 1087 rav65819_ch53_1087-1114.indd 1087 rav65819_ch53_1087-1114.indd 1087 12/7/06 4:36:08 PM 12/7/06 4:36:08 PM 53.1 Fertilization TA B L E 53.1 Stages of Animal Development (Using a Mammal as an Example) In all sexually reproducing animals, the first step in development is the union of male and
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rav65819_ch53_1087-1114 - ; chapter 53 i ntroduction Animal...

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