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Hypoblastand endoblastformation:
StaqeX (diameterca. 2mm; ca. 6 hr beforeegg
oytsX
area pellucida
60,000cells
Kollels
sickle
gem wail
margin
hypoblasVendoblast
epiblast
#rffi
area opa€
epiblasl
X
Yolkycellsare shedintothe fluidspacebetweenthe
blastodisc
and the yolk mass.
StaqeXll:
Sheddingcontinues.
Shedcellsadherelooselyto
makea hypoblastsheet.As sheddingends,we have
2 layers:the epiblastaboveand the hypoblast
below,adheringlooselyto it. The epiblastis a single
cellthick.The embryowill developentirelyfromthe
epiblast.
The hypoblast
will makeyolk sac endoderm
(seelatersection).
StaqeXlll (aboutthe time of egg laying):
Underthe epiblast,endoblastcellsmigrateout of the
posterior
marginalzoneand displacethe hypoblast
in an anteriordirection.
Staqe2: (afteregg laying)
Endoblast
cellscontinueto migrateout,and the
hypoblast
is displacedhalfway acrossthe epiblast
diameter.
The primitivesteakbeginsto format the
posterior
sidefromepiblastcells.
Staqe3+ (12-15hours after egg laying):
Primitive
streakcontinuesto elongatein the epiblast.
Epiblastcellsmove towardthe streakfrom the sides, 3 +
andthenthroughit, to form endodermand
mesooerm.
2
View from
underside
View from
outside
Posterior'
rnarginal
zone
hypoblast
area opa€
endodem
xil@
.-i,}'--_ie5';1ii,:n]r.
,fa
_:;
..q
j.
xilrw
-*r-\
:.
'a
.l
endoblast
movingin
-,'.i'
endoblast
primitivestreak
(middlelayer)
streak
q$;--}ffi
Roleof the hypoblastand endoblastin patterningthe epibl?st. Experiments:
and
1. Transplant
PMZ intothe marginalzoneon the oppositesideof anotherstageX embryo.Get a secondendoblast
primitivestreak.
2. Injectvgl mRNAintomarginalzoneoppositePMZ.Get secondendoblastand primitivestreak.
Eachformsa primitivestreak,eventhough
3, Beforeprimitivestreakforms,cut epiblastand hypoblastintoquadrants.
onlyone had the PMZ.
4. Removehypoblastbeforethe endoblastand primitivestreakform.
Get 2 or moreprimitivesteaksin the epiblast.
Vgl and WntS
Current understanding of Vg1; WntS; Nodal, Bmp2,4, and
e
n
d
o
bl
ast.
a
n
d
7 ; FGF, and Chor din i n th e h y p o b l a s t
v
n
StageX: Vg1 mRNAis expressedonlyin the PMZ.WntS(and
protein)and Bmp2,4are producedaroundthe
beta-catenin
stabilized
PMZ
at
low levelsat first. The epiblastdoesn't
zone,
entiremarginal
Cerberus Nodal
center-it will
producethese.The PMZ is ratherlikethe Nieuwkoop
intoit or intothe
inducean organizer(node)but not be incorporated
Xl
bodyaxismesoderm.
and in conjunction
StageXll: Vg1 signalingfromthe PMZ increases,
proteinof the marginalzone,inducesnearby
withbeta-catenin
epiblastcellsto expressthe nodalgeneand secreteNodalprotein
(fikexnrl ,2,4,5,6of Xenopus,butjust one kindof Nodal).Nodal
t
&
inductionin the epiblast,bu'!-theeffectdoesn't
startsendo-mesoderm
spreadveryfar in the epiblastbecausethe hypoblast(underneath)
of Nodal
produceshighlevelsof Cerberusprotein,an antagonist
as well as of WntSand Bmp.
signaling
Nodal
FGF+ Chordin
awayfromthe PMZ.The
Stage2 (afteregg laying):Endoblastcellsmigrateout of the PMZand displacethe hypoblast
intensifies
and
Nodal
signaling
it,
the
above
epiblast
in
the
Therefore,
antagonist.
the Cerberus
Ooesnot proOuce
enOoOtasi
loose
and-theprimitivestreak,a ridgeof accumulating
inducingendo-mesoderm
by autoactivation,
spreadsin the ep'rblast
mesoderm.
nearthe PMZ is exposedto WntSand Bmp,remainingas posterior
Endo-mesoderm
celts.Thiscontinues.
movingintothe centerof the epiblastgets awayfrom the WntS-Bmpsourcesat the peripheryand
Endo-mesoderm
whichfavor
Also,it is exposedto FGFand Chordinfromthe hypoblastand endoblast,
becomesanteriorendo-mesoderm.
moves
Endo-mesoderm
streak.
end
of
the
the
anterior
forms
at
node
Hensen's
neural
ectoderm.
and
nodemesoderm
epiblastbehind,the mostcentmlof whichis neural.
throughthe streak,leavingectodermal
epiblastis freeof Bmpbut the peripheryreceivesit fromthe marginalzone.
