TITLE--GAMETOGENESIS

FORCLASS--BScSEMESTER4

ByK.K.SHUKLA,AsstProfessor,

DepartmentofZoology,

JNPGCOLLEGE,LUCKNOW

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GAMETOGENESIS

Formationofembryoanditsdevelopmentinto

youngonesiscalledembryology.Everyaniamal

haslifetime(periodbetweenbirthanddeath).So

to continue its race each animalreproduce

offsprings like him.There are two type of

reproduction in animals;asexualand sexual

reproduction.

ASEXUALREPRODUCTION

Onlysingleparents.Nomaleandfemaleparents

EXAMPLE--Fission,Budding,Gemuleformation

SEXUALREPRODUCTION

Two parents,onemalewithmalegonadand

otherfemalewithfemalegonad.Malegonador

testis produce male gamete or

spermatozoa.Female gonad orovaryproduce

femalegameteorovum

Hermaphroditeormonoecious--Maleandfemale

gonadsfoundinsameanimale.gEarthworm

Dioecious--Bothmaleandfemalegametesare

producedbydifferentindividuals.

GAMETOGENESIS--Productionofgametes

Spermatogenesis--Formationofspematozoa

Oogenesis---Formationofovum

Body cells are diploid.Germ cells produce

gameteswhicharehaploid.Maleandfemale

gametesfusedtogethertoform zygote(diploid)

and zygote develop into embryo.Coming

togetherandfusionofmaleandfemalegamete

iscalledfertilization.Sodiploidparentsproduce

haploidgametesthroughmeiosis.

Maleandfemaleindividualsaredifferentiated

from eachotherbyecrtaininternalandexternal

characterscalledprimaryandsecondarysexual

characters.

PRIMARY SEXUAL CHARACTER-Testis and

sperms.

SECONDARY SEXUAL CHARACTER--Accessory

sex organs,rudimentary breast,low pitched

voice,abundantbodyhairs

SPERMATOGENESIS

OCCURINTESTISANDHASTWOSTAGES

1-Formationofspermatids

2-Spermiogenesis

FORMATIONOFSPERMATIDS-THREEPHASES

1-Multiplcationphase

2-Growthphase

3-Maturationphase

Testishasseminiferoustubuleswhichhasgerm

cells and sertolicells and in between the

seminiferoustubulesinterstitialcellorcellsof

Leydigarefound.

Seminiferous tubules-lined by PGC--Produce

sperm

Sertolicells---Nourishspermatozoa

Interstitialcells---Producehormone

PGC--------------MITOSIS-----SPERMATOGONIA---MITOSI

S----PRIMARY

SPERMATOCYTES--------MEIOSIS--------SECONDARY

SPERMATOCYTES----MEIOSIS 2

-------SPERMATID----SPERMIOGENESIS--------SPERMAT

OZOA

SPERMIOGENESIS

PROCESSOFGROWTHANDDIFFERENTIATION

OFSPERMATIDINTOSPERMATOZOA

CHANGESDURINGSPERMIOGENESIS

1-Nucleus looses water,nucleolus and RNA.

Shape of nucleus becomes elongated from

sphericaldependingupontheshapeofheadof

sperm

2-Golgibodieschangestoform acrosomewhich

getlocatedattipofsperm head

3-Centrioleisdividedintoproximalanddistal

centriole.The formergetlodged in to the

concavityofnucleusandlaterform theaxial

filamentoftail

4-Mitochondriagetconcentratedinthemiddle

pieceofsperm.

STRUCTUREOFSPERM

SIZE---40uTO12mm

SHAPE--Variable. spherical in teleosts, rod

shaped in amphibians,spoon shaped in man

andothermammals,hookshapedinmouseand

rat

Spermatozoa consists ofhead,neck,middle

pieceand tail.Head consistsofnucleusand

acrosomeatitstip.Acrosomeissurroundedby

acrosomalmembranewhichcontainsproteases

andsomepolysacchrades.Coneshapedaxial

cone oracrosomalcone is found between

acrosome and nucleus in some animals eg

Nereis,cricket,fowl. Neckhastwocentriole.

Middlepiececontainmitochondria.Tailcontains

a centralfilamentcovered by a cytoplasmic

sheath.Atthetipofthetail,axialfilamentis

naked,Axialfilamentislikecilia.

Sperm may be motile ie flagellated e.g all

vertebratessomeinvertebrates.mostlysperm is

monoflagellated but in some animals eg

Opsanus(toadfish)itisbiflagellated.non-motile

ienon-flagellatedsperm e.gAscaris,Arthropods

OOGENESIS

Formationoffemalegametesorovum iscalled

oogenesis.Itoccurs in female gonad called

ovary. Most vertebrates have paired ovary

exceptbirdswhichhassingleovary.

