developmental biology

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D E V E L O P M E N T A L B I O L O G Y
ORIGINOF SEXUAL REPRODUCTION
• Reproduction
• The goal of reproduction,foranyorganism, isto ensure the survival of itsgeneticlineage
Two ways to do this:
• ASEXUAL: offspringare exact(almost) geneticcopies of asingle parent
• SEXUAL: chromosomesof twoparentsare segregatedandrecombinedsothatnotwo offspringare identical
to each otheror to eitherparent
• Most organismsare sexual
• Of the 1.8 millionknownspeciesonly2000 of themare totallyasexual
Whydid sexevolve?
• Life originatedwithoutsex (asbestwe cantell) sosexual reproductionissomethingthathadto evolve
• There are a large numberof disadvantagestosexual reproductionwhichmakesthe evolutionof sex a
conundrum
• Ancientasexuals:Bdelloidrotifers
• Bdelloidrotifersdate back~100 millionyears
• Despite bdelloids'asexuality,they've diversifiedinto380 species
The Cost of Sex
• The cost of males
• The cost of recombination
• The cost of mating
• The Cost of Males
Passing on genesislike tossing coins
 Two copiesexistforeachgene
 Whetheryoupasson a certaincopy of a gene isan independent
eventforeach child
 If you have twochildren,sometimesyouwill passonthe same copy
to both children(leavingthe secondcopypassedon toneitherchild)
FITNESS:
o the numberof offspringanindividualproducesthatsurvivetoreproduce themselves
o Fitness=1.0 meansthat individualsof thisphenotype are successfullypassingon100% of their
genes,onaverage

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How is fitnesscalculated
 Fitness = the numberof genespassedonto the nextgeneration
 Because diploidorganisms(I.e.,mostorganisms)onlypassonhalf of theirgenestoeachchild,theymust
have twooffspringlivingtoreproductive age tohave Fitness=1
 Fitness=1 doesnot exactly meanthat youhave passedon 100% of your genestothe nextgeneration
(Remember:sometimesyousendtwocopiesof the same gene andzerocopiesof the other)
Cost of recombination
Asexual Sexual
F F
F F F F M M
Fitness of females 2 1
The Cost of Mating
• Cost of sexual mechanisms
– Chemical attractants
– Sexual organs
• Cost of matingbehaviour
– Courtshipiscostly
– Potential exposure topredators
• Disease Transmission
Sexualitymust have itsadvantages
• Hardlyany asexual lineagesseemold,andfossil evidencehassuggestedthatasexualityisadeadend
• The prevalence of sexualityamongstspeciesiscausednotbecause asexual speciesdon'tevolve,butbecause
theydon't last
Red QueenHypothesis
The Red QueenHypothesiswasfirstsuggestedbyLeighVanValen.
Accordingto thistheory,organismshave torunfast-justtostay still! Thatis to say,theyconstantlyhave to
“run to try to improve”(andthe developmentof sex wouldbe one wayof accomplishingthat).
An animal constantlymustrun to chase its prey,elude predators,andresistinfectionfromdisease-causing
organisms
Evidence for Red QueenHypothesis
 In topminnows,sexual andasexuallineages coexist
 Sexual lineagesare the leastsusceptible toparasites
 Geneticvariationneededtokeepupwithevolutionof parasites
Whyare babiesborn young?
 Somaticcellsdie,butthe germline seemstobe practicallyimmortal

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 In a landmarkarticle published in1989, Bernstein,Hopf,andMichodsuggestedthattheyhaddiscoveredthe
answer:
 ‘We argue that the lackof ageingof the germ line resultsmainlyfromrepairof the geneticmaterialby
meioticrecombinationduringthe formationof germcells.
Muller’sRatchet hypothesis
• Vastmajorityof mutationsare detrimental
• Mutationacquisitionisaone-wayprocessinthe genomesof asexuals
• In Salmonella typhimurium 444 lineagesstartedfromasingle colony
• After1700 generations,1%of lineagesshoweddecrease infitness(growthrate) butnolineagesshowed
increasedfitness
Sexincreasesvariation
 Genesfrommaternal andpaternal parentget“shuffledup”whengametesare made
 Causessome gametestohave “superfit”genotypesandotherstohave “superunfit” genotypes
Theory for the Originof Sexual Reproduction
Viral Eukaryogenesis:Eukaryoticcellsarose fromacombinationof a DNA virusand a bacterium.The virus
incorporatedgeneticmaterial fromthe bacteriumandtookoverthe role of informationstorage forthe cell.Meiosis
arisesbecause of selective pressure on
the virusto spreadhorizontally
throughoutthe populationbycell-to-cell
fusion.Twocellsinfectedwithrelated
but differentvirusesfusedbecause they
recognisedeachotherasuninfected.
Afterthe fusionof the twocells,
incompatibilitiesbetweenthe twoviruses
resultina meiotic-likecell division.
SPERMATOGENESIS
Spermatozoadevelopfrommale primordial germcellscalledspermatogonia.
Duringembryoniclife these cellsoriginateinthe endodermof the yolksacand migrate to the regionwhere testes
are to develop. here theyincreaseinnumberbymitoticdivisionandfinallysettledowninthe developing
seminiferoustubules.
TESTES

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Spermatogoniaremaindormantduringinfancyandchildhoodof the individual.
Whenthe personachievespuberty,the interstitialcellsof the testesbegintosecrete testosterone,underthe
influenceof thishormone the processof spermatogenesisisoccur.
Withthe onsetof pubertythe spermatogoniabegin
to divide mitoticallyandsoonthree varietiesof
male primordial germcellscanbe identifiedinthe
seminiferoustubules:
(i) Type A darkspermatogonia
(ii) Type A pale spermatogonia
(iii) Type Bspermatogonia
Type A dark spermatogonia
These cellsare consideredtobe reserve cells.they
occasionllydivide mitoticallytomaintaintheirown
numberandto give rise to type A pale
spermatogonia.
Type A pale spermatogonia
These cellsdividebymitosisregularlytogive rise to
othertype A pale spermatogoniaaswell astype B
spermatogonia.
Type B spermatogonia
 Each type B spermatogoniumdivides
mitoticallyafewtimestoproduce a
numberof daughtercellswhichdonot
divide furtherbutenlarge tobecome
primaryspermatocytes.
 The spermatogoniaandthe earlyspermatocyteshave the same chromosomalconfigurationasthatof all
ordinarysomaticcell.
 Each primaryspermatocyte replicatesitsDNA andduvidesbymeiosis-Itogive rise totwodaughtercells
calledsecondaryspermatocytes.
 Each secondaryspermatocyte undergoesmeiosis-IItoproduce twodaughtercellscalledspermatids.
 The spermatidscome tolie withingroovesonthe luminal surface of sertoli cellswhere theyenterthe next
stage of spermatogenesis,whichiscalledspermiogenesis.

