Gastrulation

Embryology

1. Gastrulation
Embryology
Bio 415 LECTURE 5

2. Embryo development stages
جاشملأا نيوكت(Gamete formation)
باصخلإا(Fertilization)
ةحقلالا نيوكت (توجيزلا()Zygote)
جلفتلا(Cleavage ) نيوكتوةجلفلماةلوتسلابلا وأ
نيطبتلا(Gastrulation) نيوكتةلورتساجلا
ي ضعتلا(Organogensis)ءاضعلاا نيوكت

3. Gastrulation and the three-layer formation
Theterms"gastrula"and"gastrulation"
werecoinedbyErnstHaeckel,inhis1872
work"BiologyofCalcareousSponges".

4. Characters of the gastrulation stage
Thegastrulaphaseischaracterizedbythefollowing:
1.Itisthefirststagesofcellulardifferentiation.
2.Embryoniccellsbegintofollowinspecificlayersthroughmorphogenetic
movements.
3.Thethreeembryoniclayers(externalEctoderm,mediumMesodermand
Endoderminternallayer).
4.Cellulardivisionslosesynchronizationbetweenthem.
5.Themetabolismchangeswithinthecellsandtheoxidationprocessbecomes
thedominant.
6.Nucleusatthisstagehasmoreimpactthanbeforeintheprocessofcontrol
ofdevelopmentandshowschromosomesorpaternalgenesanditseffect.

5. Gastrulation
Gastrulationrearrangesthecellsof
ablastulaintoathree-layered
embryo,calledagastrula,whichhas
aprimitivegutorcalledarchenteron.
Thethreelayersproducedby
gastrulationarecalledembryonic
germlayers
1.Theectodermformstheouter
layer
2.Theendodermlinesthedigestive
tract,andlungs
3.Themesodermpartlyfillsthe
spacebetweentheendodermand
ectoderm

6. •Theblastulaconsistsofnumerouscells,thepositionsofwhichwereestablished
duringcleavage.
•Duringgastrulation,thesecellsaregivennewpositionsandnewneighbors,and
themultilayeredbodyplanoftheorganismisestablished.
•Thecellsthatwillformtheendodermalandmesodermalorgansarebroughtto
theinsideoftheembryo,whilethecellsthatwillformtheskinandnervous
systemarespreadoveritsoutsidesurface.
•Thus,thethreegermlayers-outerectoderm,innerendoderm,andinterstitial
mesoderm-arefirstproducedduringgastrulation.
•Inaddition,thestageissetfortheinteractionsofthesenewlypositionedtissues.
Gastrulation

7. Thethreelayers(Endoderm,Mesoderm,and
Ectoderm)areformedandorganizedintheir
properlocationsduringgastrulation.
ThisfatemapdiagramofaXenopusblastula
shows:
cellswhosefateistobecomeectoderm
inblueandgreen,(skinandneuraltube)
Cellswhosefateistobecomemesoderm
inred,(muscles,bonesheart,blood,kidney…)
Cellswhosefateistobecomeendoderm
inyellow.(digestivesystem,liverlunges…)
Noticethatthecellsthatwillbecomeendoderm
areNOTinternal,yet.Itwillbecomeinternal
duringgastrulationstage.
Gastrulation

9. •Gastrulationinvolvessomecombinationofseveraltypesofmovements.
•Thesemovementsinvolvetheentireembryo,andcellmigrationsinonepartof
thegastrulatingembryomustbeintimatelycoordinatedwithothermovements
thataretakingplacesimultaneously.
•Althoughpatternsofgastrulationvaryenormouslythroughouttheanimal
kingdom,thereareonlyafewbasictypesofcellmovements.
Type of cell movement during gastrulation

10. 1.Invagination:Theinfoldingofaregionofcells.(e.g=seaurchingastrula)
2.Involution:Theinwardmovementofanexpandingouterlayersothatitspreads
overtheinternalsurfaceoftheremainingexternalcells.(e.gfroggastrula).
3.Ingression:Themigrationofindividualcellsfromthesurfacelayerintothe
interioroftheembryo.Thecellsbecomemesenchymal(i,e"theyseparatefrom
oneanother)andmigrateindependently.(e.gDrsophliaandseaurchinmesoderm)
4.Delamination:Thesplittingofonecellularsheetintotwomoreorlessparallel
sheets.(chickembryogastrolation)
5.Epiboly,Themovementofepithelialsheets(usuallyofectodermalcells)that
spreadasaunit(ratherthanindividually)toenclosethedeeperlayersofthe
embryo.(e.gformationofectoderminfrog,seaurchinandtunicatesembryos).
Type of cell movement during gastrulation

