BIOL 366 Midterm 1 2024-10-10 PDF

CELL O CELL COYYUN CA ON communication betweencells can bedirect or indirect JUXTACRINESIGNAL.IN G is the communication between neighboringcells indirectconta thecells are incontact throug...

CELL O CELL COYYUN CA ON communication betweencells can bedirect or indirect JUXTACRINESIGNAL.IN G is the communication between neighboringcells indirectconta thecells are incontact through their receptors not their plasma membranes homophilic binding iswhen thereceptorin the membrane of one cellbindsthesametypeof receptoronanother heterophilicbinding is whenthereceptor inthemembrane ofonecellbinds a differentreceptoron theothercell it couldalsoinvolvecommunicationbetween a cellreceptor theextracellular matrix it gets thesignal ming from outside of thecell AR A CR I N E S I G N ALI N G is the communication betweencellsacross distances through the secretion of proteins nto the extracellularmatrix paracrinefactors ligands are proteins that are secretedfroma signalingprotein ligandsbindreceptors on neighboring cellsreceiving the signalinginformation IGNAL TRANSDUCTION C A SC A D ES are the series of enzymatic reactions that are triggered byreceptoractivation also called signal transduction pathway oroteinproteininteractions either between ligand receptor or receptor receptor or xtracellularmatrix protein receptor often lead to a change inproteinconformation ofthereceptor thischange inproteinconformationgives thereceptor a newproperty allowsit to relay a signalin the cytoplasm DIFFERENTIAL CELL AFFINITY Townes Holtreterin lass conducted cellrecombinationassays tostudy morphogenesis development ofshape amphibianembryonictissues were dissociatedintoindividualcellsafterbeingplaced in an alkaline solution  cellsreaggregated on theirown becamespatiallyseparated epidermal cells on the outside while the neural cells were on theinside thesepositionsreflected their positions in the embryo 7 Townes Holttretercontinued theirexperiments when the threegerm layersaremixedtogether theysortthemselves intopositions that reflect their ositions in theembryo the endoderm isin themiddle separatedfrom the epidermis ectoderm y the mesoderm theyattributed theseresults to thecellsexhibitingdifferential affinity differentialaffinityisalsoobservedwhen celltypeswithinthe same germ layer are recombined e g neural plate the epidermis are bothectodermalcelltypes theywillalsorepositionthemselveslikein theembryo if theneuralplate cells the mesodermalcells the epidermiscells are mixedtogether theyarrange themselves with the neuralplate at thecenter coveredby themesoderm covered by the epidermis DIFFERENT ADHESION HYPOTHESIS Townes Holtfreter foundthat cellmovementduringmorphogenesis is notrandom buttheywere unable to tell what was directing this cell movement n 1964 MalcomSteinbergproposedthedifferentialadhesionhypothesis all thatisrequiredforsortingto occuris that celltypes differinthe the strength of their adhesions the hypothesis proposes that cell segregation arisesfromtissuesurfacetensions whichin turn arise from difference in cellular adhesiveness ells will interact toforman aggregate with the smallestinterfacialfreeenergy a group ofcellswith lowsurfacetension will not tend to sticktogetherasmuchas agroup ofcells with highsurface tension lowsurface tensioncells willenvelopthehighsurfacetensioncells cell types withhighersurfacetension migratecentrallywhencombinedwithcellswith lesssurface tension dyne cm is theunit traditionally used to measuresurface tension differencesin surfacetension are mainlydrivenby the presence of differenttypes of adhesionmolecules on thesurfaceof thecells CA D H E R I N S the majortype of celladhesionmoleculeis aproteincalled cadherin calcium dependentadhesionmore cadherinsbind to cadherinsonadjacentcells keepingthemtogether they form adherensjunctions cadherinsspan theplasmamembrane insidethecell they are anchoredtoactin filamentsviacaten cadherinsperformseveralimportant functions 1 adherence 2 connection to the actincytoskeleton 3 initiation of signal transduction pathways blocking cadherin synthesis byRNAinterference or mutationsprevents theformation of epithelialtissues causescellsto disaggregate pithelialtissues are highlyenriched in cadherinsbecause epithelialcellsneedtosticktogether to formtightboundaries several types of cadherinshavebeenidentified in vertebrateembryos E cadherin all earlymammilianembryocellsveryimportant in mediatingadhesion of epithelialcells P cadherin placenta helpskeep itstuck to theuterus N cadherin expressed in cells of the developing nervoussystem R cadherin criticalin retinaformation Proto cadherins lack the attatchment to theactincytoskeleton inside the cell surfacetension for homotypic aggregates cellshave all thesametypeof adherins is linearly related to theamounts of cadherins molecules on he cell membranes this thermodynamicprinciplealso applies in heterotypicaggregates in which the relativeamounts of different cadherin types still predict cellsorting behavior in vitro  E cadherinsin