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Spermiogenesis.
The processby whichspermatidsare transformedintospermatozoaiscalledspermiogenesis.
Spermiogenesisinclude:
 condensationof nuclearchromatin
 Formationof acrosome
 Developmentof aflagellum
 Loss of excesscytoplasm
o Duringspermiogenesisnofurtherchromosomal change buriedinthe groovesonthe surface of the sertoli
cellsof the seminiferoustubulesoccurbuta greatmodificationinthe shape of the cell takesplace.
o The spermatidslie buriedingroovesonthe surface of the sertoli cellsof the seminiferoustubules.
o A spermatidisa small,roundcellcontainingacentrallylocatedspherical nucleus.
o Importantorganellesare alsopresent:
o There isa prominentgolgi apparatus
o A pairof centriole
o Numerousmitochondria
The formation ofspermatozoon involveselaborate changesin all these cellularelements
 The golgi apparatusgive rise to a large vesicle,calledacrosome,whichcome tolie atthe anteriorpole of the
nucleusinthe formof cap.
 The acrosomal material isrich incarbohydratesandcontainshydrolyticenzymes.
 The nucleusitself becomeprogressivelycondensed andassume ahighlyflattenedandelongatedshape.
 The area occupiedbythe nucleusandacrosomal cap becomeste headregionof the developing
spermatozoon.
 As the acrosome isformng,the twocentriolesmove tothe caudal pole of the nucleus.
 One of the centriole give rise toflagellumwhichformthe tail of the developingspermatozoon.
 The cell graduallyelongates;thiselongationoccurdue to displacementof the cytoplasmcaudally.
 The mitochondriagraduallymove towardthe caudal regionof the cell.theareacontainingthe mitochondria
isknownas middle piece of the spermatozoon.
 In the final stagesof the maturationof the spermatid,the residualcytoplasmispartitionedoff fromthe
remainderof the spermatid.
 Whenthe spermis releasedfromthe seminiferousepithelium,theexcesscytoplasmbecomesdetachedfrom
the spermasmembrane-boundstructure calledresidual body,whichisphagocytosedbythe sertoli cells.

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The net resultof these changesis that a fullydevelopedspermatozoon
i.attaina verysmall size
ii.iscapable of attainingmotality
Iii.attainsonlythose cellularstructures
whichare essentialtomake itcapable
of fertilizinganovum.
OOGENESIS
 The processof formationanddevelopmentof ova iscalledoogenesis.
 Like spermatogonia,the oogoniaalsooriginate inthe endodermof the yolksacduringdevelopment.
 From here theymigrate toand settle downinthe developingovaries.
 In the ovariesthe oogoniapassthrougha phase of proliferationinwhichtheyincrease innumberbya series
of mitoticdivision.
 Later greatnumbersof these oogoniadegenerate anddisappear.
 By the 12th
weekof the developmentall of the persistingoogoniaenlarge insize tobecome primary
oocytes,afterwhichnoproliferationoccurs.the primaryoocytesbecomesurroundedbyasingle layerof
follicularcellstoformspherical orovoidstructure calledovarianfollicles.
 A primaryoocyte containsa relativelylarge,vesicular,eccentrallysituatednucleuswithaprominent
nucleolus.
 The oocyte nucleusinmanyspeciesincreasessignificantlyinvolume andbecomesveryactive inRNA
synthesis.
 Cytoplasmicorganellesare muchmore abundantinoocytes.

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 For instance a fullygrownXenopusoocyte contains200,000 timesasmany ribosomesasan average somatic
cell.
 Animal eggscontainproteins,lipidsandglycogentonourishthe developingembryo.these materialswhich
are collectivelycalledyolk,accumulateinthe eggcytoplasm.
 The amountsof yolkare minimuminmammalianeggs,whichhave tosustainembryogenesisonlytoavery
immature stage.
 In contrastthe large eggsof birdsand reptileshave large amountof yolktosupportembryonicdevelopment
to advancedstages.
 In the course of embryogenesis,yolkisbrokendownintosmall molecules,suchasaminoacids,whichare then
usedbyembyoniccellsintheirsyntheticprocesses.
 Like RNA,yolkaccumulatesinthe oocyte duringmeioticarrestandcontributestothe oocyte’senormous
growth.
 Major egg proteinsare calledvittelins,whichare formedfromprecursorcalledvitellogenins.
 In vertebratesvitellogeninsynthesisoccurinliver.
 In insectsthe majorproductionsite isthe fatbody.
 Initiallythe plasmalemmaof the oocyte issmoothandinclose appositionwiththe surroundingfollicular
cells.however,asthe oocyte enlargesandthe zonapellucidaformsaroundit,auniformcoverof microvilli
developsoverthe oolemma.
 The golgi apparatusproducesa special type of granulescalledcortical granules.theyare boundedbyaunit
membrane andcontainenzymeswhichplayaveryimportantrole inblockingpolyspermy.
 At the endof the growthperiod,all the primaryoocytesreplicate theirDNA andthenenterthe prophase of
mieosis-I,butthe completionof the prophase of thisdivisiondoesnotoccurdurinngthe restof the intra-
uterine life.duringthisperiodandfora longtime afterbirth,thesecellsremainarrestedin the prophase of
meiosis-I.thisprolongedperiodof suspendedprophase persistsforseveral years.
 Eventually,withthe onsetof pubertythe processof oogenesisisreactivated.
 Duringthe follicularphase of eachovariancycle manyovarianfolliclesgrow underthe influence of FSHbut
onlyone of themovulates,othersdegenerates.
 Shortlybefore ovulation,the primaryoocytecompletesmeiosis-I anddivide intotwodaughtercells.
 Unlike spermatogenesis,thedivisionof cytoplasmismarkedlyunequal.
 one of the daughtercellsrecievesmostof the cytoplasmandisknownas secondaryoocyte;the othercell
calledfirstpolarbodyrecievesverylittlecytoplasm.itisasmall non-functionalcell whichmaydivide again
but eventuallyundergoesdegeneration.
 As soonas meiosis-Iiscompleted,the secondaryoocyte entersthe meiosis-IIbutthe divisionprogressesonly
uptothe metaphase whenthe processisonce againarrested.thissocalledovumisreleasedatovulation.
 The secondmeioticdivisionwill onlybe completedif the ovumisfertilizedbythe sperm.
 Eggs of differentanimal speciesare ovulatedandfertilizedatdifferentstagesof mieosis.
 As soonas the cell membrane of the ovumispenetratedbyasperm,the oocyte completesthe second
meioticdivisionandtwodaughtercellsare produced.once again,mostof the cytoplasmgoestoone cell
whichiscalledootid.the othercell calledsecondpolarbody,issmallandfuctionanlessthatsoon
degenerates.
 Eggs are protectedbyelaborate envelopes.
 The plasmamembrane of eggcell iscoveredbya glycoproteinlayer,calledzonapellucidainmammalsand
generallyreferredtoasvitelline envelope whichplays animportantrole infertilization.

