Implantation in mammals

The term implantation is used to describe the attachment of the developing embryo to the endometrium.
After fertilization, the embryo reaches the uterus in the blastocyst stage. Then attached to the wall of the uterus. Though the implantation may occur at any period between the sixth to the tenth day after the fertilization generally it occurs on the seventh day after fertilization.

1. M.G. COLLEGE ARMORI
DIST. GADCHIROLI
Class: B.Sc. II Sem IV
Paper: I Unit: III
Topic : Implantation
By: Pro. S.B. Kumre
( Dept. Of Zoology )

2. Implantation
•Thetermimplantationisusedtodescribetheattachmentofthedevelopingembryotothe
endometrium.
•Afterfertilizationtheembryoreachestotheuterusintheblastocyststage.Thenattachedtothewallof
theuterus.Thoughtheimplantationmayoccuratanyperiodbetweensixthtotenthdayafterthe
fertilizationbutgenerallyitoccursonseventhdayafterfertilization.
•Itinvolvesareceptiveendometriumandhormonalfactorssuchasestrogen,progesterone,prolactinas
wellascelladhesionmolecules,growthfactors.
•Further, the uterus is ready to accept the implanting embryo only during a limited period of time known
as the ‘window of implantation’, outside of which the endometrium may be indifferent or even hostile to
the embryo.

3. Process of Implantation
•Implantationisaprogressiveandversatileprocessinwhichtheblastocystattachesand
finallyinvadestheunderlyingendometrialsurface.
•Afterfertilizationandcleavage,themorulareachestheuterusbetweenthreeorfourdays
ofdevelopment.
•Bythefifthday,blastocysthatchesoutoftheprotectingzonapellucidabyenzymatically
boringaholeandsqueezingitselfout.
•Adjacentcellsoftheendometrialstromarespondtothepresenceoftheblastocystandto
theprogesteronesecretedbythecorpusluteumbydifferentiatingintometabolicallyactive
secretorycellscalleddecidualcells.Thisresponseistermedasthedecidualreaction.
•Secretionsofthedecidualcellsandendometrialglandsincludemetabolitesthatsupport
thegrowthoftheimplantingembryo.
•Subsequently,theblastocystattachestotheendometrialepithelium.
•Thisistheinitialphaseoftheimplantationprocesscalled“adplantation”.Thisfirst
phaserequiresthenewlyhatchedblastocysttolooselyadheretotheendometrial
epithelium,often“rolling”totheeventualsiteofimplantationwhereithasfirmlyadhered.

4. Process of Implantation continue..
•Assoonasitattaches,thetrophoblaststartstoproliferaterapidlyanddifferentiateinto:
Theinnerlayerofcytotrophoblast
Theouterlayerofsyncytiotrophoblast
•Thefingerlikeprocessesofthesyncytiotrophoblastextendthroughtheendometrialepithelium
andinvadetheconnectivetissue.
•Bytheendofthefirstweek,theblastocystissuperficiallyimplantedinthecompactlayerofthe
endometrium.
•Subsequently,theuteroplacentalcirculationdevelopswiththeformationoflacunarnetworks
andvilli.

6. Major Events in Implantation
•Major Events in Implantation
•Day 5:The zona pellucida degenerates.
•Day 6:Blastocyst adheres to the endometrial epithelium.
•Day 7:Trophoblast differentiates into two layers: syncytiotrophoblast and cytotrophoblast.
•Day 8:Synctiotrophoblast erodes endometrial tissues, and the blastocyst starts to embed in the
endometrium.
•Day 9:Blood filled lacunae appear in syncytiotrophoblast
•Day 10:Blastocyst sinks beneath the endometrial epithelium and defect is filled by a closing plug
•Day 10 and 11:Lacunar networks form by fusion of adjacent lacunae
•Day 11 and 12:The syncytiotrophoblast erodes endometrial blood vessels, allowing maternal blood
to seep in and out of lacunar networks, thereby establishing a uteroplacental circulation
•Days 12 and 13:The defect in the endometrial epithelium is repaired
•Days 13 and 14:Primary chorionic villi develop
•The implanted 12-day embryo produces a minute elevation on the endometrial surface that protrudes
into the uterine lumen.

7. Types of Implantation
1) Interstitial Implantation
2) Eccentric Implantation
3) Centric or Superficial Implantation

8. •InterstitialImplantation:Theformofimplantationdiffersfromonespeciesto
another.Inprimatesandguineapigs,theblastocystburrowsthroughtheuterine
epitheliumtotheuterinestromawheretheembryodevelops.Thisformofimplantation
isreferredtoasinterstitialimplantation.

9. •Eccentric Implantation : In rodents, implantation involves the blastocyst becoming
lodged in a uterine cleft with proliferation of the surrounding uterine mucosa. This form of
implantation is known as eccentric implantation.

10. •CentricorSuperficialImplantation:Inhorses,cattle,sheep,pigs,dogs,catsandrabbits,
thefluid‐filledsacssurroundingtheembryoexpandsothattheextra‐embryonicmembranes
becomeapposedtotheendometriumandattachtoit.Thisformofimplantation,themostcommon
formofattachmentinmammals,isreferredtoascentricorsuperficialimplantation.

11. •Significance of Implantation
•The implantation process allows the developing blastocyst to obtain nourishment and
growth factors from the eroded maternal tissues.
•If implantation has not proceeded sufficiently during the menstrual cycle to allow
hormonal feedback to the ovary, then the next cycle may commence leading to conceptus
loss.
•A conceptus with major genetic defects does not develop or implant correctly leading to
their loss during the first and second weeks of development.
•In recent years with the development of Assisted Reproductive Technologies (ART or
IVF) there is a growing interest in this process, with techniques that introduce the
blastocyst into the uterus to allow normal implantation to occur.

12. Thank You