zebrafish in many embryos the onsetof gastrulation is markedbyepiboly epibolyis a morphogenic movement characterizedbytissuespreading thinning Itypically the ectodermcellsdivide spread become thinner toenvelopthe entireembryo expression of E cadherinisrequired todrivethiscellspreading thinning epibolyisdriven by radialintercalation a cellularmovement that involvescellsfromdeeperlayers movingtowardstheouter layer thismakesthetissuethinnerand itmakesthe tissuespread n zebrafish theinnerepiblastcellsmove outwardsintothe more superficialepiblastcells thismakestheepiblast thinner powerstissuespreading this processisdependenton a highconcentration of E cadherin in the enveloping layer Evi hecellsof the envelopinglayerare anchoredthere theycannotmoveto the centerof the embryo spite having a highsurfacetension instead theyattractcellstowardsthe more superficiallayers ofthe epiblast powering the radialintercalation he epiblastcellsfail to thin spread in an E cadherinmutant c 1 EXTRA CELLULAR MATRIX is theinsolublenetworkof macromolecules secretedbycells itis important forcell adhesioncellmigration theformationofepithelialsheets collagenis the mostabundantprotein in the extracellularmatrix glycoproteins are oligosaccharides attachedto aprotein e g fibronectin lamin fibronectin is a dimer that forms quaternarystructurescalledfibronectinfibrils thatplay an important role in cellmigration likeroads proteoglycans glycosaminoglycans attached to a protein a subclassof glycoproteins  3 AS A L L AM IN A is a type of extracellular matrix that lies underneaththe pithelial cells it looks like a sheet is made up of laminin collagen extracellularmlafr.is 1 N I E G R INS are a familyofcellularreceptorproteins thatbind to roteins including fibronectin lamin integrinsbindtotheaminoacidsequence arginine glycine aspartate RGD in extracellularmatrixprotein integrins connecttheextracellular matrixto theactin cytoskeleton insidecells viatalin α actinin integrinshave twomainfunctions 1 attachmentof cells tothe extracellularmatrix a signaltransduction from theextracellularmatrix to thecells NIEGRINS GENE EXPRESSION integrinscansignalfromoutsideof thecellto theinsideofthecell alteringgene expression mousemammarycells areplacedincellculture withoutabasallamina thecellswill notdifferentiateinto mammarytissue genesnormallyexpressed in differentiatedmammaryglands casein lactoferrin whey acidicproteins are not expressed mouse mammarycellsareplacedin cellculture with a basallamina gene expression changes genes thatcontrol differentiation developmentof themammaryglandare expressed EPITHELIAL VS MESENCHYMAL CELLS cells in embryos are either epithelial or mesenchymal epithelial cell cells that are tightlyconnected to eachotherinsheetsor tubes mesenchymalcells unconnected or looselyconnected to one another canoperate as independentunits PITHELIAL TO MESENCHYMAL TRANSITION EMT helial to mesenchymal transition is an importantdevelopmentalphenomenonthatoccurs manytimes throughout gastrulation organogenesis  body patterning ithelial to mesenchymal transition involves a seriesofeventsthat nstorms epithelial cellsinto mesenchymalcells EMT is what allowsindividual cellsto migrate fromonearea from theembryotoanoth signalsfrom paracrine factors can lead to thedownregulationof cadherin integrin cateninprotein expression downregulation of theseproteins will make anepithelialcell lose connections withother epithelialcells as well as connections with the basallamina allowing thetarget cell toescapefromtheepithelium mesenchymalcells while epithelial to mesenchymal transitionis essential for development whenit is reactivated in adultcells it can allow cancercells to migrate becomeinvasive cancermetastasis cells thathavebeenpartof a solidtumormassleave the umor epithelium toinvade othertissues as migratory mesenchymalcells thatformsecondary tumorselsewherein the body CELL INDUCTION COMPETENCE nduction is when an organ or tissue is formed bygroups ofcellsorganizingchangesin the developmental trajectory of their neighbors ducer is the tissue thatproduced a signal orsignals that changes the cellular behavior of othertissues the signal isoften a paracri actor butit can also involvejuxtacrine interactions sponder is the cell or tissue beinginduced mpetence is theability to receive respond to a specificinductivesignal exampleof induction vertebrate eye morphogenesis the brain ectoderm bulgesout opticvesicles towards theheadectoderm ectoderm the opic Ésicles release paracrine factors theparacrinefactorsbind toreceptors on theheadectoderm is induced to form the lens of theeye only the head iidierm iscompetent to respond to paracrine factorssecretedby the opticvesicles to form thelens ifyou transplant the opticvesicle to anotherspot on the head ectoderm a lens will form theheadectodermis the onlyregion  competent to respond to the opticvesicle ifyoutransplant theopticvesicle to thetrunkectoderm a lenswill not formbecause it is notcompetent removal of theopticvesicleprevents thelensfrombeingformed only