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1 Name:______________________________________________________ Date Performed/Submitted:_________ Section/ Group Name:_________________________________________ Score:_________________________ Exercise 3 Gastrulation Introduction: During gastrulation, highly coord
  1  Name:______________________________________________________ Date Performed/Submitted:_________ Section/ Group Name:_________________________________________ Score:_________________________ Exercise 3 GastrulationIntroduction: During gastrulation , highl coordinated cell mo!ements transform the blastula  from a ball of  blastomeres  to a highl organi ed embro called a gastrula  #ith three distinct laers from #hich all organs andtissues srcinate$ %he three laers of the gastrula are collecti!el called embryonic germ layers , and include the ectoderm, mesoderm , and endoderm. &hile presumpti!e 'future( ectoderm cells remain on the exterior of thegastrula, blastomeres that #ill become mesoderm and endoderm migrate internall$ %he ne# locations of thesecells allo# tissues and organs to form in their proper positions in a process called organogenesis $ %he ectodermgi!es rise to the epidermis and components of the nervous system , the endoderm generates the epithelium of the gut and gut derivatives  'e$g$, li!er, pancreas(, and the mesoderm forms the circulatory system, muscles,kidneys, gonads, dermis, and skeletal components $ )t the end of gastrulation, the gastrula is characteri ed ban inner laer of endoderm, an outer laer of ectoderm and the middle laer of mesoderm$ *nderstanding ho#cells differentiate into the three germ laers and exhibit speciali ed mo!ements is one of the ma+or areas or research in de!elopmental biolog toda$ %his exercise explores the process of gastrulation using se!eral modelorganisms to demonstrate different gastrulation strategies$ Basic Cell Movements Gastrulation mo!ements in!ol!e the entire embro$ ell mo!ements of an area in the gastrula aresnchroni ed #ith mo!ements of other areas of the embro$ %he pattern of gastrulation can !ar amongstdifferent species but the embro exhibit similar tpes of basic cell mo!ements$  Invagination : infolding or buc-ling in#ard of a sheet of cell.analogous to a finger po-ing a balloon 'e$g$ sea urchin ectoderm($  Involution : in#ard migration of an expanding outer sheet of cellsso that it spreads o!er the basal 'internal( surface of the remainingexternal cells 'e$g$ amphibian mesoderm($  Ingression : migration of indi!idual cells into the embrosinterior 'e$g$ urchin mesoderm($  Delamination : splitting of a single sheet of cells into t#o separate but parallel sheets 'e$g$ chic- and mammalian hpoblastformation($  piboly : expansion of an outer sheet of cells b mitosis to co!er the entire embros surface. mo!ementis analogous to srup being poured o!er an orange 'e$g$ ectoderm in sea urchins, tunicates, amphibians,fish, and birds($  Convergence : migration of cells to#ard a common point such as the midline of the embro 'e$g$ cellmigration o!er the frog dorsal blastopore lip, and chic- primiti!e strea- formation($  !tension : rearrangement and elongation of a cluster of cells to form a thin, stretched cellular group$)nalogous to a group of people that form a large cro#d #hile #aiting for a bus, but mo!e in single file#hen boarding it 'e$g$ sea urchin endoderm during archenteron formation, and cell migration o!er thefrog dorsal blastopore lip $ ea #rchin Gastrulation   Sea urchins exhibit a simple tpe of gastrulation that occurs soon after the blastula hatches from the fertili$ation envelope $ )t the onset of gastrulation, primary mesenchymal cells  'deri!ed from micromeres( ingressinto the blastocoel, and later differentiate into mesoderm$ %he !egetal side of the blastula flattens into a !egetal platethat in!aginates forming the archenteron  'embronic gut, also called gastrocoel ($ %he opening of the archenteron isthe blastopore,  #hich in sea urchins is the site of the future anus $ %he archenteron elongates b con!ergent extension,reaches the inner surface of the blastocoels #all, and fuses #ith the ectoderm to form the stomodeum %mouth ($ %heanus mar-s the future dorsal side %aboral side ( and the stomodeum forms on the !entral side %oral side&.  2 %he gastrula next progressesthrough the prism  and pluteus larvalstages.  %he prism stage does not ha!elar!al arms, #hereas the pluteus hast#o pairs: a prominent long !entral pair ' anal or dorsal arms (, and ashort dorsal pair: near the mouth %oral or ventral arms ($ picules made of calcite 'a03, 1g03( pro!ide s-eletal supports for thelar!al arms$ )t the prism stage, thespicules begin to lengthen gi!ing theembro a 2prism2 shape$ urther spicule elongation enables lar!alarms to gro# out in the pluteus$ %hearchenteron differentiates into threeregions of the gut 'esophagus,stomach, intestine( #hich appearsshaped li-e 456 in the pluteus$ %he pluteus is co!ered #ith cilia #hichallo# it to s#im freel #ith its  armsfor#ard$ %he pluteus #ill later metamorphose into a small sea urchin, #hich gro#s spines, becomes sedimentar and resembles the adult form$ 'rog Gastrulation %he pigmented animal hemisphere of amphibian embros 'e$g  Rana pipiens, Xenopus laevis ( ma-es cellmo!ements during gastrulation !isible as the drasticall change the embro7s appearance$ Gastrulation begins onthe