SHAPEOFOVARY---VARIABLE

PISCES---Elongated

Amphibia---Lobular

REPTILESANDBIRDS---Bunchesofgrapes

Mammals----Flat,ovalorrounded

SIZEOFOVARY--40uto150u

Teleosts----Enormous

Human,Cow----Small

TYPESOFOVARY

Compactovary--

Found in cyclostomes, elasmobranchs,

osteichthes,fishes,reptiles,birdsandmammals

Ithas centralmedulla containing blood and

lymphvesselsandoutercortexcontainingovum

invariousstagesofdevelopment

A tunica albugenea or connective layer

surroundsthecortex.

Ovaryislinedbygerminalepithelium whichform

PGCorprimordialgerm cellsforoogenesis

Saccularovary

FoundinAmphibia

Ithascortexlikecompactovarybutmedulla

regioninsaccularovaryisrepresentedbylarge

lymphspaces

Histologyofovary

Ovaryhasthreekindsofcells

Germ cells-----produceova

Somaticcells---form generaltissueframework

forprotectionandnourishment

Endocrine cells---Produce sex

hormone---responcible for secondary sexual

characters

Primary sexualchracters---female gonad ie

ovaryandfemalegametesieovum

Secondarysexualcharacters

Broadeningofpelvis,developmentandlactation

ofmammary glands,Growth ofuterus and

vagina,growthofpubichairs,Changeinvoice

quality,onsetofmenstrualcycle

OOGENESIS

AnOvum cellcontainsyolk,glycogen,nucleic

acidandproteins

Aneggcellhasthreebasicfunction

1-Supplynucleustofutureembryo

2-Supplycytoplasm tofutureembryo

3-Supplynourishment tothefutureembryo

Soaneggcellisnonmotile,largesized,well

specialised,packagedandprogrammedcells.

Packaging---Growth of primary oocytes and

accumulationofyolkinit.

Programming of egg---Eggs contain

determinative and directionalinformation for

developmentoffutureembryo.

OOGENESIS---THREEPHASES

1-Multiplicationphase

2-Growthphase

3-Maturationphase

MULTIPLICATIONPHASE

Premordialgerm cells detach from germinal

epithelium ofovaryandenterincortexofovary

PGCbecomeseggmothercelloroogonia

Oogonia divide by mitosis to form primary

oocytes

GROWTHPHASE

Veryprolonged

Sizeofprimaryoocytesincreasesconsiderably

from 10udiameterofPGCto200udiameter.

Howeverinfrog diameterofprimaryoocytes

increasesupto2000uandinbirdsupto40000u.

Rateofgrowthvarriesfrom slowtofast.Infrog

ittakesthreeyear.

MODEOFGROWTH

Previtellogenesisgrowthperiod

Vitellogenesisgrowthperiod

PREVITELLOGENESISGROWTHPERIOD

Tremendousincreaseinvolumeofnucleusand

cytoplasm ofprimaryoocytesoccur

Nuclearsapincreasesandnucleusisfilledwith

fluid called germinalvesicles.Chromosomes

acquire characterstic shape called lampbrush

chromosomes.Eachlampbrushchromosomes

manufacturenumerousmRNA bytranscription

which are used in protein formation during

growth.HoweversomemRNA arewrappedin

protein and become inactive and stored as

informosomes which are used during early

clevageofeggwhenchromosomalDNAisbusy

initsownduplication.

NucleolusalsoincreasesgreatlyinnumberegIn

Triturus600,in Siredon 1000.Transcriptional

activityincreasesgreatly(geneamplification)

Quantity and quality ofcytoplasm increases

during previtellogenesis. Number of

mitochondria increases greatly and in

Amphibiansandbirdsmitochondriaaggregated

toform mitochondrialclouds.ERnumberalso

icreases. Golgi complex becomes well

developedandform corticalgranules.

Corticalgranulesaresmallmembranebound0.8

u to 2.0u in diameter containg

mucopolysacchrides. Cortical granules form

fertilization membrane.Corticalgranules are

foundinbivalvemolluscs,someannelids,fishes

frog and some mammal(rabbit,man).No

corticalgranuleingastropoda,insects,urodela,

birdsandmammals.

VITELLOGENESISGROWTHPERIOD

During this phase yolk is synthesised

(vitellogenesis). Vitellogenin controls the

vitellogenesis. Chemically yolk is

phospholipoproteins.Yolk may be protein or

fattyyolk.Itmaybesmallgranularorinform of

large yolk platelets.Yolk platelets may be

phosvitinorlipovitellin.