theoptic vesicleis capable of beingtheinducer cellinductionoccurssequentially a previous inductionevent made a tissue competent for the nextinduction event lens development eforetheopticvesicleis formed unidentifiedinducers triggerthesynthesis of transcription factor called 0 2in the presumptive lensectode inducers from the anterior neuralplate theninduceexpression ofthePax 6 transcriptionfactorin the presumptive lensectoderm which required for the competence to respond to theopticvesicle'ssignals eye malformations inhumans havebeenlinkedwithPax6 mutations the paracrine factors secreted by theopticvesicle Fgte BMP4 triggerthe transcription translation of transcriptionfactors sox L Mat whichtogetherwith Paxo areneededfor the activation oflensspecificgenes Paxe mutantmouse has no eyes itsfaceis deformed as for Paxomutantflies theyalsolackeyes eyeless Paxo mutant flies can berescued with the mousePaxogene it shows that Paxbgeneis a highly conserved gene Paxe isonlyrequiredinthesurfaceectoderm for properlens induction ARACRINE FACTORS INDUCER MOLECULE S aracrinefactors workin a range ofabout 40 200sum around iscell diameters this is unlike endocrine factors like hormones hich travel large distances autocrineinteractions in which the same cells thatsecreted thesignal alsorespond to it are not common it is seen in theplacentawhere the cellssynthesize a growth factorwhosereceptor is on the placental cells  themselves theresultis an explosiveproliferation of tissue M O R P H O G E N s are paracrinefactors that dictatecell fategene expression changes through concentration gradients a morphogenis a diffusiblechemical molecule that determines the taleof a cell byits concentrate mostmorphogens are smallsignaling proteins butothers canbelarger e g in the syncytium of Drosophilaembryos transcription factors like Bicoidacting as morphogens dictate thetale of nuclei 4 O R P H O G E N GR AD IENT specification ofmesodermal cell types in xenopus beads that secrete the paracrinefactor activin at differentconcentrations were aced in themiddleof unspecifiedcells activinis a paracrinefactorfromtheTOF β family dependingon howmuchactivin wassecretedfrom thebeads the surroundingcellsstarted express different transcriptionfactors bra goosecoid when exposed to a highconcentration of activin cellsrespond by expressinggoosecoid when exposed to a slightly lowerconcentration ofactivin cellsrespond byexpressing bra it the bra thresholdis notmet neithergeneis activated SIGNAL TRANSDUCTION C ASCADES almost all majorsignaltransduction pathways no mattertheligand thereceptorinvolved appearto be variations on a common theme receptorscontain anextracellulardomain transmembrane domain and intracellular domain ligand bindingtriggers a conformationalchange in the receptor whichallows it to dimerize dimerization activates thereceptor'senzymaticactivity usually kinaseactivity the activated kinasereceptor can phosphorylate andactivate a respondingprotein most receptors thathave kinaseactivitywillspecifically phosphorylate tyrosineresidues on target proteins these receptors  re called receptortyrosine kinases or Rtks PARACRINE FACTOR FAMILIES FIBROBLAST GROWTH FACTOR FAMILY FGFgenes cangeneratehundredsof protein isoforms throughalternativesplicin Gfs cansometimessubstituteforoneanother buttheexpressionpatterns oftheFoes andtheirreportsgivethemseparatefunctions GFs canfunction through two pathways Rik pathway Jak stat pathway of theligand binds a setof Rtks calledfibroblastgrowthfactorreceptors FGFRs when FGF bindsfibroblastgrowthfactorreceptors the receptordimerizes then autophosphorylatesitself activating its kinaseactivity the adaptorprotein recognizes the phosphorylatedtyrosines on theRTK activates GEF GEFactivates the Ras a protein by allowing phosphorylation of theGDPboundRas at the sametime Gapproteinstimulates the hydrolysis of the phosphate bond returningRasto itsinactivestate the activeRas activates the rat proteinkinaseC which phosphorylates a seriesofkinase suchas MEK the activated kinase ERKaltersgeneexpression in the nucleus of the responding cell by phosphorylating certaintranscriptionfactors the pathway endswith the phosphorylation of transcription factors or translation factorswhich alter the types ofgenes thatare express ligand RTK GEF Ras Rat MEK ERK transcription factor transcription FGF and the Rtk pathway Fgf8 is important for manyprocessesincluding limbdevelopment lens induction Fate is a paracrinefactorproduced by the opticvesicle toinduce theouterheadectoderm to form a lens Fgt8 binding in the respondingcell triggers the expression of the transcription factor LMat expression of L Mat can beinduced in theouterheadectoderm byectopic expression of Fgf8 in the surroundings The JAK STATpathway example of Jak stat pathway prolactin hormone signaling casein gene activation FOFRs canalsosignal through the Jak stat pathway  the Jak stat pathway the Jak kinaseis linked tothecytoplasmicpartof the receptor whenprolactinbinds thereceptor dimerizes it the two Jak proteins