11. Sea urchin gastrulation

12. Thefrogblastulaismanycelllayerswithblastocoelattheendofcleavagestage.
Theblastulastarttoformthedorsallipwerethecellintheanimalpolestartto
movefromtheoutsidetotheinsideviathedorsallipofBlastopore.
Cellsofthedorsalliporiginateinthegraycrescentarea(inthefertilizedova)
andinvaginatetotheinsideoftheembryo.
Cellscontinuetomovefromtheembryosurfacetotheinsideoftheembryoby
involution.
Theseinvagenatedcellsbecometheendodermandmesoderm.
Theblastocoelisreplacedbyanewcavityinsidethegastrulastagecalled
archenteronwhichwillbecomethecavityofelementarycanallater.
Thesurfaceoftheembryoisnowectoderm,theinnermostlayerisendoderm,and
themiddlelayerismesoderm.
Theblastoporeencirclesayolkplugwhengastrulationiscompleted.Witchwill
becometheposteriorareaoftheembryo.
Gastrulation in the frog

13. •GastrulationbeginsondorsalsideBelowtheequator,inregionofgreycrescent.
•Cellsinvaginatetoformaslenderblastopore.
•Dorsallipofblastoporewillbecomeimportantorganizingregionofembryo
(Spemannorganizer)
•Cellsbecomeelongatedastheycontacttheinnersurface(Bottlecells).
Future ectoderm
Key
Future endoderm
Future mesoderm
SURFACE VIEW
Animal pole
Vegetal pole
Early
gastrula
Blastopore
Blastocoel
Dorsal lip
of blasto-
pore
CROSS SECTION
Dorsal lip
of blastopore
Bottle cell
Cell Movements in Amphibian Gastrulation

14. •Asmallgroupofcellschangeshape,
narrowingattheexterioredgeofthe
blastula.
•Thischangeincellshape,called
apicalconstriction,createsalocal
invagination,whichpushesmore
interiorcellsupwardsandbeginsto
rollasheetofcellstowardsthe
interior.
•Theconstrictedcellsarecalledbottle
cells,duetotheirshape(likean
upside-downbottleintheseimages).
How does the blastopore lip form?

15. Gastrulation in the frog

16. Future ectoderm
Key
Future endoderm
Future mesoderm
SURFACE VIEW
Blastocoel
shrinking
CROSS SECTION
Archenteron
Next steps:
•Involutionofthecellsatthemarginalzone(outersheetspreadsoverinner
sheet).
CellsfromAnimalpoleundergoepiboly
Convergeattheblastopore,Whenreachblastopore,travelinwardBottlecells
continuetomigrate,formleadingedgeofarchenteron(primitivegut).
Cell Movements in Amphibian Gastrulation

17. OncethepaddedorgastrophilaisformeduntilthePericardialplug
beginstodecreaseasaresultofthecrawloftheanimalpolecells
arounditfromeachdirection,thenthelipseventuallybecomeasmall
longitudinalmischiefknownastheprimitivelinehastwofineholes
thatclosetheabdomenandthedorsalremainsopenforawhileand
thencloselater.
Aftertheprocessoflining,thefetustransformsfromahollowballwith
asinglelayer,intoathree-layerouterbodyknownasectoderm,
internalknownasendoderm,andcellsthatmigratefromtheoutside
totheinsidearethemiddleclass,whichisknownastheMesoderm.
Fromthesethreeembryoniclayers,allorgansofthefetusariselater.
Gastrulation in Amphibian

18. SURFACE VIEW
Blastopore
Late
gastrula
Blastopore
Blastocoel
remnant
Yolk plug
CROSS SECTION
Ectoderm
Mesoderm
Endoderm
Archenteron
Future ectoderm
Key
Future endoderm
Future mesoderm
•Massofyolkleftbysurroundingblastopore=yolk.
•plugMassofyolkleftbysurroundingblastopore=yolkplug.
•Cellsfromthedorsallip(thefirstcellsthatmigratedinward)
becomeprechordalplate(willformheadmesoderm).
•Nextcellsthatinvoluteformchordamesoderm(willbecome
notochord)Importantforpatterningthenervoussystem.
Gastrulation in Amphibian

19. Future ectoderm
Key
Future endoderm
Future mesoderm
SURFACE VIEW
Animal pole
Vegetal pole
Early
gastrula
Blastopore
Blastocoel
Dorsal lip
of blasto-
pore
CROSS SECTION
Dorsal lip
of blastopore
Late
gastrula
Blastocoel
shrinking Archenteron
Blastocoel
remnant
Archenteron
Blastopore
Blastopore
Yolk plug
Ectoderm
Mesoderm
Endoderm
Gastrulation in Amphibian