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 In contrast,manytypesof eggsdepositedonland,suchasthose of reptilesandbirds,have hardshells.for
instance the yolkychickeneggissurroundedinitiallybyafragile vitellineenvelope.above thisenvelope several
layersof “egg white”consistedof ovalbuminisdepositedabovewhichshell membranesare added.
Fertilization
 Fertilizationisthe processwherebytwosex cells(gametes) fusetogethertocreate a new individual .
 Fertilizationaccomplishestwoseparate ends:sex (thecombiningof genesderivedfromthe twoparents)
and reproduction(the creationof neworganisms).
 Thus, the firstfunctionof fertilizationistotransmitgenesfromparenttooffspring,andthe secondisto
initiate inthe eggcytoplasmthose reactionsthatpermitdevelopmenttoproceed.
 Althoughthe detailsof fertilizationvaryfromspeciestospecies,conceptiongenerally
consistsof fourmajor events
1. Recognitionbetweenspermandegg.
2. Regulationof spermentryintothe egg.
3. Fusionof the geneticmaterial of spermandegg.
4. Activationof eggmetabolismtostartdevelopment.

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Recognitionof spermand egg
 The interactionof spermandegg generallyproceedsaccordingtofive basicsteps
1. The chemoattractionof the spermto the eggby soluble moleculessecretedbythe egg
2. The exocytosisof the acrosomal vesicle toreleaseitsenzymes
3. The bindingof the spermtothe extracellularenvelope(vitelline layerorzonapellucida) of the egg
4. The passingof the spermthroughthisextracellularenvelope
5. Fusionof egg and spermcell plasmamembranes
 Enclosingthe cytoplasmisthe egg plasma membrane.Outside the plasmamembrane isthe vitelline
envelope.Thisenvelopecontainsglycoproteinsandisofteninvolvedinsperm-eggrecognition.Itis
supplementedbyextensionsof membrane glycoproteinsfromthe plasmamembrane andbyproteinaceous
vitelline poststhatadhere the vitelline envelopetothe membrane.The vitellineenvelope isessential forthe
species-specificbindingof sperm.
 In mammals,the vitellineenvelope isaseparate andthickextracellularmatrix calledthe zonapellucida.
 The mammalianeggisalso surroundedbyalayerof cellscalledthe cumulus,which ismade up of the
ovarianfollicularcellsthatwere nurturingthe eggatthe time of itsrelease fromthe ovary.Mammalian
spermhave to getpast these cellstofertilize the egg.The innermostlayerof cumuluscells,immediately
adjacentto the zona pellucida,iscalledthe coronaradiata.
External fertilizationinseaurchins
 Many marine organismsrelease theirgametesintothe environment.
 These organismsare facedwithtwoproblems:How can spermandeggsmeetinsuch a dilute concentration,
and howcan spermbe preventedfromtryingtofertilizeeggsof anotherspecies?
 Two majormechanismshave evolvedtosolve theseproblems:
 speciesspecificattractionof spermandspecies-specificspermactivation.
Sperm attraction:
 One chemotacticmolecule,called resact,hasbeenisolatedfrom the eggjellyof the seaurchin Arbacia
punctulata .
 Resact diffusesreadily in seawaterandhasa profoundeffectatverylow concentrationswhenaddedtoa
suspensionof Arbacia sperms.
 Resact isspecificforA.punctulata and doesnotattractspermof other species. A.punctulata spermhave
receptorsin theirplasmamembranesthatbindresact and can swimupa concentrationgradientof this
compounduntil theyreachthe egg.
The acrosomal reaction in sea urchins
 Acrosomal reactioninseaurchinsis initiatedbycontactof the spermwiththe eggjelly.
 In sea urchins,the acrosomal reactionisthoughttobe initiatedbyafucose-containingpolysaccharide inthe
eggjellythatbindstothe spermandallowscalciumto enterintothe spermhead.
 In most marine invertebrates,the acrosomal reactionhastwocomponents:
 the fusionof the acrosomal vesicle withthe spermplasmamembrane and
 the extensionof the acrosomal process

