First-week-of-development.ppt of midwifery

Embryology

1. The Beginning of Human Development:
First Week

2. The Beginning of Human Development
FirstWeek
•Fertilization
•cleavage
•Blastocystformation
•ImplantationStart

3. Transportation Of Gametes

4. •Theescapingfollicularfluid
•Thesweepingactionofthefimbriae
•Fluidcurrentsproducedbytheciliaof
themucosalcellsofthefimbriae
•Peristalsis-movementsofthewallof
thetube
TransportationOfoocyte

5. Sperm Transport
•Peristalticcontractionsofthethick
muscularcoatoftheductus
deferens.
•Closureofthevesicalsphincterat
theneckofthebladder,contraction
ofurethralmuscle,andcontraction
ofthebulbospongiosusmuscles.
•From200to600millionspermsare
depositedaroundtheexternalosof
thecervixandinthefornixofthe
vaginaduringsexualintercourse.
Thepassageofspermatozoathroughthemale
reproductivetract

6. •Theenzymevesiculase,coagulates
someofthesemenorejaculate.
•Whenovulationoccurs,thecervical
mucusincreasesinamountand
becomeslessviscid,makingitmore
favorableforspermtransport.
•Thespermspassslowlythroughthe
cervicalcanalbymovementsoftheir
tails.
•Muscularcontractionsofthewallsof
uterus&uterinetubes.
•Prostaglandins
•Chemoattractantsproducedby
cumuluscellssurroundingtheoocyte

7. •Thespermsmove2to3mmper
minute,butthespeedvarieswith
thepHoftheenvironment.
•Theyarenon-motileduring
storageintheepididymis,but
becomemotileintheejaculate.
•5-45minutestocompletethe
journey.
•Onlyapproximately200sperms
reachthefertilizationsite.
•Mostspermsdegenerate.

8. Viability Of Gametes
•Oocyte-12-24hours
•Sperm–3to5days
•CryopreservationofEmbryos
Spermsandoocytescanbestoredfrozenfor
manyyearstobeusedinassistedreproduction.

9. •Freshlyejaculatedspermsareunabletofertilizeoocytes.
•TheyshouldundergoprocesscalledCapacitation&
AcrosomalreactiontofertilizetheOocyte
1.Capacitation
•Processofconditioningofsperminthefemalegenitaltract
•Epithelialinteractionbetweenfemalemucosalliningand
sperm
•Capacitatedintheuterusoruterinetubesbysubstances
secretedbythesepartsofthefemalegenitaltract.

10. •Theglycoproteincoatandseminalplasma
proteinscoveringtheplasmamembraneofsperm
intheacrosomalregionareremoved.
•Onlycapacitatedspermundergoesacrosome
reactionandfertilizestheovum.
•Capacitatedspermsshownomorphologicchanges,
buttheyaremoreactive

11. 2. Acrosome Reaction
•Occurswhencapacitatedspermcomesincontactwith
thezonapellucida(i.e.,acrosomereactionisinduced
byzonaproteins).
•Thespermplasmamembrane,calciumions,
prostaglandins,andprogesteroneplayacriticalrolein
theacrosomereaction

12. Fertilization
•Complexsequenceofcoordinated
moleculareventsthatbeginswith
contactbetweenaspermandan
oocyteandendswiththe
interminglingofmaternaland
paternalchromosomesatmetaphase
ofthefirstmitoticdivisionofthe
zygote
•Defectsatanystageinthesequence
oftheseeventsmightcausethe
zygotetodie.
•Takesapproximately24hours

13. Steps/Phases of Fertilization
•Penetrationofcoronaradiata
•Penetrationofzonapellucida
•Fusionofspermandoocytecellmembranes
•Completionofsecondmeioticdivisionofoocyte
•Formationoffemalepronucleus
•Formationofmalepronucleus
•Formationofzygote

14. Steps/Phases of Fertilization
•Penetrationofcoronaradiata:
•Thecapacitatedspermcanfreelypenetratethroughthe
coronaradiatatoreachthezonepellucidabythe
movementsofitstail.

15. •Penetrationofzonapellucida:
•Theheadofthespermiscappedby
anorganellecalledacrosome.
•Theacrosomecontainsatrypsin-like
proteindigestingenzymeand
hyaluronidase.
•Whentheheadofspermcomesin
contactwithzonapellucida,an
acrosomereactionisinducedbythe
zonaproteins.