20. What causes gastrulation?
Whatfactorsareresponsibleforallofthecomplexand
orchestratedmovementsthatoccurduringgastrulation?
Thereareatleast2hypothesesandtheyarenot
mutuallyexclusive.
1.Cellularbehaviorsobservedineachgastrularegion
arecausedbylocalaccumulationsofgeneproducts
thatarealreadypresentinthefertilizedoocyte.
2.Oneregionoftheblastulacouldbedeterminedto
organizethebehaviorofallotherregions.The
dorsallipoftheblastoporewillbecomethe
spemannorganizerthatappearstobeparticularly
importantinregulatingthefateofotherareas.2
genes,goosecoidandnogginfromtheembryonic
organizerareaseemtocausethecellinvaginationin
theblastoporeareatomediatethiseffect.

21. How does the blastopore lip form?

22. Gastrulation in the birds
The epiblastand the hypoblast:
•By the time a hen has laid an egg, the blastoderm contains
some 20,000 cells.
•At this time, most of the cells of the area pellucida remain
at the surface, forming an "upper layer,“ called the epiblast.
•While other area pellucida cells have delaminated and
migrated individually into the sub germinal cavity to form
hypoblast islands.
•The resulting two-layered blastoderm (epiblast and
hypoblast) is joined at the marginal zone of the area opaca,
and the space between the layers forms a blastocoel.

23. •Theavianembryocomesentirelyfromtheepiblast.
•Thehypoblastdoesnotcontributeanycellstothedevelopingembryo.
•Rather,thehypoblastcellsformportionsoftheexternalmembranes,
especiallytheyolksacandthestalklinkingtheyolkmasstotheendodermal
digestivetube.
•Hypoblastcellsalsoprovidechemicalsignalsthatspecifythemigrationof
epiblastcells.
•Thethreegermlayersoftheembryoproper(plustheamnion)areformed
solelyfromtheepiblast.
Gastrulation in the birds

24. The primitive streak
•The major structural characteristic of avian, reptilian
and mammalian gastrulation is the primitive streak.
•The primitive streak becomes the blastopore lipsof
amniote embryos.
•The primitive streak first arises from a local
thickeningof the epiblast at the posterior edge of the
area pellucida, called Koller's sickle, and the epiblast
above it.
Gastrulation in the birds occur via

25. The Formation of
primitive streak

26. The Formation of primitive streak
1.ThestreakisFirstvisibleascellsaccumulateinthemiddlelayer,Followedbya
thickeningoftheepiblastattheposteriormarginalzone.
2.Thisthickeningisinitiatedbyanincreaseintileheight(thickness)ofthecells
formingthecenteroftheprimitivestreak.
3.Thepresumptivestreakcellsaroundthembecomeglobularandmotile,andthey
appeartodigestawaytheextracellularmatrixunderlyingthem.
4.Thisprocessallowsthesecellstoundergointercalationandconvergentextension.
5.Convergentextensionisresponsiblefortheprogressionofthestreakdoublingin
streaklengthisaccompaniedbyaconcomitanthalvingofitswidth.
6.Ascellsconvergetoformtheprimitivestreak,adepressionformswithinthe
streak.Thisdepressioniscalledtheprimitivegroove,anditservesasanopening
throughwhichmigratingcellspassintothedeeplayersoftheembryo.

27. Gastrulation
in birds

28. •Assoonastheprimitivestreakhas
formed,epiblastcellsbeginto
migratethroughitandintothe
blastocoel.
•Thestreakthushasacontinually
changingcellpopulation.Cells
migratingthroughtheanteriorend
passdownintotheblastocoeland
migrateanteriorly,formingthe
endoderm,headmesoderm,and
notochord.
•cellspassingthroughthemore
posteriorportionsoftheprimitive
streakgiverisetothemajorityof
mesodermaltissues.
Formation Of Endoderm And Mesoderm

29. Across-sectionthroughtheembryo
allowsustoobservethethreegerm
layersthatformduringgastrulation:
ectoderm,mesoderm,andendoderm.

30. Mammalian Gastrulation
•Birdsandmammalsarebothdescendantsofreptilianspecies(albeit
differentreptilianspecies).
•Itisnotsurprising,thatmammaliandevelopmentparallelsthatofreptiles
andbirds.
•thegastrulationmovementsofreptilianandavianembryos,whichevolved
asanadaptationtoyolkyeggs,areretainedinthemammalianembryo
evenintheabsenceoflargeamountsofyolk.

31. Imagesofhumanembryosduringgastrulation,13-19dayspostovulation.
Noticetheprimitivestreak,whichisanalogoustotheblastoporeofXenopus.

32. Human embryo
implantation
and Gastrulation

33. Schematic diagram showing The derivation of tissues in human and rhesus monkey embryos