phosphorylate achother the dimerized receptor activating the dormant kinaseactivityofthereceptors Jak phosphorylates a tyrosineresidue of the stat family of transcription factors stats on allows it todimerize enter thenucleus to bind to DNA regulatorysites withother transcription factors the stats proteinactivates the transcriptionof thecaseingene gand receptor Jak stat stat dimerization transcription of andthe Jak stat pathway Fgers the Jak stat pathway iscriticalin theregulation ofhumanfetalbonegrowth Fgers with itsligand signals the chondrocytes thecartilage producingcells stopthe expansion of thegrowthplate FgtrsactivationtriggersSTAT1 activationwhich triggerstranscriptionof the cell cycleinhibitorpal it arrests celldivision stopsgrowth ain offunctionmutationsinEgfr makingitactiveevenin theabsenceof aligand causethanatophoric dwarfismbecausethe growthplates p dividing bones do notgrow HEDGEHOG hh FAMILY was discoveredin Drosophila named hedgehog because a lossoffunctionmutationin thegenecauses wae to grow denticles chairlikestructures on its cuticle vertebrateshave at least three hedgehoghomologs i desert hedgehog cann indian hedgehog inn sonic hedgehog shh whichhasthegreatestnumberoffunctions of thethree vertebrate homologs hedgehogproteins function asmorphogens theyinducedifferentialgene expression at differentthreshold concentrations that result indistinctcellidentities  hedgehog processing secretion before Hedgehog proteins can be secreted as morphogens they are processed assembled differentmodesof processing assembly can affecttheamountof hedgehogthat is secreted thegradient that isformed I Finstation of thehedgehoggene in the endoplasmic reticulumproduces a hedgehog protein withautoproteolyticactivity thatcleaves to the carboxyl terminustoreveal a signal sequence thatmarkstheprotein for secretion secretion also requires theadditionof cholesterol palmitic acid to the hedgehogprotein SECRETION yclearingoff its carboxyl terminus associatingwith bothcholesterol palmiticacidmoieties hedgehogproteins can beprocessed cretedas i monomers requiresonly cholesterol 2 multimers requiresboth cholesterol palmiticacid 3 multimerin a lipoproteinassembly requires the addition of heparansulfateproteoglycans Caspes 4 exoresicles edgenog signaltransductionpathway he cholesterol moiety on hedgehogis necessary for hedgehog tobind a receptor liedpatched Patcheditselfdoesnot transduceanysignal butinstead functions as a epressorfor a proteincalled smoothened in the absence of hedgehog smoothened is degraded due topatched the transcriptionfactorCi coli in vertebrate homologs istethered to the microtubule a portion of Ci Gli is clearedbyproteases and thiscleavedfragmenttranslocatesintothenucleuswhereitacts as a transcriptionalrepressor no transcription fromhedgehog targetgenes in thepresenceof hedgehog hedgehoginhibits patchedto activate the hedgehog signalingpathway conformationalchangesin patcheduponhedgehogbinding releasethe inhibitionofsmoothened parched hedgehog are endocytosed degraded moothenedreleases cioli from the microtubules probablyby phosphorylation i I t oldies in the nucleuswhere itactsas a transcriptionalactivatorof thesamegenes itusedto repress imb facial abnormalities in sonic Hedgehog mutant the Hedgehogpathway isextremelyimportant in vertebrate limbpatterning neural differentiation and pathfinding retinal development craniofacialmorphogenesis andmore in micewith homozygousnullmutationsinsonic Hedgehogdisplaymajorlimb facial abnormalities oneopticvesicle isformedinshnmutantembryos cyclopia encephalyis a conditioncausedby thefailure of the prosencephalon embryonic n human holopros orebrain todivideinto twolobes inseverecasesthiscancausecyclopia whichisthe most severe form of holoprosencephaly holoprosencephaly can be causedbyreduced hedgehog signaling this is due to mutations in genes that encodeeither sonichedgehog or the enzymes that synthesize cholesterol lackof modifications reducedsecretion WNT Family are cysteine rich glycoproteins that were discovered in Drosophila the mutationsingene wingless preventthe formation of the wing wnt fusion of wingless thevertebratehomolog integrated there are a separate wnt genesfoundinhumans wnt processing secretion like hedgehog wht is synthesized in the ER andmodifiedbylipidaddition palmitic palmit oleicacid catalyzedbytheenzyme Porcupine lipidaddition isimportant for wnt secretion as itallows it to be transported to the cell membrane wht can besecretedby free diffusion exoresicles beingpackagedinlipoproteinparticles uponsecretion wnt associates with glypicans catypeof heparansulfateproteoglycan in theextracellularmatrix whichrestrictsits diffusion leadsto a greaterconcentration closertothesite of production  wnt attaches to a receptorcalledFrizzled on the responding cell the binding of wnt toFrizzledrequiresthelipids palmit oleicacid to beattached to wnt and the coreceptors LRPS 6 calargetransmembraneproteinthatactsas areceptor uponFrizzledactivation the cellexpresses a proteincallednotum