16. •Formation of zygote:
•Themaleandfemalepronuclei
loosetheircellmembraneand
chromosomesoftwonuclei(23in
each)mixtogethertoform
diploid(i.e.,46chromosomes).
•Theootidbecomesazygote.
•Thechromosomesinzygote
becomearrangedonacleavage
spindleinpreparationfor
cleavageofzygote.

17. Results of Fertilization
•Stimulatesthepenetratedoocytetocompletethe
secondmeioticdivision.
•Restoresthenormaldiploidnumberofchromosomes
(46)inthezygote.
•Resultsinvariationofthehumanspeciesthrough
minglingofmaternalandpaternalchromosomes.
•Determineschromosomalsexoftheembryo.
•Causesmetabolicactivationoftheootidandinitiates
cleavage(celldivision)ofthezygote.

18. Cleavage Of The Zygote
•Consistsofrepeatedmitoticdivisions
ofthezygoteintosmallerunits.
•Resultsinarapidincreaseinthe
numberofcells&arecalled
blastomeres.
•Theybecomesmallerwitheach
successivecleavagedivision.
•Thedivisionofzygotestartsjustafter
fertilizationandcontinuesasthe
zygotepassesalongtheuterinetube.

21. •During cleavage the zygote is surrounded
by a rather thick zona pellucida.
•The zygote divides to form two cells of
which one is smaller than the other (two-
cell stage of embryo).
•The larger cell divides first giving rise to
three-cell stage.
•The smaller cell divides next and embryo
consists of four cells that divide to form
eight cells.
•Blastomereschange their shape and tightly
align themselves against each other—
compaction.

22. •The8cellsfurtherdividetoform16cells.
•A16-cellstageembryoresemblesamulberry
fruitandistermedmorula.
•Inmorula,theblastomeresareverysmall
andcontainverylittlecytoplasmtosurvive.
•Theblastomeresareenclosedbythezona
pellucida.
•Thecellsnowgetarrangedintotwogroups:
•Groupofcellsinthecenterofmorulais
calledinnercellmass(embryoblast)
•Groupofcellspresentattheperipheryof
morulaarecalledoutercellmass
(trophoblast).

23. Blastocyst Formation
•Atabout4
th
dayafterfertilization,
morulaentersintotheuterine
cavity.
•Theendometrialfluidpenetrates
thezonapellucidaandentersinto
theintercellularspacesofthe
morula.

24. •Graduallytheintercellularspacesbecome
confluentandfinallyasinglelargecavityis
formed.
•Thiscavityiscalledblastoceleandatthisstage
theembryoiscalledblastocyst.
•Theblastoceleisfilledwithfluidrichin
nutrients,whichissecretedbytheendometrium
oftheuterus.
•Thisfluidisalsotermeduterinemilk.

25. •Asthecavityenlargesthe
outercellsformingtrophoblast
becomeflattened.
•Theinnercellmasscalled
embryoblastbecomescompact
andisattachedtothe
trophoblastatonepole—the
embryonicpole.
•Theresultantwholestructure
isnowcalledblastocyst.

26. •Theembryoblastgivesrisetotheembryoandthe
trophoblastprovidesnutritiontotheembryo.
•TheBlastocystfloatsintheuterinesecretionfor
about2days.
•Zonapellucidadegenerates&disappears.
•Now theblastocystbecomesreadyfor
implantation.

27. Implantation
•Approximately6-7daysafter
fertilization,blastocystsattachesto
theuterineendometriumusually
adjacenttoembryonicpoleof
blastocyst.Thisprocessiscalledas
implantation

28. Development of theSecond Week
1.Implantation
Iscompleted
Atday10blastocystis
completelyembeddedand
siteofimplantationis
closedbyaclosingplugof
fibrincoagulum.
Siteofimplantationwill
completelyrecoverandthe
defectiscoveredby
epitheliumatday12.

30. 2.Decidua
Isendometriumafterimplantation
Showsdeciduarxns
Endometrialcellularandvascular
changes
Appearanceofdecidualcellsthat
containlargeamountoflipidand
glycogen,andproducehormones
thatarenutritivetothedeveloping
embryo.

31. •Has three regions
1. Deciduabasalis
Located deep to the
conceptus.
Make the meternalpart
of the placenta
2. Decidua
parietalis(decidua
vera)
Located away from the site
of implantation
3. Deciduacapsularis
Located superficial to the
conceptus
Gradually fuses with
duciduaparietalisand
degenerate at 22 wks.

