Chapter 47: Animal Development Lauren Bookstaver Block: F

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

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”

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

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)

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)

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

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

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

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

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

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

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

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