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Species-specificrecognitioninseaurchins
 Once the sea urchinspermhas penetratedthe eggjelly,the acrosomal processof the spermcontactsthe
surface of the egg. A majorspecies-specificrecognitionstepoccursat thispoint.The acrosomal protein
mediatingthisrecognitioniscalled bindin.
 it iscapable of bindingtodejelliedeggsof the same species.
 Thus,species-specificrecognitionof seaurchingametesoccursat the levelsof spermattraction,sperm
activation,andspermadhesiontothe eggsurface.
 Once the sea urchinsperm has penetratedthe eggjelly,the acrosomal processof the spermcontactsthe
surface of the egg.
Gamete Fusionand the PreventionofPolyspermy
1) Fusionof the eggand spermplasma membranes
o Recognitionof spermbythe vitellineenvelope is followedbythe lysisof thatportionof the envelope of the
spermheadby the acrosomal enzyme.Thislysisisfollowedbythe fusionof the spermplasmamembrane
withthe plasmamembrane of the egg.
2) The preventionofpolyspermy
o The fast blockto poly-spermyisachievedby changingthe electricpotential of the eggplasmamembrane.
o Thismembrane providesaselective barrierbetweenthe eggcytoplasmandthe outside environment,and
the ionicconcentrationof the eggdiffersgreatlyfromthatof itssurroundings.
o Within1 3 secondsafterthe bindingof the firstsperm, the membrane potentialshiftstoapositive level,
about+20 mV . This change iscausedby a small influx of sodiumionsintothe egg.
3) Slow block to polyspermy
o The slowblock to polyspermy.
o Directlybeneaththe seaurchineggplasmamembrane are about15,000 cortical granules.
o Upon spermentry,these cortical granulesfuse withthe eggplasmamembrane andreleasetheircontents
intothe space betweenthe plasmamembrane andthe vitelline envelope proteins.

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 Several proteinsare releasedbythiscortical granule exocytosis.The firstare proteases.Theseenzymes
dissolve the proteinpoststhatconnectthe vitellineenvelope proteinstothe cell membrane,andtheyclip
off the bindinreceptorandanyspermattachedto it .
 Mucopolysaccharidesreleasedbythe cortical granulesproduce anosmoticgradientthatcauses
 waterto rush intothe space betweenthe plasmamembraneandthe vitelline envelope,causingthe
envelopetoexpandandbecome the fertilizationenvelope.A thirdproteinreleasedbythe cortical granules,
a peroxidase enzyme,hardensthe fertilizationenvelope.
 Finally,afourthcortical granule protein,hyalin,formsacoatingaroundthe egg .
 The egg extendselongatedmicrovilli whosetipsattachto this hyaline layer.This layer providessupport for
the blastomeresduringcleavage.
Fusionof genetcmaterial
 In seaurchins,the spermnucleusentersthe eggperpendiculartothe eggsurface.Afterfusionof the sperm
and eggplasmamembranes,the spermnucleusanditscentriole separate fromthe mitochondriaandthe
flagellum.The mitochondriaandthe flagellumdisintegrate insidethe egg,soveryfew,if any,sperm-derived
mitochondriaare foundindevelopingoradultorganisms.
 Thus,althougheachgamete contributesahaploidgenome tothe zygote,the mitochondrial genome is
transmittedprimarilybythe maternal parent.
 Afterthe seaurchinspermentersthe eggcytoplasm, the male pronucleusrotates180° sothat the sperm
centriole is betweenthe spermpronucleusandthe eggpronucleus.The spermcentriole thenactsasa
microtubule organizingcenter,extendingitsownmicrotubulesandintegratingthemwitheggmicrotubules
to forman aster.
 These microtubulesextendthroughoutthe egg andcontact the female pronucleus,andthe twopronuclei
migrate towardeach other.
 Theirfusionformsthe diploid zygote nucleus.
The Activation ofEgg Metabolism
 Althoughfertilizationisoftendepictedasmerelythe meanstomerge twohaploidnuclei,it hasanequally
importantrole ininitiatingthe processesthatbegindevelopment.These eventshappenincytoplasmand
occur withoutthe involvementof the parental nuclei.
 The release of calciumionsthatoccurs whenthe spermentersthe eggiscritical for activatingthe egg’s
metabolismandinitiatingdevelopment.
 Calciumionsrelease the inhibitorsfrommaternallystoredmessages,allowingthese mRNAstobe translated.
 Theyalsorelease the inhibitionof nucleardivision,therebyallowingcleavagetooccur.
IP3 Pathway
 The membrane phospoholipidphosphotidylinositol4,5-bisphosphate issplitbythe enzyme phospholipaseC
to yieldtwoactive compounds i.e IP3(inositol 1,4,5-triphosphate)andDAG(diacylglycerol).
 IP3 isable to release calciumionsintocytoplasmbyopeningthe calciumchannelsof endoplasmicreticulum.
 DAG activatesproteinkinase Cwhichinturnactivate Na+/H+exchange pump.
 These processesresultinthe liberationof Ca2+ and the alkalinizationof eggandtherefore areinvolvedinthe
developmentof egg.

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CLASSIFICATIONOF EGGS
The Structure of a typical Ovum
• Ovumis the female gamete.ltstoresfoodrequiredforthe entireprocessof developmentinthe formof
yolk.lthas three importantf unctions:
1. lt suppliesahaploidsetof chromosomestothe future embryo.
2. lt contributesalmostall cytoplasmtothe zygote.
3. lt suppliesfoodtothe developingembryo
Shape and Size
• Typically,the eggsare spherical orovoidinshape.Butina few animalslike insects,the eggsare elongated
and cylindrical .Eggsare generallylargerthanthe spermsandaverage somaticcells.
The egg iscoveredexternallybyaplasmamembrane orplasmalemma.Withinthe plasmamembraneisthe
granularcytoplasm.
Organization of Egg Cytoplasm
• The cytoplasmof egg is knownasooplasm.ltisgranular andcontainsin additiontothe usual cellular
organellescertainotherinclusionsIikeyolkandcortical granules.The peripherallayerof ooplasmismore
viscousandgelatinous.ltis knownasthe eggcortex whichis providedwithmanymicrovilli andcortical
granules.The microvilli are formedbythe outpushingsof the plasmalemmaandtheyhelpintransportation
of substancesfromthe outside intothe ooplasmduringthe developmentof egg.
• Theyare membrane boundandare formed fromgolgi complex.Theycontainhomogeneousandgranular
polysaccharides.Cortical granulesare presentinthe eggsof seaurchins,frogs,fishes,bivalvemolluscs,
several annelidsandcertainmammals.
Yolk:
• Nutritive substancesare storedinthe cytoplasmof egginthe form of yolk. Thisstoredfoodis utilizedbythe
embryoforits earlydevelopment.The processof formationof yolkisknownasvitellogenesis.The yolkisa
complex material consistingof proteins,fats,carbohydratesetc.The yolkmaybe called"proteinyolk"when
it hasmore proteinsthanlipids,or" fattyyolk"whenithas more fat contentsthanthe proteins.Most
animal eggscontainbothkindsof yolk.
ClassificationofEgg
• 1. On the Basis of the Amountof yolk
• 2. On the Basis of the distribution of yolk

