Chapter 47: Animal Development

1. Chapter 47:Animal Development Lauren Bookstaver Block: F

2. Overview • Development determined by zygote genome & distribution of maternal substance (cytoplasmic determinants) in egg • Location of cells affects early development • Selective gene expression leads to cell differentiation, the specialization of cells in structure & function • Morphogenesis produces body shape

3. Overview (continued):MAJOR CONCEPTS • “After fertilization, embryonic development proceeds through cleavage, gastrulation, & organogenesis” • “Morphogenesis in animals involves specific changes in cell shape, position, & adhesion” • “The development fate of cells depends on their history & on inductive signals”

4. Fertilization • Activates egg & brings together nuclei of sperm & egg, triggering embryonic development • Combines haploid sets of chromosomes • Acrosomal reaction releases hydrolytic enzymes that digest material around egg • Gamete contact/fusion depolarizes egg cell membrane & sets up fast block to polyspermy • Membrane fusion initiates cortical reaction, where rise in calcium ions stimulates cortical granules to release contents outside egg • Forms fertilization envelope that functions as slow block to polyspermy

5. Cleavage • Rapid cell divisions where embryo becomes partitioned into many small cells (blastomeres) • Creates multicellular ball (blastula), which contains fluid-filled cavity (blastocoel) • Meroblastic cleavage- incomplete division of egg in animals w/ yolk-rich eggs (i.e. birds & reptiles) • Produces cap of cells (blastoderm) on top of large, undivided yolk • Holoblastic cleavage- complete division of eggs w/ little amounts of yolk (i.e. mammals, frogs, sea urchins)

6. Gastrulation • Changes in cell motility, shape, & adhesion, resulting in 3-layered embryo (gastrula) • 3 embryonic germ layers: outer ectoderm, middle mesoderm, & inner endoderm lining embryonic digestive tract • Has digestive cavity (archenteron)

7. Organogenesis • Organs develop from embryonic germ layers • Vertebrates: formation of notochord from condensation of dorsal mesoderm, neural tube from folding of ectodermal neural plate (will develop central nervous system), & coelom from splitting of lateral mesoderm

8. Developmental Adaptations of Amniotes • Amniotes (i.e. reptiles & mammals) create aqueous environment necessary for embryonic development • Develop in fluid-filled sac contained in shell or uterus • Germ layers give rise to 4 extraembryonic membranes: amnion, chorion, yolk sac, & allantois

9. Mammalian Development • Fertilization occurs in oviduct, & embryonic development begins on way to uterus • Eggs small & store few nutrients • Cleavage is holoblastic & show no obvious polarity • Gastrulation & organogenesis similar to birds & reptiles • Blastocyst implants uterus after fertilization & cleavage in oviduct • Trophoblast initiates formation of fetal portion of placenta & embryo develops from single layer of cell (epiblast) in blastocyst • 4 extraembryonic membranes homologous to birds & reptiles

10. Cytoskeleton, Cell Motility, & Convergent Extension • Changes in cell shape & position involve reorganization of cytoskeleton • Convergent extension- cells of tissue layer crawl between each other, causing sheet of cells to become narrower & longer

11. Roles of Extracellular Matrix & Cell Adhesion Molecules • Fibers of ECM provide anchorage for crawling cells & tracts that direct movement of migrating cells • Cell adhesion molecules help regulate movement & tissue building by holding cells together • Cadherins are important adhesion molecules

12. Fate Mapping • Developed by Vogt, fate maps of embryos have shown that specific regions of zygote or blastula develop into specific parts of older embryos • “Founder cells” identified that will generate specific tissues • Cell’s developmental potential restricts as development proceeds

13. Establishing Cellular Asymmetries • In nonamniotic vertebrates, major axes established in egg or at fertilization • Unevenly distributed cytoplasmic determinants in egg important in formation of body axes & differences of blastomeres • In amniotes, polarity not established until later • Environment differences play influence initial differences between cells & later body axes • As embryonic development continues, potency of cells becomes limited

14. Cell Fate Determination & pattern Formation by Inductive Signals • Cells influence development of other cells w/ signals that switch on or off sets of genes • Cells & offspring differ based on positional info- molecular cues indicating position • Info in form of signal molecules in “organizer” regions of embryo • (i.e.) Dorsal lip of blastopore in amphibian gastrula (shown above), AER & ZPA of vertebrate limb bud • Influence gene expression in cells that receive them, leading to differentiation