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Chapter 47: Animal Development

Chapter 47: Animal Development. Theories of embryonic development Preformation Belief that egg or sperm contain a miniature embryo/adult form Epigenesis Embryo develops gradually from an egg Developmental plan influenced by Zygote genome Maternal mRNA Cytoplasmic determinants

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Chapter 47: Animal Development

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  1. Chapter 47: Animal Development Theories of embryonic development Preformation Belief that egg or sperm contain a miniature embryo/adult form Epigenesis Embryo develops gradually from an egg Developmental plan influenced by Zygote genome Maternal mRNA Cytoplasmic determinants Cell signaling influences gene expression

  2. Fertilization • Activates the egg • brings nuclei of egg & sperm together begins embryo metabolic reactions • Acrosomal Reaction • Acrosome at sperm tip releases hydrolytic enzymes which digest egg jelly coat/vitelline membrane • Ion channels open in egg membraneNa+ flows in depolarizing membrane prevents other sperm from fusing with egg (fast block polyspermy) • Cortical Reaction • Fusion of egg & sperm stimulates a series of changes in egg cortex • Formation of fertilization envelope to function as a slow-block to polyspermy

  3. Activation of the egg • Sharp rise in Ca2+ in egg causes: • Increased cell respiration & protein synthesis • Sperm & egg nuclei to fuse • DNA replication & first cell division • 90 minutes in sea urchins & frogs • 12-36 hours in mammals • Can be stimulated to occur without sperm • Fertilization in mammals • Fertilization is internal (fallopian tubes) • Secretions of female tract enhance sperm motility • Sperm migrates into zona pellucida (3D matrix of egg) • Sperm binds to & depolarizes egg to prevent polyspermy • Diploid nuclei form after 1st cell division (12-36 hours after sperm bind)

  4. Cleavage • Partitions the zygote into smaller cells • Rapid succession of mitotic cell divisions without G1 & G2 • First 5-7 divisions form cluster of cells= morula • Fluid cavity blastocoel forms in morula • Hollow ball of cells with fluid within= blastula • Polarity & planes • Vegetal pole • high yolk concentration • forms at posterior end of embryo • Animal pole • low concentration of yolk • forms the anterior end of embryo • Gray crescent • near equator • Types of cleavage • Meroblastic- incomplete division; found in yolk-rich eggs • Holoblastic- complete division; found in eggs with little or moderate amounts of yolk

  5. Gastrulation • Rearranges the cells in the blastula • forms a 3 layered embryo with a primitive gut (archenteron) • Invagination distributes the cells into layers • open end of archenteron= blastopore (future anus) • Germ layers • Ectoderm • outermost layer • forms nervous system, skin, hair, nails, eye lens • Endoderm • inner layer • gives rise to digestive lining, pancreas, liver, thyroid, lungs, bladder • Mesoderm • middle layer • gives rise to muscles, skeleton, gonads, excretory & circulatory systems • 3 layer embryo= gastrula Organogenesis • Formation of organs from germ layers • Localized morphological changes in tissue & cell shape • Neural tube & notochord first to appear & elongate in chordate embryo • Blocks of somites from which vertebrae & muscles will form appear along the notochord

  6. Amniotes • Embryos develop in a fluid-filled sac within a shell or uterus • Formation of extraembryonic membranes (life support system for embryo); occurs simultaneously with gastrulation • Amnion • fluid protecting the embryo from temperature & movement shocks • Chorion • gas exchange • Allantois • gas exchange & metabolic waste storage • incorporated into umbilical cord • Yolk sac • blood vessels • carry food to the embryo

  7. Avian development • Yolk contains food supply • Blastodisc= embryo forming portion of egg on upper surface • Primitive streak- longitudinally thickens along blastodisc • Extraembryonic membranes • Mammalian development • Fertilization & cleavage occurs in oviducts • Blastocyst reaches uterus & implants at day 7 • Extra-embryonic membranes • Organogenesis begins with formation of neural tube, notochord, & somites

  8. Morphogenesis • Involves specific changes in cell shape, position, & adhesion; reorganization of the cytoskeleton • Morphogenetic movements partially guided by extra-cellular matrix • Cell Adhesion Molecules (CAM) contribute to selective association of cells with one another • Cytoplasmic determinants help establish body axes & differences among cells of the early embryo • Inductive signals drive differentiation & pattern formation in vertebrates • The “organizer” influences & induces the development of other cells • Inducers turn some genes on & other off

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