BIPN 144 Week 3 Notes(part 2)

BIPN 144 Week 3 Notes(part 2) - BIPN144Week3Notes(part2)

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Unformatted text preview: BIPN144Week3Notes(part2) Note:ThishasnotbeenreviewedbyDr.Bierandmayhaveerrors.Always trusttheprofessoroverthesenotes. Dr.Bierstartedoffbydiscussingtwoverycommondevelopmentalwordswhichhe doesnotlike:DeterminedandFate.Hefeelsthatthesewordsaremisleadingascell identityisneverabsolutelylockedintoanyonefate.Theirradiatedfrogegg/ nucleartransferexperiment(GurdonExperiment)provedthis.Soratherthan lockedin,Dr.Bierconsiderscellstobe"settled"andthatmostcellsinanadultwill remainsettledundernormalcircumstances. Hediscussedtheconceptofa"fatemap,"andexplainedhowpeopleusedtoadhere afleckofsomematerialtoaspecificcellorregioninadevelopingembryo,andthen watchwhereitendedupintheabsenceoffurtherembryonictampering. Experimentsofthistypeyielddatathatcanbeusedtoproducea"fatemap"which indicatesmoreorlesswhatanygivenregionislikelytobecomeiftheembryo developsnormally.Whiletheseareusefulexperiments,thetermfateisstillabit strong,andexperimentalresultsmaydifferdependingonwhichscientistperforms them. Hethendiscussedthetermdeterminedinalittlemoredetail.Forexample,some peoplewillsaythattheSpemannOrganizerisdeterminedtomakenotochord.While alargechunkofSpemannOrganizerwillusuallyproducenotochord,thisalso dependsonexperimentalconditions.Onceagain,theGurdonexperiment demonstratedthatevena"terminallydifferentiated"skincellcangiverisetoa complete(thoughsickly)frog.DrBierthendescribedthecommunityeffect.To helpexplainthisconceptheemployedtheNewYorkeratUCSDanalogy: AlargegroupofNewYorkbuddieswhomovetoUCSDtogetherwillreinforceeach other'sNewYorkhabits,accentsandmannerisms,andwillbeonlyminimally effectedbytheirnewSo.Cal.environment.WhereasaloneNewYorkerwithno reinforcementmaybefarmorelikelytobeassimilatedintoSanDiegoculture.The sameistrueofcellstransplantedfromoneregionofoneembryotoadifferent regionofadifferentembryo. Fateordeterminationisreallyamatterofdegree.Determiningwhencellshave lockedintheirfateispointless,theyneverreallydo.Toomuchof"fate"dependson howtheexperimentwasdone. DrBierthenbeganpreparingtodiscusshowsubdivisionsforminthe neuroectodermoftheflyembryo.HeremindedusthatASCgenesareTF'sthatturn onneuralgenes.Hethendefinedthefollowingterm: Proneuraldomainagroupofcellsthathavecompetencetobecomeneural.ASC genesareproneuralgenes.(NoASC=NoNeural).ASCgeneexpressionisrequired forneuralcompetence.Tofurtherexplaincompetencehecomparedittoasculptor shapingablock.Itmaynotbedeterminedexactlywhichpartoftheblockwill becomethehead,butageneralregionofheadcompetencecanbeascertained.Dr Bierthenmovedontoexplainingthemechanismsofsubdividingthe neuroectoderm. Thesethreelayers(r1,r2,r3)giveriseto3primaryrowsofneuroblastsinthe centralnervoussystem(similartovertebratesspinalcord). VentralDominantRepression(illustratedabove)meansthattheventralTF's,if expressed,willinhibitthemoredorsalTF's.SoVNDinhibitsINDandMSH.AndIND inhibitsMSH.ThesethreeTF'sarealsoinfluencedbyDPPandDorsal. Letsexaminethemlayerbylayer: r1Ofthethreelayersr1hasthehighestconcentrationofDorsal(andthelowest concentrationofDPP).ThishighconcentrationofDorsalactivatesVND.VNDinturn repressesINDandMSH. r2NotenoughDorsaltoactivateVND.ThereisenoughDorsaltoactivateIND. LevelsofDPParetoolowtorepressIND.ActiveINDrepressesMSH. r3EnoughDPPtorepressIND.NotenoughDorsaltoactivateVND.NotenoughDPP toInhibitMSH,soMSHisactive.NoINDorVNDtorepressMSH.MSH(likeASC)is controlledbyrepression. Ingeneral,DPPrepressesINDmoreeffectivelythanitrepressesMSH.(lessDPPis requiredtoblockINDthanwouldberequiredtoblockMSH). Ingeneral,moreDorsalisrequiredtoturnonVNDthanisrequiredtoturnonIND. AlwaysrememberVentralDominantRepression. TaketimetoconsiderpossiblemutantssuchasaVNDmutant,aVND/INDdouble mutant,oravnd/ind/snailtriplemutant. DrBierthenwentontodescribetheformationofthevertebrateneuraltube. Inthemouse,Msx1=MSH,Gsh1=IND,andNkx2.2=VND.BMP=DPPandShh (SonicHedgehog)=Dorsal. Followinggastrulationthemesodermformsthenotochord.Theneuralplate,which isdorsaltothisregion,theninvaginates,formingtheneuraltube.Verydifficultto visualizewithoutapicture(sorry). ...
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