PHOSVITIN--Mol wt 35000, phosphorus--8.4

percent,

LIPOVITELLIN--Mol wt-40000, lipids- 17.5

percent.

Twomoleculeofphosvitinandonemoleculeof

lipovitellintogetherform oneyolkplatelet.

Vitellogenesisisextraovarian.Ininsectsitoccur

infatbodywhileinvertebratesinoccurinliver.

MATURATION

Simultaneously with vitellogenesis primary

oocytes undergo meiosis to form secondary

oocytes and a firstpolarbody.Secondary

oocytesandpolarbodydivideinto ootidand

polarbody.sooneprimaryoocytesproduces

oneootidandthreepolarbody.Duringgrowth

phase nucleus ofprimaryoocytes remain in

prolonged meiotic prophase till fertilization

when maturation resumes and meiosis

completed.

STRUCTUREOFOVUM ANDTYPESOFEGG

Eggisroundedorspherical,0.06to85mm in

size,protectedbymembranescalledprimary,

secondaryandtertiarymembranes.

Primary membranes---cellmembrane,vitelline

membrane,zonaradiata,zonapellucidaandjelly

coat.

Secondary membrane--chorion and corona

radiata. Chorion is found in insects and

ascidiansandformedbyfolliclecells.Chorion

hasmanyporescalledmicropilesthroughwhich

sperm enters. Corona radiata is found in

mammals formed by cells outside zona

pellucida.

Tertiary membranes-White albumen, shell

membraneandshell.Itissecreateddbyoviduct.

The ovum has cytoplasm which contains

cortical granules, yolk granules, pigments

besidescommoncellularorganelles.Nucleusis

shiftedtoonepolecalledanimalpole.opposite

poleiscalledvegetalpoleduetoconcentration

of yolk. So ovum has animal-vegetal

axis(Polarity).

Significanceofeggmembranes

1-Protectsovum

2-Preventpolyspermy

3-Helpinformationoffertilizationmembrane

Typesofegg

A-Amountofyolk

1-Alecithal-NoyolkegMammal

2-Microlecithal-LittleyolkegAmphioxus

3-Mesolecithal--ModerateyolkegAmphibians

4-Megaormacrolecithal-Largeamountofyolk

egFish,reptiles,birds

B-Distributionofyolk

1-HomoorIsolecithal-Yolkuniformlydistributed

egAmphioxus,echinodermata

2-Telolecithal-Heavyyolkatvegetalpole,less

yolkatanimalpoleegAmphibia,birds

3-Centrolecithal---Yolkincentereginsects

C-Protectivecover

1-Non-cledoic--Noprotectivecover.egglaidin

waterorinmotherwombegfish,amphimbia,

mammal

2-Cleidoic--With protective cover calcareous

shell.egglaidonlandegReptilesandbirds

D-Determinativeandindeterminativeegg

1-Determinateormosaicegg-Eachpartofeggis

destinedtoform particularpartofembryo.eg

Annelida,mollusca

2-Indeterminate egg-No predetermination eg

Amphibia,reptilesandbirds

COMPOSITIONOFYOLK

Water--48.7%

Phospholipidandfats--32.6%

Proteins--16.6%

Carbohydrate--1.0%

SEMINATION, OVULATION AND

TRANSPORTATIONOFGAMETES

SEMINATION--process of release of mature

sperm from testis

OVULATION--Process ofrelease ofova from

ovary

SEMINALFLUID--

Consistsofsperm andseminalplasma.Seminal

plasmacontainsenzymes,fructose,proteins.In

manymammalssementendstocoagulateafter

dischargefrom reproductivesystem.Inmouse,

rat,opossum etc semen coagulate to form

vaginalplugtopreventsemenfrom seepingout

of vagina. Vesiculase enzyme catalyse

formation ofvaginalplug.Fibrinogenase and

fibrinolysinpreventsemenfrom coagulation.

Oviparous--Egglayingmammalegcyclostomes,

mostfishes,amphibiansreptiles,birds

Ovoviviparous and viviparous--Egg layed in

femalereproductiveorganuterus

OVULATIONANDTRANSPORTATIONOFOVA

Dischargeofmatureovafrom thefolliclesis

calledovulation.

In amphibian after mauration the ovum is

releasedintothecoelomeandmovestothe

openingoftheoviductsenterintheoviductsand

getsurroudedbyjellycoatssecretedbyoviducts

andfinallyreleasedinwaateratthetimewhen

malefrogreleasedsperm duringfertilization.

In mammals graffian follicle enlarges and

mature ,moves close to the surface,which

bulges,a smallamoutofblood followed by

somefollicularfluidcontaingeggemergesfrom

follicle and released in coelome near

infundibulum of oviduct. After ovulation

collapsedfolliclesform corpusluteum.

