Lecture 7+8 Rapid Specification Snails and Nematodes.pdf

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Cell and Systems Biology
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William Navarre

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CSB328H1 © Lisa Zhao 20| Page 1 L E C T U R E 7 & 8 : R A P I D S P E C I F I C A T I O N S N A I L S & N E M A T O D E S Reading: Chapter 5  eukaryotic organism – cell contains a nucleus & chromosomes that undergo mitosis  multicellular eukaryotic organism – cells from mitosis remain together & function as a whole  metazoan – animal that undergoes gastrulation (Fig 5.1)  all animals gastrulate & nothing gastrulates that isn’t an animal  35 metazoan phyla = 35 patterns of animal dvpmt Metazoan Divisions Defining traits Germ layers Ectoderm Endoderm Mesoderm archeocyte (a somatic cell) can differentiate into no Sponges all cell types yes yes (no organs, indiv cells can form new organisms, cells are species-specific circulatory system, nerves, muscles) Ctenophores ex. jellyfish radial symmetry yes yes no Cnidarians Ecdysozoans molt schizocoelous gastrulate ex. arthropods,nematode exoskeleton mouth formation of the forms Protosomes spiral first coelom (body cavity) cleavage, from the forms out ofpreviously Triploblasts Lophotrochozoans trochopore blastopore solid cord of mesodermal aka cells yes yes yes Bilaterians (larval form) 3 germ layers have notochord & anus enterocoelous Chordates pharyngeal forms formation of the Deuterostomes arches first body cavity from the from mesodermal pouches Echinoderms blastopore extending fromthe gut Page 2|CSB328H1 © Lisa Zhao2013 EARLY DEVELOPMENTAL PROCESSES CLEAVAGE  cleavage – mitotic divisions, egg cytoplasm divided into smaller nucleated cells  cleavage-stage cells = blastomeres blastomere: a cell derived from cleavage blastula: embryonic stage composed of blastomeres blastocyst: mammalian blastula blastocoel: cavity in the blastula blastopore: invagination where gastrulation begins  rate of cell division & placement of cells under control of oocyte (maternal) factors (w exception to mammals)  cytoplasmic volume doesn’t increase  rapid cleavage – eliminate gap phases (G1, G2) Cell Cycle 1. biphasic cell cycle of early blastomeres (Fig 5.2A)  driven by gain & loss of mitosis-promoting factor (MPF)  MPF initiates entry into mitotic (M) phase – breakdown of nuclear envelope, DNA condensed into chromosomes  MPF degradation initiates entry into DNA synthesis (S) phase  MPF consists of a large subunit, cyclin B – accumulated during S & degraded after the cells reach M  cyclin B regulates the small subunit, cyclin-dependent kinase (CDK/Cdc2)  CDK activates mitosis – phosphorylates histones, nuclear envelope lamin proteins  presence of cyclin B required for CDK function  presence of cyclin B is first controlled by regulators found in the egg cytoplasm – produces rapid & synchronous cell divisions  when the cytoplasmic components are used up, the nucleus begins to synthesize regulators 2. embryo enters a mid-blastula transition (MBT)  zygotic genome is activated  gap stages are added to the cell cycle (Fig 5.2B)  not synchronous – diff cells synthesize diff regulators of MPF  transcription of mRNAs for gastrulation & cell specification CSB328H1 © Lisa| Page 313 Patterns of Embryonic Cleavage  diff patterns of embryonic cleavage are determined by 2 factors: (Fig 5.4) 1. amount & distribution of yolk protein in the cytoplasm 2. mitotic spindle orientation & timing of formation  yolk determines the location of cleavage & size of blastomeres  yolk inhibits cleavage  vegetal pole – yolk-rich pole  animal pole – yolk-free, fast cell divisions, location of nucleus  isolecithal eggs (sea urchin, mammal, snail) have sparse yolk  embryos must have another way of obtaining food ex. voracious larval form, maternal placenta  holoblastic cleavage – cleavage furrow extends through entire egg  insect, fish, reptile, & bird eggs have yolk sufficient to nourish embryo throughout dvpmt  meroblastic cleavage bc yolk platelets impede membrane formation  telolecithal eggs (fish, reptile, bird)  only one small area of the egg is free of yolk & cell divisions only occur here – discoidal cleavage  centrolecithal eggs (insect)  yolk is in centre & divisions of cytoplasm occur only in the rim around the periphery of the cell – superficial cleavage  4 major types of holoblastic cleavage: 1. radial 2. spiral – snails 3. bilateral 4. rotational – nematodes Page 4|CSB328H1 © Lisa Zhao2013 GASTRULATION & AXIS FORMATION  the 3 germ layers are produced during gastrulation  during gastrulation, blastomeres move to new positions & acquire diff neighbours  gastrulation proceeds by a combination of:  embryos must develop 3 axes: (Fig 5.5) 1. anterior-posterior (anteroposterior) axis 2. dorsal-ventral (dorsoventral) axis 3. right-left axis  snails & C. elegans are characterized by Type I embryogenesis:  immediate activation of zygotic genes  rapid specification of the blastomeres by the products of zygotic & maternal genes  small number of cells at the start of gastrulation EARLY DEVELOPMENT IN SNAILS CLEAVAGE IN SNAIL EMBRYOS  annelid worms, polyhelminth flatworms, molluscs  spiral holoblastic cleavage – cleavage at oblique angles  produces stereoblastulae (blastulae) w NO blastocoel  first 4 blastomeres (macromeres) are diff sizes (D largest, Fig 5.6)  each macromere buds off a small micromere in each cleavage  3 cleavage: A  macromere 1A & micromere 1a (right) th 1 2  4 cleavage: 1A  macromere 2A & micromere 2a (left) & 1a  micromeres 1a & 1a (left)  spindles appear to alternate clockwise & counterclockwise, causing alternate micromeres to form obliquely to the L & R of their macromeres CSB328H1 © Lisa| Page 513  fates of the cells are specified by cytoplasmic localization & induction:  first-quartet of micromeres  head structures  second-quartet of micromeres  statocyst (balance organ) & shell  cytoplasmic factors of the oocyte control the orientation of the cleavage plane to the L/R  dextral coiling – coils opening to the right of their shells  sinistral coiling – coils opening to the left  direction is usu species-dependent – mutationd can arise  orientation of the cells are diff after the 2 cleavage – diff position of the 4d blastomere (mesentoblast – heart & larval muscle [mesoderm], gut tube [endoderm])  right-coiling allele is dominant to the left-coiling allele  direction of cleavage is determined by the genotype of the mother (a dd female snail can only produce sinistrally coiling offspring even if the offspring’s genotype is Dd) Autonomous Cell Specification & the Polar Lobe  autonomous dvpmt – loss of an early blastomere causes loss of an entire structure  morphogenetic determinants responsible for certain organs are very localized 1. mRNAs for some transcription factors & paracrine factors are placed in particular cells by associating them w certain centrosomes (Fig 5.10)  the 3’ tail (untranslated regions [UTRs]) of mRNAs associate w specific centrosomes  cells of the same micromere tier share mRNAs w similar 3’ UTRs  mRNA associates w the centrosomic region that will generate the micromere tier & becomes localized to particular blastomeres by the 24-cell stage (Fig 5.11A)  ex. association of decapentaplegic (dpp) w specific centrosomes (Fig 5.10)  (A) no Dpp accumulation  (B) 4- to 8-cell stage: dpp mRNA (black) accumula tes at one centrosome of the pair forming the mitotic spindle  (C) dpp mRNA attends the centrosome in the macromere (not micromere) 2. patterning molecules are bound to a certain region of the egg that will form the polar lobe  the polar lobstis an extrusion of cytoplasm formed immediately before the 1 cleavage (Fig 5.12)  it is connected only to the CD blastomere  looks like another cell – three-lobed structure is called the trefoil stage embryo  Crampton (1896) – polar lobe required for normal dvpmt Page 6|CSB328H1 © Lisa Zhao2013  removal of polar lobe at trefoil stage  cells divide normally, but larva is incomplete (missing endoderm & mesodermal organs)  same abnormal larva produced by removing D blastomere  thus, the polar lobe contains the endodermal & mesodermal determinants which give the D blastomere its endomesoderm-forming capacity  Van den Biggelarr (1977) – morphogenetic determinants in the polar lobe are located in the cytoplasm  removal of polar lobe cytoplasm – normal embryo  addition of polar lobe cytoplasm to B blastomere – no duplication in structures  Clement (1962) – D-quadrant macromeres are involved in inducing other fates of other cells  D blastomere receives polar lobe contents  removal of D, 1D, or 2D cells, or polar lobe – incomplete larva  D blastomere doesn’t directly contribute cells to the missing structures (heart, eyes, foot, intestine, velum, shell gland)  removal of 3D shortly after the division of 2D into 3D & 3d – incomplete larva like when D, 1D, or 2D removed  removal of 3D at a later time – almost normal larva, no heart or intestine  removal of 4D after the division of 3D into 4D & 4d – normal dvpmt  all essential determinants for heart & intestine formation are in 4d (mesentoblast)  the mesodermal & endodermal determinants of the 3D macromere are transferred to the 4d blastomere  the inductive ability of the 3D blastomere (for eyes & shell gland) is needed during the time 3D is formed but not afterwards  3D activates the MAP kinase signalling pathway in the ectodermal (eye- & shell-gland forming) micromeres above it  after 3D divides into 4D & 4d, MAP kinase activity persists only in 4d to produce the heart  morphogenetic determinants specific in the 4d blastomere (mesentoblast):  β-catenin – mediate autonomous specification  Nanos mRNA & protein – specification of germ cell progenitors  importance of the non-diffusible polar lobe cytoplasm that is localized to the D blastomere:  contains the determinants for the proper cleavage rhythm & cleavage orientation of the D blastomere  contains determinants (those entering the 4d blastomer
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