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• 3. On the basisof potentialitiesof eggforfurtherdevelopment
Classificationofegg on the basisof amount of yolk
• Eggs are groupedintofourtypesonthe basisof the amountof yolk presentinthem.
a. Alecithal egg
b.Microlecithal egg
c.Mesolecithal egg
d.Macrolecithal egg
1) Alecithal egg:
 Whenthe egg contains negligibleamountof yolk,itis calledalecithal egg.
Eg. The eggsof Eutherianmammals
2)Microlecithal egg:
 Whenthe egg containsmall amountof yolkit issaidto be microlecithal.
 Romerand Balinskynamedthese eggsasoligolecithal eggs.
 Eg.Amphioxus,Tunicates
3)Mesolecithal egg:
 Whenthe amountof yolkpresentismoderate andisnothigh,these eggsare alsonamedas mesolecithal
eggs
 e.gamphibian,Dipnoi ,xenopus andPetromyzon
4)Macrolecithal egg:
 Whenthe egg containslarge amountof yolkit issaidto be macrolecithal ormegalecithal egg.
 It isalso calledPolylecithal egg.
 Eg. bonyfishes'amphibians,reptilesandbirds. Etc
Alecithal egg microlecithal egg mesolecithal egg macrolecithal egg
On the Basis of the distribution of yolk
a. Isolecithal egg
b. Telolecithal egg
c. Centrolecithal egg
a. Isolecithal orHomolecithal Egg:
 lnisolecithal eggs,the verylittleamountof yolkpresentisuniformlydistributedthroughoutthe ooplasm
 (e.gechinoderms,Amphioxus,mammals).Thisconditionisusuallyobservedineggswithverylittleamount
of yolk.

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b. Telolecithal Egg:
 In eggscontainingmoderate orlarge quantityof yolk,the distributionof yolkisnotuniform.ltis
concentratedmore towardsthe vegetal pole.Suchatype of egg,in whichthe yolkisconcentratedtowards
one pole,is calledtelolecithal egg.
 Telolecithal eggsmayfurtherclassifiedintothree types
1. SlightlyTelolecithal This type of egg containsonlya small quantityof yolkwhich isdistributedunevenly.
The vegetal pole hasthe highestconcentrationandthe animal pole the lower(e.g.eggsof fishes).
2. ModeratelyTelolecithal egg Thistype of eggcontainsa moderate quanilty of yolkwhichis
Distributed unevenly.Due tohighconcenteration of yolk inthe vegetal hemisphere,the nucleusis
shiftedmore towardsthe animal hemisphere (eg.amphibianegg).
3. ExtremelyTelolecithalEgg lnthistype of egg,due to the heavydepositionof yolk,the entire vegetal
hemisphereandamajor portionof the animal hemisphere are occupiedbyyolk.Due tothisextremely
unevendistributionof yolk,the ooplasmandnucleusare displacedtowardsthe animal pole(e..g
reptilianandavianeggs).
c. Centrolecithal egg:
• Egg of manyarthropodsand some coelenteratesare describedascentrolecithal.Theyare relativelylarge
and elongate andhave averygreat amountof yolk.The nucleusliesatthe geometriccentre of the yolk
mass,surroundedbya small amountof cytoplasm.A thincytoplasmiclayercoversthe surface of the yolk.
Fine strandsof cytoplasmextendfromthe peripheral layertothe zone occupiedbythe nucleus.
On the basis ofpotentialitiesforfurther development
1)Mosaic Egg:
ln certaineggs,everyportionispredeterminedwithrespecttoitspotentialitiesforfurtherdevelopment
lf a small portionof suchan eggis removed,adefectiveembryoisformed,Thisisbecause removalof aportion
resultsina permanentlossfromthe egg.The remainingportionof the eggcannotmake compensatorydevelopment
for the lostpart. Suchan egg, iscalledmosaicor determinateegg(e.gannelids,Molluscs).
2)Regulative Egg
lnvertebratesandmostof the invertebrates,the developmentalpotentialitiesare notpredeterminedinthe eggs.
Removal of a small portionof the egg,or evenone or twoearlyblastomereswill notaffectthe normal development.
Thistype of eggin whichthe future developmentalpotentialitiesare notpredeterminedisknownasregulativeor
indeterminate egg.

15. P a g e | 15
Cleavage
Cleavage isarapid seriesof mitoticdivisionsthatoccur justafterfertilization.
There are twocritical reasonswhycleavage issoimportant:
 Generationof a large numberof cellsthatcan undergodifferentiationandgastrulationtoformorgans.
 Increase inthe nucleus/cytoplasmicratio.Eggsneeda lotof cytoplasmtosupportembryogenesis.Itis
difficultorimpossibleforone nucleustosupportahuge cytoplasm, andoocytesare one of the largestcells
that exist.One small nucleusjustcannottranscribe enoughRNA tomeetthe needsof the huge cytoplasm.
 A largernucleustocytoplasmicratioisoptimal forcell function.Celldivision occursrapidlyafterfertilization
to correct thisproblem.
Cleavage differsfromnormal mitosesin 2 respects
1. Blastomeresdonotgrowin size betweensuccessivecell divisionsastheydoinmostcells.Thisleadstoa
rapidincrease inthe nucleus/cytoplasmicratio.Cellsundergoingcleavagehave mainlySandM phasesof
the cell cycle (little ornoG1 or G2).
2. Cleavage occursveryrapidly,andmitosisandcytokinesisineachroundof cell divisionare completewithin
an hour.Typical somaticcellsdivide muchmore slowly(several hourstodays) andeventhe fastestcancer
cellsdivide muchslowerthanoccursin a zygote duringcleavage.
Cleavage differsindifferenttypesof eggs.The presence of large amountsof yolkaltersthe cleavagepattern,leading
to incomplete cleavage thatcharacterizesbirdsandreptiles.
Eggs are classifiedby how much yolk ispresent
1. Isolecithal eggs(iso=equal) have asmall amountof yolkthatis equallydistributedinthe cytoplasm(most
mammalshave isolecithal eggs).
2. Mesolecithal eggs(meso=middle) have amoderate amountof yolk,andthe yolkispresentmainlyinthe
vegetal hemisphere (amphibianshave mesolecithal eggs).
3. Telolecithal eggs(telo=end) have a large amountof yolkthatfillsthe cytoplasm, exceptforasmall area
nearthe animal pole (fish,reptiles,andbirds).
4. Centrolecithal eggshave alotof yolkthatis concentratedwithinthe centerof the cell (insectsand
arthropods).
The pattern of cleavage of the zygote dependsuponthe pattern of yolk distribution
1. Holoblasticcleavage:occursinisolecithal
eggs(mammals,seaurchins).The entire egg
iscleavedduringeachdivision.
2. Meroblasticcleavage occurswheneggs
have a lot of yolk.The egg doesnotdivide
completelyateachdivision.Twotypes:
Discoidal cleavage islimitedtoa small
discof cytoplasmat the animal pole.All
of the yolkfilledcytoplasmfailsto
cleave (characteristicof telolecithal
eggssuch as birds).
Superficial cleavage islimitedtoa thin
surface area of cytoplasmthatcovers
the entire egg.The inside of the egg
that isfilledwithyolkfails tocleave
(centrolecithal eggssuchasinsects).