ahydrolase thatissecreted creates I I attached townt wnt Frizzleddissociate because the lipidsareessential for wnt tobindtoFrizzledwhich preventsexcessive wntsignaling negativefeedbac an experimentshowed that theenzymaticactivityof notumisrequired for appropriatedelipidation of whisa notumlackingenzymaticactivityis unable toremove thelipidgroup from whisa the canonical wnt pathway culminates in theactivation of theβ catenin transcription factor modulation of geneexpression alsocalled β catenindependentpathway in the absence of Wnt 1 B catenin isconstantlybeingdegraded by aproteindegradationcomplexcontainingseveral proteinsincluding Apc axin ask3 glycogensynthase kinase3 2 asks phosphorylates atein sothatit willberecognized degradedby the proteasome TCF LEF transcriptionfactors keep wnt responsivegenes repressed in the presence of Wnt the lipidated wntprotein bindsto the Frizzledreceptor interactswithLRP 516transmembraneprotein Frizzled Lrps 6 bind to Disheveled Disheveled bindsasks attheplasma prevents it from phosphorylating β catenin β catenin isstabilizedentersthe nucleus binds to the LEF of transcriptionfactorwhichconverts it froma repressor to anactivator inhibits Phosphi UMMARY Wnt Frizzled Lapse Disheveled asks B catenin transcription the noncanonical wnt pathways in addition to regulatingchanges in geneexpression in thenucleus wnt pathwayscanalso cause changesin thecytoplasm toinfluence cellfunction shape behavior he noncanonical wnt pathways can bedividedinto a types theplanarcellpolaritypathway thewnt calcium pathway  theplanarcell polaritypathwayPcp regulatesactin microtubulecytoskeleton which influences cell shape motility 1 Wnt activates Disheveled by bindingto Frizzled Ror transmembraneprotein 2 Disheveledactivationleadsto the activationof Rho Gtpases 3 theseactivated RhoGtpases coordinate changes in thecytoskeleton organization as wellas gene expression changes mediatedthrough Jak Forfamily this pathwaydirectscelltodivide moveinthesameplane wnt calciumpathway leadsto therelease ofcalciumstoredwithincells thatactsas animportantsecondarymessenger Wntactivates Rykreceptors whichtriggers a cascade thatendswith therelease of calciumfrom the ER releasedcalciumcanactivateenzymes transcriptionfactors translationfactors THE TGF β SUPERFAMILY regulate some of themost importantinteractions in development the TGF β superfamily includes the TGF β familycar thenodal i theactivins a familiesthebonemorphogenicproteins BMPs familyc thev91 family severalotherrelatedproteins embers of the TGF β superfamily activatemembers of thesmadfamily of transcription factors the TGF Bligandbinds to atype II toe βreceptor which allows thatreceptor tobindto a type I TGFBreceptor the type II toe β phosphorylates thetype I TGF βreceptor whichactivatesit the activatedtype I receptor phosphorylates thesmadproteins the smad proteins bindtosmad4 dimerization form a transcriptionfactor that will regulate geneexpression UMMARY TGF β superfamily ligand type II Tor βreceptor type I tof βreceptor smadactivation smaddimerization transcrip psupeiiamti.in ecapparacrinefactors Families 1 Fibroblastgrowth factorfamily FGF 2 Hedgehogfamily 3 Wnt family 4 tof epidemalgrowthfactorEGF hepatocytegrowthfactor neurotrophins stemcellfactoraresomeparacrinefactorsthatdonotfit inanyabovegro  IF FUSION TRANSPORT OF PARACRINE FACTORS free diffusionof paracrinefactorsis hindered byothercells by theextracellularmatrix paracrine factormovementawayfromthecellsecretingit can behelpedorinhibitedbyotherproteinsor extracellularmatrixfibers heparan sulfate proteoglycans citspos glypicans in the extracellularmatrix can modulate thestabilityreception diffusionrates and ncentrationgradient of fibroblastgrowthfactors bonemorphogenicproteins BMPs and wants Fgf8 gradient is shapedthroughmultiplemechanisms other paracrinefactors mayusesimilar mechanisms 1 mrna ofFgte isregulated thedifferencein therare fatetranscription and fate mrnadecaycaninfluencetheamount ofFgteproteinultimately secreted Fatecan freelydiffuse thisdoesnotrepresent alargeportionofFate Fate can travelrapidlyalongHspofibers longfiberspresentin allanimaltissues fordirected diffusion denseareas of Hspascanslow down Fgtediffusion presentinthe areasof thebody that needa higher concentration ofsignals the Fgto EGFR complexcanalso be internalizedbyendocytosis targeted for endosomaldegradation decreasesignaling INGLESS has lipidmodificationsmakingtheproteinhydrophobic unabletotravelinthe extracellularenvironment a protein called dailylikeprotein dip shields the palmitoleate moiety andallowswingless totravel in the xtracellular environment UX T A CRINE SIGNALING has threemajorfamilies notchproteins bindtoligandslike theDeltaprotein a celladhesionmolecules likecadherins 3 Ephreceptors bindto ephrinligand NOTCH SIGNALING signalingcellsexpress Delta Jagged orserrateproteins in their cellmembranes whichbindnotch proteins in neighboringcells i when bound to one ofitsligands notchundergoes a conformationalchange thatenables the eavage of