Stagea and S:The centerof the ectodermal
startsin the centralepiblastthat has receivedlittleor no Nodalor
EpiJermisdevelopsat the edge.Neuraldevelopment
suchas Chordinand
nodefurtherproducesBmp antagonists
Also,
the
Bmp2,a(buthas receivedFGF and Chordin).
now veryanterior,
Cerberusand FGFfromthe hypoblast,
to epidermis).
Nogginthat preventBmp effects(ectoderm
in the epiblast.
continueto favoranteriorneuraldevelopment
(D)
Gastrulation:
'
THETWO MAJOR
GASTRULATION
are:
MOVEMENTS
1) Watertallmove.
ment of epiblast
cells throughthe
primitivestreak
2) BegIesslan ot
Hensen'snode
Area oPaca
'
Margin
Area
pellucida
Thickening
areaof
blastoderm
\L,/
Koller's
sickle
Posterior
Area opaca
o
Area
pellucida
Anterior
(F)
\u/
H.nr..t
Hensen's
node
Ectoderm
Area
Neural
pellucida
ectoderm
Neural fold
Somite
Head
Segmental
Notochurd
|
^
plate
Hensen's
trode
Primitive
streak
(l )
Neural fold
Foregut
Lateral plate
Notochord
mesoderm
Extraembryonic
m esoderm
p
F i g u re 1 l.lo
Cell movementsof the primitive streakof the chick embryo.(A,C) Dorsal view of the formation and elongationof the primitive steak The blastodermis seenat (A) 3-4 hours,(B) 7-8
hours,and(C) l5-l6hoursafterfeniliation.Theearlymovemenrs'of
themigratingepiblast
cellsareshown by arrows.(D-F) Formation of notochord and mesodermalsomitesasthe
primitive streakregresses,
shom at (F) l9-22 hours,(E) 23-24 hours,md (F) the four-somire
shge. Fatemaps of the chick epiblot are shown for tlvo stages,the definitiveprimitive streak
stage(C) and neurulation(F). The endodermhasalreadyiagressedbeneaththe epiblast,and
convergentertensionis seenin *re midline. (Adaptedftom severalsources,especiallySpratt
1946and Smith and Schoenwolf1998.)
Ingressionof cells of the primitive streak
Segmental
plate
Primitivestreak
o
o
u,
o
o
tD
.t
6
o
(Waterfall
movem6ntsl
o
o
epiblosl
.:
=
E
:
node
Notochord
\
Paraxial
mesoderm
(somites)
Primitive
gloove
of
head fold
process
-=v
)zEpidermal.
ectoderm \
opaca
Anteriorborder
of mesoderm
,_
Area
Regressionof
Hensen'snode
Process
(H)
--"----_.
node
3
r^
\t
Head
Waterfallmovementsof
epiblastcellsthroughthe
primitivestreak
Prinritive
streak
takingshape
lnvolution
of the headprocess
(prechordalplate,
pharyngealendoderm)
i
primilive
CL
tl
C. Chick: leftight section, sloge 4
o
CD
c
o
t0
o
e
tt
o
rlJ
" Eo
stoge 4
medion
section
l'
o
z
Regression of Hensen's node, and streak regression
-
Notochord,
Medial somites,
Floor plate
Gut roof
ffi
Hensens node
ffi
#{
ffi
q fw
vd
PO II "&Y
pnmlrvesr'eak
. r .' .' - "' i.rr:
,.: ]
head lold '
.li" ni i
.iil
A
:\
.' .