16. P a g e | 16
Sea UrchinsHave Isolecithal Eggs And UndergoHoloblastic Cleavage
 Cleavage plane:thisisthe plane inwhichcleavageoccurs.Itis orientedatrightanglestothe metaphase
plate.Inseaurchins,the firstcleavage ismeridional.
 Meridional cleavage runsfromone pole toanother(toptobottom),like the meridianonaglobe.
 The secondcleavage isalsomeridional.
 Equatorial cleavage encirclesthe zygote like the equatoronthe globe.The thirdcleavage inthe seaurchinis
equatorial.Thiscreatesananimal andvegetal half.
 The fourthcleavage isunique.Equal cytokinesisoccursinthe four blastomeres of the animal pole,givingrise
to 8 mesomeres(allthe same size).
 Unequal cytokinesisoccursinthe vegetal pole.Thiscauses4large macromeresand4 small micromeres
 The 5th
cleavage ismeridional.All mesomeresdivideequallyasdothe macromeres.
 As cleavage progresses,all blastomeresadhere atthe outersurface,butattachmentislostat the inner
surface.The blastocoel isacavityformeddue to the unequal adherence of blastomeres.
Amphibianeggs have moderate amount of yolk,however,theyare still able toundergoholoblasticcleavage.
The 1st cleavage ismeridional,asisthe 2nd.The 3rd
cleavage isequatorial.The cleavageisdisplacedtowardthe
animal pole due tothe yolk.Thisresultsin4 small animal blastomeresand4 large vegetal blastomeres.
Morula (morum= mulberry) atthe 16 to 32 cell stage the embryoiscalledamorulabecause itlookslike amulberry.

17. P a g e | 17
The blastocoel is displacedto the animal pole in
amphibians
 Blastula= fromthe 128 cell stage onwardthe
amphibianembryois ablastula.The outersurface of
the amphibianblastulahascellsconnectedby
specializedcell junctions.
 Tightjunctionscreate a seal thatisolatesthe outside
of an embryofromthe innerlayer.Tightjunctions
polarize the apical andbasal surfaces.The basal
portionsof cellsstartsecretingintothe blastocoel.
Desmosomesattachthe
blastomerestogetheronthe
outside.
 Gap junctionsconnectall
surface blastomeres.
Mammalian eggshave rotational
cleavage that is holoblastic
 The mammalianeggisa little
slow.Itbeginstocleave in
the oviductand continues
until itimplantsinthe wall of
the uterus(1 cleavage /24
hr).
 Asynchronouscleavage:
mammalianembryosare
unusual inthat theyhave
asynchronouscleavage.Not
all blastomeresdivideatthe
same time.
 The firstcleavage ismeridional,andthe secondcleavage isrotational.The 2 blastomeresdivideindifferent
planes(one isequatorial andone ismeridional.
Mammalian embryosundergo compaction at the 8 cell stage
 At first,the blastomeresof mammalianembryoshave aloose arrangement,andtouchonlyatthe basal
surfaces.
 Aftercompaction,blastomeresadheretightly,maximizingthe areaof contact.
 Duringcompaction,eachblastomere undergoespolarization.Tightjunctionsdeveloponthe outersurface,

18. P a g e | 18
 allowingproteinstospecialize.Cellstake upfluidsfromthe uterine environmentand Secrete intothe
blastocoel.
 Gap junctionsformonthe outercellstoaidin intercellularcommunication.
A blastocoel developsas cleavage proceedsto the 32-64 cell stage
 Aftercompactionat the 8-16 cell stage,there are 2 typesof blastomeres.Outsideblastomeresare tightly
joinedandnumberabout9-14. Theysurround2-7 inside blastomeres thatare looselyjoined.
 Cavitation:the outside blastomeresstarttotake up fluidfromthe uterusandpumpit intothe center,
creatingthe blastocoel.The blastocystisthe characteristicof earlyembryonicdevelopmentinmammals.
 Innercell mass:this givesrise tothe embryo,anddevelopsfromthe insideblastomeres
 Trophoblast:a structure consistingof outside blastomeres,thiscontributestoformingthe placenta.
Embryonic stem cellscan be
culturedfrom the innercell mass
 Cellsinthe innercell mass
are undifferentiated,they
multiplyindefinitely,andare
knownas embryonicstem
cells.Stemcellsare
totipotent=theyhave the
potential toformanytissue.
These cellsare of great
scientificandmedical
importance.
 Theycan be removedfrom
the embryo,genescanbe
introducedintothe cells,and
thentheycan be placedback in
the blastocyst.Thisishowone
constructstransgenicor “knock
out” mice.
 The embryonicstemcellsare
alsousedto grow certaintypes
of tissue inculture.
Theoretically,itshouldbe
possible togrowstructures
such as ears,muscles,nerves,
and skinfortransplantationto
sickindividuals.
 Interestingly,if youinject
adult,differentiatedcellsback
intothe environmentof the
morulaor blastula,they
become undifferentiated,and
theycan redifferentiate to
formmany parts of the body.
Developmentofmonozygoticor identical twins
Monozygotictwinsdevelopfromone zygote bysplittingatvariousstagesof development(fromthe 2cell to the
blastocyststage).
The stage of splittingeffectsthe overallstructure of the embryoandextra embryonicmembranes.Conjoinedtwins
• are identical twinswhodevelopfromasingle fertilizedovum.
• are alwaysthe same sex and race.