itscytoplasmicdomainby aprotease theclearedportionentersthenucleus where itbindsa dormanttranscriptionfactorof the csc family Notchdisplaces thereceptorproteins bindsactivators of transcription includingthe histone acetyltransferase p300 opens chromatin when bound to thenotchprotein esr transcriptionfactorsactivatetheirtargetgenes STEM CELLS efore they are committed differentiated undifferentiated embryoniccellsarecalledstemcells stem cell has theabilityto divide recreateitself has the potency toproducedifferenttypes of differentiated celltypes T E M CELL DIVISION single cell asymmetry thestemcell creates onedaughtercell thatgoeson to itterentiate one daughtercell that isidentical replenishesthestemcellpool sopulation asymmetry symmetricaldifferentiation in apopulationofstemcells somegroups in dividesymmetrically to produce twodaughtercellsthatare committed to differentiate othergroups willdivideto producetwo self renewingstemcells O T E N C y refers to theabilityof astemcell togeneratedifferentcelltypes thehigherthepotency themorecelltypesitcan for i totipotency theycan formall celltypes in mammals only the zygote fertilizedeggs thefirst 4 8cells aretotipotent theycan generateboththeembryoniclineages theextraembryoniclineages placenta theextra embryonicmembranes arenecessary forthenutrients growthortheembryo it is composed of the chorion amnion yolksac allantois 2 pluripotent theycan formcelllineages in mammals theembryodevelops an outerlayer extraembryonictissue andinner  ell mass clams onlythecells of the inner cellmass are pluripotent multipotent theycan formmultiplecell types withrestrictedspecificity for thetissueinwhichtheyreside indefinitely unlike totipotent pluripotentstemcells adultstemcells or multipotentstemcellsinculturecannot self replicate anking thestemcells according to potency i zygote 2 humanembryonicstemcellsafterplacentaldevelopment 3 hematopoieticstemcells bloodstemcells adultstemcells are foundin manyadultorgansandtheyaretypicallymultipotent when multipotentstemcelldivides asymmetrically thedifferentiating aughter cellgoesthroughatransitionstage called the progenitorcell or he transitamplifyingcell progenitorcells can onlyself amplify a coupleoftimesbeforedifferentiating theyserve to amplifythepool of progenitors ST E M C E L L N IC H E is the microenvironment surrounding a stem cell or a group ofstemcells the stemcell niche regulates the stemcellsbetween selfrenewal differentiation differenttissueshave differentniche architectures but thebasicprinciples of regulation are shared extracellular mechanisms mechanical force cell cell cellmatrix adhesions endocrine paracrine juxtacrinesignaling intracellular mechanisms transcriptional regulation epigenetic regulation cytoplasmic determinants 4 AM M A LI AN INNER CELL MASS ICM mammals only thefertilizedegg zygote the first 4 8 cells are tipotentc can formbothextraembryoniclineages embryonic lineages after the 8 cellstage themammalianembryodevelops an outer troph ectoderm placenta and an innercellmasswhichgenerates  the embryo a cavitycreated in the morula to create theblastocyst ectodermcellssurround theinnercellmassclamsand afluidfilledcavity troph called the blastocoel the 1cm becomes the epiblast primitiveendoderm the tropn ectodermbecomes theplacenta the primitive endoderm becomesthe extraembryonicstructure yolksac thechorion theepiblastbecomesthe embryoproper thecells ofthe1amare pluripotent theycangenerateeverycellin thebody exceptthe placenta ifyouremovecellsfromtheinnercellmass culturethem invitro theyarecalled embryonicstemcells Escs culturedembryonicstemcellsretain their pluripotency can similarly generate all celltypesof thebody mechanisms promoting pluripotency in 1cmcells hreetranscriptionfactors arenecessaryin theinnercellmass to maintainpluripotency i oct4 2 nanog 3 Soxa coxa is for the troph ectoderm'sfate expression of thesethreetranscription factorsbegin to decrease in the innercellmass the epiblast development the transcriptionfactorCaxa is necessary in the morula'soutercellstopromotetrophectodermdifferentiation repress theepiblastdevelopmen whatmechanisms are atwork tocontrolthe temporal spatialexpression patterns of genes within the presumptive ion trophectoderm cellpolarity asymmetric partitioning of E cadherins basalaxisyieldsdaughtercellsthat perpendicular asymmetric divisionsalongtheapico segregate to theinside outside of theembryo  basalaxispropagate cellsinthe trophectoderm paralleldivisions to theapico ectodermcellswillcause theinner cells to becomepluripotent because theoutercells have polarity the outertroph C EL L P O L AR I T Y refers to differences inshape structure andfunction within a cell epithelial cellsare polarizedsuch that theyhave an apical domain a basal domain theapicaldomain istypicallyfacingtheoutside andthebasalmembrane is contacting theinterior ASYMMETRIC PARTITIONING OF E CADHERI at themorula stage E cadherin localizes to the basolateralmembr presentPaying simdeemstines thisis where thefuture outer layercellsare contacting the