dorsal side in the gra crescent at the marginal one, an area surrounding the e8uator of the embro #heremicromeres and macromeres meet$ ) group of cells %bottle cells&  in the dorsal marginal one constrict at one end, becoming bottle9shaped and in!aginate to form theleading edge of the archenteron$ %his causes a pigment difference on the embros surface #hich becomes !isible as the dorsal lip  of the blastopore ,o!er #hich marginal one cells 'future mesoderm(in!olute$ n some frogs, the archenteron roof isinitiall lined #ith in!oluted mesoderm %chordamesoderm ( that later becomes co!ered #ithendoderm$ %he archenteron  floor is lined #ith!egetal cells that become endoderm$ %he blastocoelis displaced as the archenteron enlarges$ E!entuallthe blastopore lip #idens into a crescent forminglateral and !entral lips as additional marginal onecells in!olute$ %he dorsal lip of the blastopore  has been called the frog organi$er % pemann(sorgani$er (, since it induces cell fates and 2organi es2 cells into specific tissues$ t is analogous to the fishembryonic shield $ %he blastopore is the site of the future anus  in amphibians$ Exterior cells of the animalhemisphere proliferate and mo!e b epibol to co!er the entire embros surface #ith pigmented micromeres$ Near the end of gastrulation, the dar- bro#n embro has a small, light ello# yolk plug  'patch of endoderm cellsthat has not et been co!ered b pigmented cells, and is bounded b the lips of the blastopore($ nternali ation of the ol- plug b epibolic mo!ements of the ectoderm signals the end of gastrulation$  3 )vian Gastrulation )!ian gastrulation begins in the embros thic-ened caudal margin ' marginal $one or belt ( #here thearea pellucida and area opaca meet$ %he direction of cell mo!ements during gastrulation is sho#n in ig$ 3$;$ells migrate into the embro through the primitive streak  , a midline thic-ening of epiblast located in the caudalt#o9thirds of the embro$ *rimitive folds  'primiti!e ridges( flan- a midline depression do#n the primiti!e strea- called the primitive groove $ uture endoderm cells are initiall located in the epiblast, but later ingress throughthe primiti!e strea- to populate the h!poblast$ Prospecti!e mesoderm cells migrate through the primiti!e strea- and populate the area bet#een the epiblast and hpoblast$ ells that ingress through the primiti!e pit 'most rostralregion of primiti!e groo!e( form: <( a thic-ened area rostrall called +ensen(s node ' primitive knot (, and =( the head process  #hich becomes the anterior portion of the notochord. +ensen(s node is the avian organi$er $2%he primiti!e strea- is first !isible from >9<? hours of incubation after egg9laing$ t elongates and reaches its fulllength b <@ hours of incubation$ %he primiti!e strea- then regresses in the cranial region of the embro as futuremesoderm and endoderm cells migrate through it$ %he primiti!e strea- is !isible onl in the caudal region of the339hour embro stage 'stage <?(, and has regressed completel b A@ hours$ ig$ 3$; sho#s the cut a#a !ie# of a chic- gastrula sho#ing migration of presumpti!e mesoderm 'red(and endoderm 'ello#( cells through the primiti!e strea-$ )rro#s sho# the direction of migration from theepiblast 'blue( to the blastocoel 'gra( and hpoblast 'purple($ells remaining in the epiblast after gastrulation areectoderm$ 1igrating endoderm cells e!entuall replace the hpoblast$ Dorsal lip is up$   ig$ 3$A Sagittal section of froggastrulae: late blastula ')9B(. earlgastrula #ith dorsal lip '(. mid gastrula#ith !entral lip and ol- plug 'D(. lategastrula #ith ol- plug 'E9($  Cfertili ation en!elope=Canimal pole3 C blastocoelAC!egetal pole;Cdorsal lip of blastopore>CarchenteronCblastopore@Col- plugC!entrol lip of blastopore<?Couter laer of ectoderm<< Cinner laer of ectoderm<=Cchordamesoderm 'roof of archenteron(Double9headed arro#C marginal one <C epiblast, =C hpoblast, 3C subgerminal ca!it, AC de!eloping neural tube, ;C head process, >C ingressed endoderm, C ingresses mesoderm, @C Fensen7snode, C primiti!e groo!e, <?C primiti!e groo!e, <<C area opaca, <=C ol- ig$ 3$ %rans!erse sections through a <>9hour chic- embro: )( cranial blastoderm. B( head fold 'rostral notochord(. ( Fensens node. D( rostral primiti!e strea-. E( caudal primiti!e strea-. (, caudal blastoderm$ n thecranial and caudal blastoderm '), (, the bilaminar appearance of the blastoderm is demonstrated, and the subgerminal ca!it is !isible$ %he epiblastis thic-er in the head fold, Fensens node, and primiti!e strea-$ ngressing cellsmigrate o!er the primiti!e folds #hich are raised abo!e the blastoderm surface$ i$ 3$; ut a#a !ie# of a chic- astrula $  4 Materials: 1icroscopePrepared slidesSea *rchin: earl gastrula, late gastrula, pluteusStarfish: earl gastrularog: earl gastrula, late gastrula *rocedure: <$Hie# slides under scanner, P0 and FP0$=$S-etch/dra# obser!ed slides 'under FP0($ abel the parts$ -uestion: <$Discuss the importance of gastrulation in embronic cell differentiation$=$ist and describe some de!elopmental anomal associated #ith gastrulation$ Name:_____________________________________________ Date Performed/Submitted:_________________ Section/ Group Name:________________________________ Score:__________________________________ aborator &or-sheetExercise 39Gastrulation ig$ 3$> &hole mount of <>9hour chic- embro$
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