19. P a g e | 19
• are more oftenfemale thanmale,ataratio of 3:1.
• occur once in 40,000 births but onlyonce in200,000 live births.
• maybe causedbyany numberof factors,beinginfluencedbygeneticandenvironmentalconditions.
Gastrulation in mammals
Mammalian eggshave rotational cleavage that is holoblastic
The mammalianeggisa little slow.Itbeginstocleave in
the oviductand continuesuntil itimplantsinthe wall of
the uterus(1 cleavage /24 hr).
Asynchronouscleavage:mammalianembryosare
unusual inthat theyhave asynchronouscleavage.Notall
blastomeresdivideatthe same time.
The firstcleavage ismeridional,andthe secondcleavage isrotational.The 2 blastomeresdivideindifferentplanes
(one isequatorial andone ismeridional.
Mammalian embryosundergo compaction at the 8 cell stage
At first,the blastomeresof mammalianembryoshave aloose arrangement,and
touch onlyat the basal surfaces.

20. P a g e | 20
Aftercompaction,blastomeresadheretightly,maximizingthe areaof contact.Duringcompaction,eachblastomere
undergoespolarization.
Tightjunctionsdeveloponthe outersurface,allowingproteinstospecialize.
Cellstake upfluidsfromthe uterine environmentand Secrete intothe blastocoel.
Gap junctionsformonthe outercellstoaidin intercellularcommunication.
A blastocoel developsas cleavage proceedsto the 32-64 cell stage
Aftercompactionat the 8-16 cell stage,there are 2 typesof blastomeres.Outside
blastomeresare tightly joinedandnumberabout9-14. Theysurround2-7 inside
blastomeresthatare looselyjoined.
Cavitation:the outside blastomeresstarttotake up fluidfromthe uterusand
pumpit intothe center,creatingthe blastocoel.The blastocystisthe
characteristicof earlyembryonicdevelopmentinmammals.
Innercell mass:thisgivesrise tothe embryo,anddevelopsfromthe inside
blastomeres
Trophoblast:a structure consistingof outside blastomeres,thiscontributesto
formingthe placenta.
Beginningof implantation
The blastocystliesfree inthe uterine secretionsforabout2 days.
On the 6th
dayof developmentthe zonapellucidadisappearsandthe conceptusbeginstoimplantinto
endometrium.
The trophoblastbegintodifferentiate intotwolayers
i.the outerlayercalledsyncytiotrophoblast, consistsof multinucleatedprotoplasmicmassinwhichnocell
boundariescanbe distinguished
Ii .the innerlayer, termedcytotrophoblast, ismade upof mono-nucleatedcellswithdistinctboundaries
The syncytiotrophoblastpossessactive invasivepropertiesbecause itsecrete hydrolyticenzymes
Completionofimplantation
On day7 the blastocystbecome tightlyadherenttothe epitheliumof the endometriumdue toinvasive activityof
the syncytiotrophoblast.
The cells of endometriumrespondbydifferentiatingintolarge polyhedral cells,whichare ladenwithlipidsand
glycogen.
The endometriumisnowreferredtoasdecidua.
As implantationprogresses,the expandingsyncytiotrophoblastgraduallyspreadsoverthe outer surface of the
blastocystandby day9 the blastocystisalmostcompletelycoveredbyathicklayerof syncytiotrophoblast.
Formation of bilaminargerm disc
Withthe beginningof the secondweek,the cellsof the embryoblastbecome differentiatedintotwo layers,
Epiblastand hypoblast
Formationof amnioticcavity
Developmentof primaryyolksac

21. P a g e | 21
Gastrulation
Gastrulationinmammalsbeginswiththe appearance of primitivestreakonthe dorsal surface of the embryonicdisc
On day15 of developmentthe primitive streakcanbe seeninthe longitudinal midline of the caudal partof the
embryonicdiscasa narrowgroove withslightlyelevatedmargins.
These elevationsare producedbyproliferationandaccumulationof epiblasticcells.
The groove withinthe primitivestreakiscalledprimitive groove.
By the 16th
day a small circularswellingappearsatthe cranial endof the primitive streak.Thisswellingcalled
primitive node,showsasmall depressioncalledprimitive pit.
At itscranial end,the primitive groove become continuouswiththe primitive pit.
Afterformationof the primitivestreakandappearance of the primitivegroove,the epiblasticcellssurroundingthe
primitive streakbegintoproliferate,flattenandlose theirconnectionswitheachother.
These epiblasticcellsmigrate towardsthe primitive streak
As the primitive grooveisarrived,thesecellsslipthroughthe groove intospace betweenthe epiblastand
hypoblast.thisinwardmovementof the epiblasticcellsiscalledinvagination.
Those epiblasticcellswhichenterthe cranial partof the primitive streakinvade the hypoblastanddisplace itscells,
so that eventuallythe hypoblastbecomescompletelyreplacedbyanew layerof cellscalleddefinitive endoderm.
The definitive endodermformsthe roof of the yolksac.
Otherepiblasticcellswhichinvaginatethroughthe primitive streakcome tolie betweenthe definitive endoderm
and the epiblast.These cellsspreadlaterallyandforwardstoforma new layerknownasembryonicmesodermalso
calledintraembryonicmesoderm.
Afterformationof the embryonicmesoderm, thoseepiblastic
cellswhichremainonthe dorsal surface of the embryonicdisc
nowconstitute embryonicectoderm.
Under the embryonicmesodermliesthe definitive endoderm
whichisnowreferredtoas embryonicendodermorsimply
endoderm.
Thus,the two-layered(bilaminar) embryonicdisc(consisting
of epiblastandhypoblast) hasnowbecome three-layered(trilaminar).
The three layersof the trilaminarembryonicdisc(ectoderm, mesodermandendoderm) are calledprimarygerm
layers.All tissuesandorgansof the humanbodywill derive fromthesethree layers.