ii
.9
ti.,
,:j
l;i
|
? /node
,.,,
o
A
\/
li
't,.,,,..[,o#i"'
ls node regresses from anterior to posterior along the centerline of the primitive streak, it obliterates the
Jtreak and leaves behind it cells of the nolochord, medial side of the somites, the floor plate of the neural
as it moves, and then
fube, and the gut roof- Cells proliferate in the node, and some cells are recruited into it
a re left behind. The node has inductive activities like that of the amphibian trunk{ail organizer
Chordamesoderm)
.9
d
4
The fate map of the bird early gastrula; and a comparison with the Xenopus fate map:
anterior
anteriOf ("dorsat")
Extraembryonicepidermis
Embryonicepidermis
Neuralplate
Embryonicepidermis
Somiles
plate
Ertraembryonic mesoderm
Gut endoderm
- Embryonic Yolk sac endoderm
Underneath: Hypoblast,endoblast
extraembtyonic endoderm (Yolk
posterior
posterior ("venrral")
sac)
Prlmitive streak
Chick fate map of the epiblastat maximumstreakextension,
belorecells ingressthroughthe streakand beforenode
regression.
The mouse map is nearlythe same,but the epiblastis pushed
down at lhe centerinto a cup shaDe.
somatic mesoderm
When gastrulationand neurulationare
fnsshed,the embryo looks like this, spread
over the huge yolk mass. Later it will lift up
and elose ventrally.
gplanchnic
m€soderm
Frog (Xenopus) fate map, late blastula,
before involutionbegins.Note: sphericalsurfacs
has been squashedflat lo a disc and the gut roof
endodermlayerof the marginalzone has been
rcmoved.
Neural tube
inter'mediate
mesoderm (kidneY)
Coelom
somite
mesoderm
ectoderm
Yolk-filled
endoderm
(4,
Extra-embryonicdevelopmentin birds:
Hcrd
Reptiles,birds,and mammalsare distinguishedas
'amniotes'because
theyforman amnion(seebelow).
They also form three other Kindsof extraembryonic
tissues,namely,the chorion,allantois,and yolk sac.
Mostof thesetissuesare derivedfrom lhe epiblastand
formedaftergastrulationas extensionsof the germ
layersof the pharyngula,but yolk sac endodermcomes
largelyfrom the hypoblast,which segregatesbefore
gastrulation.All extraembryonicmesodermhas gone
throughthe primitivestreak.
Amnion: a fluid filled cavity surroundedby the amniotic
membrane,a bilayerof sheets of ectodermand somatic
(parietal)mesoderm.The embryois withinthe amnion,
an adaptationfor eggs developingout of water,on dry
land.
Chorion: a bilayerof sheets of ectodermand somatic
(parietal)mesoderm,enclosingthe entireembryo,all
otherextraembryonictissues,and the yolk, in a "chorioallantoiCcavity.The chorionmediatesgas exchange
and calciumuptake.(Seemoreunderallantois.)
Allantois:linedby the allantoicmembrane,a bilayerof
sheetsof hindgutendodermand splanchnic(visceral)
mesoderm.The mesodermis heavilyvascularized.The
allantoisstoresurinarywastes (comingfrom yolk and
albuminbreakdown.On dry land, there is no water to
excretewastes into.)and mediatesgas exchange.
\ /henthe allantoisexpandsand pressesagainstthe
choroin,they fuse to make the chorio-allantoic
membrane,
whichmediatesgas exchangeand calcium
uptakefromthe egg shell.
Yolk sac: linedby a yolk sac membrane,a bilayerof
sheetsof hypoblastand endoblastendodermand of
(visceral)mesoderm.The mesodermis
splanchnic
heavilyvascularized.
This sac containsthe egg-syolk,
whichis digestedin the sac. Nutrientsdiffuseinto the
gut or are canied in the blood steam,then absorbedin
the bpdy.
&k*mbryoni(
tclom
-
Eersbryoil<
,old
kodem
gplanchnic
H€.d
told
T.it aold
m€sodcm
Enddem
Yolk
vil€llft
Gnvelots
E<tod"m
FIGURE T8
Schetutk d..wints
ol thg qhemof the chir*. The
bryonk memb6*s
edbryo is at longitudin.lly and the
albumen etrd shell @tings are nol
shown. (A) zday eFbryo. (B) lday
diictubryo. (c) Detailed *hemtk
Iram of th€ oudrl (hind) rgion of the
<hick enbryo showing th€ tomation
ol the allantois. (D) iday embryo. (E)
9iay embryo. (Atter Carlon, 1981.)
Chodfihdodc
odty
Nol(no.d
\tlanus
pushing
into cxltaambryoni(
tips 6ming together, to fuse
Cut
slightJy later stage than @)
(amniotic cavity has formed)
c.vitt
Chodon
Yol!
(when pressedtog"th"r,{t), L^
membrane
chorioallantoic
mesoderm to m"sod"r-P