uture cells of the innercell mass byexperimentallyeliminatingthe E cadherin it disrupts theapico basalpolarity thespecificationof the1cm trophectoderm lineages because E cadherins allow for juxtacrine communication asymmetric partitioning of Ecadherin influences theblastocystcellfates ectoderm promotespluripotency in the innercellmass E cadherin in the troph theinnermasscell cadherinon thebasalsideof thefuture troph ectodermcellsactivates the Hippo pathwayin the uture inner masscells tippo signaling leads to the degradation of yap a transcriptional coactivatorthat allows the pression or coxa coxa specifies the trophectodermtales don'twant caxa in 1cmcells oct4 activates a generegulatorynetwork that promotespluripotency in the innercellmass Caxa in the outer cellspromotes the trophectodermfate in theoutercells Hipposignalingis not activated yapis abundant activates transcription ofcoxa trophectoderm fate  D U L T ST E M N I C H E s are found in many adult tissues organs germcells brain epidermis hairfollicles intestinal villi blood etc the adultstemcells are multipotent they are housed in controlled by its own adultstemcellniche whichregulates stemcell self renewal survival and differentiation of thoseprogeny that leavetheniche QUIESCENCE most adult cells are quiescent otherwise known as being in the Go phase of thecellcycle it is a reversible state of the cellcycle quiescent cells do not divide butretain the ability to re enter the cellcycle and resume proliferation in response to certainsignals quiescent adultstemcells can be triggered to re enter the cell cycle to divide either asymmetrically or symmetrically M AM M ALIAN NEURAL STEM CELLS he fatherofmodernneuroscience santiago Ramon y cajal in 1913 thought that the nervepaths neurons oncefullydeveloped were ixed immutable hethoughtthat neuronscouldnot beregenerated thefirstevidence of neurogenesis was in 1969 the evidence ofDNA replication in the adult ratbrain there are twoniches in the adultbrain i the subgranularzone esoz the hippocampus a the ventricular subventricular zone Cv svz the lateral ventricles neural stem cellsin the ventricular subventricular zone arefoundin thelateral wall of the lateral ventricles of the brain the ventricles are filledwith cerebrospinal fluid Csf MAOIs THE VENTRICULAR SUB VENTRICULAR ZONE is composedof ependymalcells CE cells along the ventricular wall 2 B cells neuralstemcells B Ba  3 progenitor c cells betweenpluripotent specialized 4 migrating neuroblast a ce B cellsprojectaprimaryciliumfrom theirapicalsurfaceintothe cerebrospinal fluid and a long basalprocess that connects to bloodvessels cell B2 cell a cell a cell neuralprecursors neuroblasts migrate to the olfactory bulb forfinalneural differentiation AINTAINING THE NEURAL STEM CELL POOL IN THE V SUZ symmetricaldivisions bothdaughtercellsdifferentiate outweigh asymmetrical divisionsconedaugthercelldifferentiates andtheotheris a Nsc mechanisms are inplaceto ensure thatthe Bcellsof thisnicheare notlost duringcalls forneurogenicgrowth orrepairin responseto injury 1 VCAM I the rosette mail clusters of B cells aresurrounded by ependymal cellsCEcells whenviewedfrom thesurfaceof the ventricularzone theyform a pinwheel likerosettestructure came is aspecific celladhesion protein thatkeeps the rosettetogether s the mammalian brainages boththenumberof observed pinwheelstructures thenumberofneural stem is in the pinwheelsdecrease whichcorrelates with a reduction in neurogenic potencylater in life the Bcellsthat are most tightlyassociated with ependymalcells are themost quiescent B cells the tighterthe adhesion to Ecells the more quiescent thecells the more looselypacked B2cells are primed activelyproliferating experimental inhibition of veant disrupts therosettestructure causes thelossof theneuralstemcell quiescence promotion of progenitor cell differentiation 2 notch signaling notch is activatedby cell cell interactions notchintracellular domain Nico is cleaved and released to function as part of a transcription factorcomplex that represses neuralgeneexpression  notchreceptors in the basal endfeet of Bcellsbind toJaggedt Jagi receptors in the endothelial cells ofbloodvessels juxtacrinecom Notch signaling represses differentiation notch reams keep the poolof neural stemcells ROMOTTING DIFFERENTIATION IN THE V SV2 bonemorphogenic ireiihesigiainjPiptaimaiinefactor endothelialcells bloodvesselcells at thebasalsideof thenichesecrete bone orphogenicprotein which promotes differentiation ependymalcells at the apicalsideof theniche secretenoggin which is a BMP hibitor the antagonisticsignaling gradientsbalanceneurogenesis as Bcells transitionsintotype ccells thenintotype Acells move closerto the basalborder of theniche theyexperience increasinglevels of BMP signaling c B cell a cell a cell neuralprecursors neuroblasts migrate to the olfactory bulb forfinalneural differentiation 2 Epidermalgrowthfactor paracrinefactor the epidermalgrowthfactor receptor is activatedin type c cells some typeof B cells which upregulates