22. P a g e | 22
Formation of notochord
Developmentof notochordconsistsof twoevents
1. Formationof notochordal process
2. Formationof notochordfrom the notochordal process
Formation of notochordal process
The epiblasticcells,whichenterthe primitive pitgive rise toamesodermal tube betweenthe ectodermand
endoderm;thistube iscallednotochordal process.
The tubularnotochordal processgrowsinlengthas the cellsproliferatinginthe regionof the primitivenode are
addedat the proximal endof the tubularprocess.
Formation of notochord
By the 18th
day of developmentthe floorof the notochordal processfuseswiththe underlyingendoderm.The fused
layersoondegenerate anddisappear,resultinginthe lossof floorof the notochordal process.
The openingatthe primitive pitnowtemporarilyconnectsthe amnioticcavityandyolksac,this communicationis
calledneuroentericcanal
Afterthe disappearance of the floor,the roof of the notochordal processformsaflattenedmid-ventral barof
mesoderm,callednotochordal plate,whichliesinthe roof of the yolksac intercalatedbetweenthe endodermal
cells.Lateron the notochordal plate detachesfromthe endodermandgraduallyrollsuptoforma soldchord of cells,
whichiscallednotochord.

23. P a g e | 23
Whenthishappensthe endoderminthe roof of the yolksac becomes continuous once againandnogap remainsin
the endodermal liningof the yolk sacroof.
Neurulation
Duringthe 3rd
weekof development,the notochordinducesthe overlyingectodermtodifferentiateintoathickplate
of pseudo-stratifiedcolumnarneuro-epithelial cells. Thisplate iscalledneural plate andthe ectodermformingthis
plate isknownas neuroectoderm. The restof the ectodermwhichliesaroundthe neural plate isnow referredtoas
surface ectoderm.
By the endof the 3rd
weekof development,the neural plateinvaginatesalongitscentral axistoforma groove called
neural groove.The foldsformingthe sidesof the groove are termedneural folds.The edge of eachfoldisknownas
neural crest.
Duringthe 4th
weekthe neural groove deepens,the neuralfoldsbecome concave androtate aroundthe neural
groove so thattheirlateral lipsmeetdorsallytoformatubularstructure,whichiscalledneural tube.
As the lipsof the neural tube fuse,the junctionbetweenthe neuroectoermandthe adjacentsurface ectodermis
pulleddorsallyandthe opposingmarginsof the surface ectodermalsomeetandfuse.Due tothisfusion,the neural
tube separatesfromthe surface ectoderm.
Neurulation
Neurulation is a part of organogenesis in vertebrate embryos.
Stepsof neurulationincludethe formationof the dorsalnervecord,andthe eventualformationof the centralnervous
system.
The process begins when the notochord induces the formationof the central nervous system (CNS) by signaling the
ectoderm germ layer above it to form the thick and flat neural plate.

24. P a g e | 24
The neural plate foldsinuponitself toformthe neural tube,whichwill laterdifferentiate intothe spinalcord andthe
brain, eventually forming the central nervous system.
Primary neurulation
Induction
Primary neurulation occurs in response to
soluble growth factors secreted by
the notochord.Ectodermal cellsare induced
to form neuroectoderm from a variety of
signals.Ectodermsendsandreceivessignals
of BMP4 (bone morphogenic protein) and
cellswhichreceiveBMP4signal developinto
epidermis.
The inhibitory
signals chordin, noggin and follistatin are
needed to form neural plate.
These inhibitory signals are created and
emitted by the notochord. Cells which do
not receive BMP4 signaling due to the
effects of the inhibitory signals will develop
intothe anteriorneuroectodermcellsof the
neural plate.
Shape Change
The cells of the neural plate are signaledto
become high-columnarandcanbe identified
through microscopy as different from the surrounding epiblastic ectoderm. The cells move laterally and away from
the central axis and change into a truncated pyramid shape.
Folding
The process of the flat neural plate folding into the cylindrical neural tube is termed primary neurulation
As a result of the cellular shape changes, the neural plate forms the medial hinge point (MHP). The expanding
epidermisputspressure onthe MHP and causesthe neural plate to foldresultingin neural foldsand the creationof
the neural groove. The neural folds meet and fuse at the midline.
The folding of the neural tube to form an actual tube does not occur all at once.
The lateral edgesof the neuralplate touchinthe midline andjointogether.Thiscontinuesbothcraniallyandcaudally.
The openings that are formed at the cranial and caudal regions are termed the cranial and caudal neuropores. In
human embryos, the cranial neuropore closes approximately on day 25 and the caudal neuropore on day 27.

25. P a g e | 25
Secondary Neurulation
In secondary neurulation, the neural ectoderm and some
cells from the endoderm form the medullary cord. The
medullary cord condenses, separates and then forms
cavities. These cavities then merge to form a single tube.
Secondary Neurulation occurs in the posterior section of
mostanimalsbutitisbetterexpressedinbirds.Tubesfrom
both primary and secondary neurulation eventually
connect.
Early brain development
The anterior segment of the neural tube forms the three
main parts of the brain: the forebrain, midbrain, and the hindbrain. Formation of these structures begins with a
swelling of the neural tube. Ion pumps are used to increase the fluid pressure within the tube and create a bulge.A
blockage betweenthe brainandthe spinalcordpreventsthe fluidaccumulationfromleakingout. Thesebrainregions
furtherdivideintosub-regions.Theneuraltube becomesthe germinalneuro-epitheliumandservesasasource of new
neurons during brain development.
Non-neural ectoderm tissue
Mesoderm surrounding the notochord at the sides will develop into the somites (future muscles, bones, and
contributes to the formation of limbs of the vertebrate).
Neural crest cells
Masses of tissue called the neural crest that are located at the very edges of the lateral plates of the folding neural
tube separate from the neural tube and migrate to become a variety of cell populations, including the cells of the
peripheral nervous system.