Hormonalcontrolofovulation

FSH--Inducefolliculargrowth

LH--ActsynergisticallywithFSHcausefollicular

maturation,ovulationandformationofcorpora

lutea.

LTH--Responcible for milk secretion and

maintainCL

ESTROGEN--Stimulate pitutary to secrete

increasedamountofLTHandLHanddecreased

secretionofFSH.CLisformedinthiscondition.

PROGESTRONE--Corpora lutea produce

progestrone.LTHmaintainsCL.

function Hormone that

control

Folliculargrowth FSH

Ovulation LH

Estrogensecretion FSHANDLH

GrowthofseminiferoustubFSH

Spermatogenesis FSHandLH

Androgensecretion LH

CLgrowthandmaintainenance LTH

Milkejection,uteruscontractionOxytocin

FERTILIZATION

Fusionofmaleandfemalegamete

BASICREQUIREMENTFORFERTILIZATION

1-Medium egwaterorbodyfluids

2-Lifespanofgameteislimited,Sotheymust

contactwithinlifespan.

3-Numberofspermsmustbehigherthanthe

eggtoensurefertilization.

MECHANISM OFFERTILIZATION

Encounterofsperm andova

Capacitationandcontact

Acrosomereactionandpenetration

Activationofovum

Migrationofpronucleiandamphimixis

ENCOUNTEROFSPERM ANDOVA

Theprimaryrequirementforfertilizationisthat

maleandfemalegametecometogether.water

orbodyfluidisrequiredfrom movementofmale

gametetoreachfemalegamete.Maledeposit

the male gamete in the viscinity offemale

gamete.

CAPACITATIONANDCONTACT

Capacityofspermatozoatofertilizetheeggof

samespecies.FRLillie1919proposedfertilizin

theory which states that sperm produces

antifertilizinandovum producesfertilizinwhich

isspecificandactlikelockandkeyofenzyme

andsperm getagglutinatedwitheggofsame

species.

Chemically fertilizin glycoprotein or

mucopolysacchrideandeachspeciespossess

specifictypeoffertilizin.Itsmolwtis30000.

Antifertilizin is protein with molwt 10000.

fertilizin of one animalspecies react with

antifertilinofsamespeciesonly.

ACROSOMEREACTIONANDPENETRATION

Acrosomeofsperm producessperm lysin or

hyaluronidase which dissolve egg membrane

follicular cells corona radiata. Acrosome

changesmorphologically.from thebaseofthe

acrosomeafilamentgrowstowardstheegg.

Thisiscalledtheacrosomalfilament.Theegg

also producesaconicalprojection to receive

acrosomalfilament.Thisiscalledfertilization

cone.Cytoplasmiccontactisnow established

between sperm and egg.Onlythe head and

middlepieceentertheegg.insomeanimals

onlyheadandcentrioleenterswhileinmanand

manyothermammalswholespermsenterthe

egg.

ACTIVATION OF OVUM OR CORTICAL

REACTION

Assoonassperm entertheeggthevitelline

membraneoftheegggetliftedfrom thecell

membrane of the egg and a space called

perivitelline space is formed between cell

membrane and vitelline membrane. The

perivitellinespaceisfilledwithperivitellinefluid.

Thecorticalgranulesintheeggareejectedinto

theperivitellinespaceandgetattachedtothe

vitelline membrane and produce fertilization

membrane. This fertilization membrane

preventspolyspermy.

The fertilized egg starts consuming more

oxygen. The egg undergoes tremendous

chemical changes. Nucleus of the sperm

enlargestobecomesperm pronucleusandthe

eggnucleusbecomefemalepronucleus.

Migrationofpronucleiandamphimixis

The sperm nucleus on entry into the egg

cytoplasm rotatesby180degreetobringthe

centriole forward. Nucleus starts moving

towardsthecenteroftheeggcell.Thisiscalled

penetrationpath.Thenitmakesasuddenturnof

90 degree and then reaches the female

pronucleus.Thisnew pathiscalledcopulation

path.Thetwonucleifusedtogether(amphimixis)

toform zygote.

Usually only one sperm enters the egg

(monospermy)butinsomecasesegarachnids

andinsectsmorethanonesperm enterstheegg

calledpolyspermy.Thoughmanyspermsenter

theeggbutonlyonesuceedinfusingwiththe

femalepronucleus,othersdegenerate.

SIGNIFICANCEOFFERTILIZATION

1-Initiatesdevelopmentofembryo

2-Bringsaboutnewgeneticcombination

3-Resteoresdiploidcondition

4-Breaksdormancyofegg.