NUMB NUMB inhibits the notchsignaling which promotes neurogenesis differentiation H EM A TOPOIETIC STEM CELL HSC NICHE everyday in yourblood more than 100billioncells are replacedwith new cell the conceptof astemcell niche was firstcharacterizedwith hematopoieticstemce the hematopoietic stemcellsnicheisfound in bonemarrow bone marrow transplants havebeenusedsince the 1950s to treat bloodbased diseases such as leukemias lymphomas immune deficiency disorders and aplasticanemia  the hematopoieticnicheis subdividedinto tworegions the endostealniche hematopoieticstemcells are in directcontact withthe osteoblasts lining theinnersurfaceof thebone the perivascularniche hematopoieticstemcells are in closecontact withcellsliningor surroundingbloodvessels with these two niches there are two subpopulationsof hematopoieticstemcells long term quiescent hematopoieticstemcellswhichpossess the greater potential forselfrenewal aretypically foundintheendostealniche activelydividinghematopoieticstemcells aretypicallyfound in the perivascular niche theycan respondrapidly to immediateneeds depending on physiologicalconditions stemcellsfromonepopulation canenterthe nextpopulation a complex cocktailof paracrine factors cell adhesionmolecules sympatheticneuralinputs hormonalchanges bloodpressure changes and extracellular matrix components all influence thebalancebetween quiescence proliferation differentiation excl 12 prevents stemcells frommaturing entering thebloodstream to mature a selectiveknockoutof exec la causes reductions in hematopoieticstemcells EMBRYONIC STEM CELLS IN THE LAB pluripotentembryonic cells can generate allcellsin thehumanbody and are extremely usefulfor research pluripotent embryonicstemcells can bederived from two sources innercellmass of theearlystageblastocyst embryonicstemcells Escs primordial germcellsfromthe fetus beforetheymigratetothegonads embryonic germcells EGCs not all the embryonicstemcellsareequal theyhavedifferentselfrenewalcapacities characteristics and can morereadilyformcertain types ofcells it is due to the time they were collected weretheydifferentiating a bit there are a embryonicstemcells states  naive mostimmature undifferentiated greatestpluripotency primed inner cell mass cell withsome maturationtowards the epiblastlineage NDUCING DIFFERENTIATION FROM ESCs embryonicstemcells can becoaxedtowardsdifferentcellfates byexposing them to specificcombinationsof paracrine transcriptionfactors N D U C ED PLURIPOTENT STEM CELLS nearlyany dividing cellin the adultmouse orhumancan be dedifferentiated backinto a pluripotentstatebyinducing fourpioneertranscription factors oct3 4 establishespluripotency blocksdifferentiation soxa establishespluripotency blocksdifferentiation c Myc allows chromatin accessibility kit4 prevents celldeath 1 entiremouseembryos canbegeneratedfromsingle inducedpluripotentstemcells buttheyarebest generating thecelltypesof theorganfromwhichtheyoriginated inducedpluripotentstemcells retainsomeepigeneticmemoryof the adultcell fromwhichtheyoriginated eg DNA methylation histonemethylation REATING A MODEL OF THE HUMAN EMBRYO enlistshavegrownanentitythatcloselyresembles an earlyhumanembryo withoutusingspermeggsor a womb implantationof the humanembryo leadstomanychanges inorganization thatare important forgastrulation implantation is a periodwith a highincidence of embryoloss miscarriage itis important tounderstand themorphogenicevents that driveimplantation in theearlyhumanembryo YEDICALORESEAR.CH USES FOR INDUCED PLURIPOTENT STEM CELLS diseasemodeling studyingdiseasepathologybycreating patientspecific inducedpluripotentstemcells  screendrugin patientderiveddifferentiatedcells combininggenetherapy with patientspecificinducedpluripotentstemcells to treat diseases usingpatientspecific ipsc derived progenitorcellsfor transplantwithouttherisk ofimmunerejection donorcellswouldhave thesame genome as thepatient IPS Cs GENE EDITING i a mouse with sickle cell anemia wascreated 2 ipsesfromthismouse weregenerated byinfectingthemwith Oct4soxakity c Mycviruses 3 the mutation was corrected byspecificgene targeting fibroblastsfromthesame mouse butafterculturing add the wildtype allele of human globin Hba to correctthedetect the cellsdifferentiate intoembryoidbodies transplant of hematopoieticstemcellsbackintotheirradiatedmice mouse is curedfromsicklecellanemia samethingcan bedone with humans affectedbyβ thalassemia DANGERS OF EXPERIMENTAL STEM CELL THERAPY EXAMPLE 1 a 26yearold man withpartialarmparalysis severelegparalysis underwentan experimentalstemcelltransplantusingstem cells from his nostrils 12years later a tumorhadformed in hisspine containedtissueconsistent withectopicnasaltissue aninternationalindustryhas developed thatsellsunproven medicalstem cell based treatments EXAMPLE 2 a 72yearold womanwithmacular degeneration underwentinjections ofadipose derivedstemcellsin hereyes this procedure was not regulatedbytheFDA was not an officialclinicaltrial vision waslostinbotheyes and imagesshow clumpsof blood floatinginsid