32. •Astheblastocystimplants,thetrophoblastcontactstheendometriumand
differentiatesinto
•Aninnerlayerofmononucleatedcellscalledcytotrophoblast
•Anoutermultinucleatedlayercalledsyncytiotrophoblast

33. •Cytotrophoblast
•Aninnerlayerofmononucleated
cells.
•Mitoticallyactive
•Formsnewcellsthatmigrateinto
theincreasingmass of
Syncytiotrophoblast
•Syncytiotrophoblast
•Anoutermultinucleatedzone
withoutdistinctcellboundaries.
•Cellsinthecytotrophoblastdivide
and migrateintothe
syncytiotrophoblast.
•SourceofhCG

34. 3. Primary chorionic villi
Is formed by projections
of cytotrophoblast
covered by
syncytiotrophoblast

35. 4. Primordium of intervilous
spaces of the placenta is
established.
Lacunar stage
isolated cavities -lacunae-
appear in the
syncytiotrophoblast.
The lacunae soon become
filled with a mixture of
maternal blood from
ruptured endometrial
capillaries and cellular debris
from eroded uterine glands.
The fluid in the lacunar
spaces-passes to the
embryonic disc by diffusion
and provides nutritive
material to the embryo.

36. •Thecommunicationoftheerodedendometrial
capillarieswiththelacunaeestablishestheprimordial
uteroplacentalcirculation.
•Whenmaternalbloodflowsintothelacunae,oxygen
andnutritivesubstancesareavailabletotheembryo.
•Oxygenatedbloodpassesintothelacunaefromthe
spiralendometrialarteries,and
•poorlyoxygenatedbloodisremovedfromthem
throughtheendometrialveins.

37. 5. Bilaminar germ disc is formed
Asimplantationoftheblastocystprogresses,Cellsofinnercell
massorembryoblastalsodifferentiateinto2layers
OuterEpiblastcellslayerHighcolumnarcells
InnerHypoblastcellslayerSmallcuboidalcells
Theepiblastformstheflooroftheamnioticcavityandis
continuousperipherallywiththeamnion.
Thehypoblastformstheroofoftheexocoelomiccavityandis
continuouswiththethinexocoelomicmembrane.
Thismembrane,togetherwiththehypoblast,linestheprimary
umbilicalvesicle.
Theembryonicdiscnowliesbetweentheamnioticcavityand
theumbilicalvesicle(yolksac).

39. 6.Amnioticcavityappears
Asimplantationoftheblastocystprogresses,a
smallspaceappearsintheembryoblast.
Thisspaceistheprimordiumoftheamniotic
cavity.
Soonamniogenic(amnion-forming)cells-
amnioblastsseparatefromtheepiblastandformthe
amnion,whichenclosestheamnioticcavity.
1
st
novalcavityproducedduringthe2
nd
wkof
development.

40. 7. Prochordal (prechordal)
plate is formed
Is formed by columnar cells
from hypoblast cells in a
circular manner and firmly
attached to the overlying
epiblast cells layer.
Used to identify the cranial
regionand the organizer of the
head region.

41. 8.Exocoelomic(Heuser’s)membraneappears
Bysimplesquamouscellsprobablyfromthehypoblast
cellsandlinetheinnersurfaceofthecytotrophoblastcells
coveringtheblastocystcavity.
Convertblastocystcavityintoexocoelomiccavity,thatwill
soonmodifytoformtheprimary(primitive)yolksac.

43. 9.Extraembryonicmesodermappears
Fromthehypoblastcellsbetween
i.Cytotrophoblastcellslayerandexocoelomic
membrane
ii.Cytotrophoblastcellslayerandamnion.

44. 10.Extraembryoniccoelome(chorioniccavity)
SplitsExtraembryonicmesoderminto:
–1)Extraembryonicsplanchnic(splanchnopleuric)
mesodermthatlinestheyolksac.
–2)Extraembryonicsomatic(somatopleuric)mesodermthat
linesthecytotrophoblastlayerandtheamnion.

46. 11. Chorion is formed
Chorion is formed by
extraembryonic
somatic mesoderm
(chorionic plate) and
the overlying
trophoblast.

47. 12.Secondary(definitive)yolk
sacappears
formedbyshrinking
primitive(primary)yolk
sacbecauseofexpansion
ofthechorioniccavity.
Alargepartofthe
primaryyalksacis
pinched off as
exocoelomiccyst.

49. prepared by w/rt Fitsum Wolde
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