Placenta structure and Classification

This explains the placenta of Eutherian including human

1. PLACENTALSTRUCTURE
AND
CLASSIFICATION
Dr.Dinesh C. Sharma
Associate Professor & Head-Zoology
K. M. Government Girls P.G. College,
Badalpur, G.B. Nagar-203207
[email protected]

2. Theplacentais a temporaryorganthat
connects thedeveloping fetusvia
theumbilical cordto theuterinewall to
allow nutrient uptake, thermo-regulation,
waste elimination, and gas exchange via
the mother's blood supply; to fight against
internal infection; and to produce
hormones which support pregnancy .
Placentas are a defining characteristic
ofplacental mammals, but are also found
inmarsupialsand some non-mammals
with varying levels of development

3. Theplacentasofalleutherianmammals
providecommon structuralandfunctional
features, but there are striking
differencesamongspeciesingrossand
microscopic structureoftheplacenta.
Two characteristics areparticularly
divergent and form bases for
classificationofplacentaltypes:
•Thegrossshapeoftheplacentaand
thedistributionofcontact sites
between fetal membranes and
endometrium.
•Thenumberoflayersoftissuebetween
maternalandfetalvascularsystems.

4. On the basis of mode of Impanation
Placenta
Superficial
Carnivore,
Dog,
Monkey
Eccentric
Squirrel, Rat
Interstitial
Hedgehog, Ape,
Human, Bat,
Guinea pig

5. 1.SuperficialImplantation:-Growthofthe
chorionicsacbringsitintocontactwith
theliningofthemainuterinecavity.This
typeofimplantationiscalledcentral
implantation,e.g.,ungulates,carnivores,
monkey.
2.EccentricImplantation:-Thechorionic
sacliesforatimeinafoldorpocket
whichloosesofffromthemaincavity,
e.g.,beaver,rat,squirrel.
3.InterstitialImplantation:-Thechorionic
sacpenetratesintothesubstanceofthe
uterinelining,e.g.,hedgehog,guineapig,
somebats,apeandman.

6. Placenta
Yolk sac
MetaTherai
Allanto-chorionic
Eutheria
Non-Deciduous
Cotyledonary
Ruminants.
Intermedia
te
Giraffe
Diffuse
Horses,
pigs
Deciduous
Zonary
dogs and
cats
Meta
Discoidal
Human
Discoidal
Primates,
Rodents
Contra-
Deciduate
On the basis of origin of Villi

7. Placenta
Yolk sac
MetaTherai
Allanto-chorionic
Eutheria

8. 1. Non-Deciduous Placenta or Semiplacenta:
Inmostmammals,theimplantationissuperficial,i.e,
theblastocystliesinthecavityoftheuterusin
contactwiththeuterinewall.Thecontactmaybe
mademoreintimatebythesurfaceoftheblastocyst
byformingfinger-likeoutgrowthswhichpenetrate
intodepressionsinthewalloftheuterus.Such
outgrowthsareinitiallyformedbythetrophoblast
(i.e.,theepitheliallayercoveringtheblastocyst),but
laterontheconnectivetissueandbloodvessels
invadetheoutgrowths.
Theseoutgrowthsarecalledchorionicvilli,theblood
vesselsofchorionicvilliarethebranchesofallantoic
bloodvesselsincaseofchorio-allantoicplacenta.(In
chorio-vitellineplacenta,vitellinebloodvesselsgive
theirbranchestochorionicvilli).

9. Atthetimeofbirth,whenparturition(the
separationofthefoetusanditsmembranes from
themother’sbody)occurs,thechorionicvilliare
simplydrawnoutfromthedepressionsinthe
walloftheuterusand,thus,maternalandfoetal
tissuesareseparatedwithoutfurtherdamageto
theuterinewallandnobleedingoccurs.
Thistypeofplacentaiscallednon-deciduateor
non-deciduousplacentaandisfoundinpigs,
cattleandsomeothermammals.Further,the
chorionicvilliofanon-deciduateplacenta,
becauselieinappositionwiththeendometrium,
but,donotfusewithit,sosuchaplacentais
alsocalledsemiplacenta.

10. 2. Deciduous Placenta or Placenta Vera:
Inothermammals, however,thedegreeofintimacy
between maternalandfoetaltissuesbecomes further
increased.Thewalloftheuterusbecomes erodedto
variousdegreesthroughtheactionofthetrophoblastand
theembryonictissuespenetrateintotheuterinewall,
establishingamoreintimatecontactandfacilitatingthe
passageofsubstancesfromthemothertothefoetusand
fromthefoetustothemother.
Herebecausethechorionicvillifusewiththeeroded
uterinemucosa,suchplacentaiscalledplacentavera
(trueplacenta).Attheendofpregnancytheuterinewallis
nolongerintactandwhenthefoetuswithitsmembranes
includingthechorionisremoved,moreorlessextensive
haemorrhage fromtheuterinewallensues(i.e.,atbirth,
whensuchplacentaisdischarged,theuterineliningalso
tearsawaywithsomebleeding).Suchatypeofplacenta
foundinhighereutherianmammals andiscalled
deciduateordeciduousplacenta.

11. Thematernaltissueswhichare
expelledatbirthinthecaseof
deciduate placenta arecalled
deciduae.The haemorrhage at
parturitionisnormallystoppedby
thesamemechanism asservesfor
theexpulsionofthenewborn,the
contractionofthemuscularwallof
theuterusconstrictstheblood
vesselsand,thus,slowsdownthe
flowofblood,untilclottingofthe
blood stops thehaemorrhage
altogether.

12. 3.Contra-DeciduatePlacenta:-
InPeramelesandTalpa(mole),
somewhat modified typeof
deciduate placenta occurs,
which is called contra-
deciduate placenta.Insuch
case,notonlythereisalossof
maternaltissuebutalsoofthe
foetalportionoftheplacenta,
bothofwhichabsorbedinsitu
bymaternalleucocytes.

13. 1. Non-Deciduous Placenta or Semiplacenta:
A.DiffusePlacenta:-Insomemammals
(e.g.,ungulates,pig,sow,mare,horse,
lemur,etc.)thechorionicvilliremain
scatteredalloverthesurfaceofthe
chorion and theirplacentae are
correspondinglyexpensive.

14. 1. Non-Deciduous Placenta or Semiplacenta:
B.Cotyledonary Placenta:-Ina
cotyledonaryplacenta,thevilliarefound
ingroupsorpatches,whiletherestof
thechorionsurfaceissmooth.The
rosettesorpatchesofvilliarecalled
cotyledons,andtheplacentaofthistype
isfoundinruminants (cud-chewing),
ungulatessuchascattle,sheepand
deer.

15. 1. Non-Deciduous Placenta or Semiplacenta:
C.Intermediate Placenta:-
It is a rare type, it shows free villi
on cotyledons. Villi are arranged in
cotyledonsas well as scattered.
Hence it is called intermediate type
placenta
In these three types of placenta
during parturition the foetus will
not damage uterus.
Camel and giraffe

16. 2. Deciduous Placenta or Placenta Vera:
A-Zonary Placenta:-In a zonary placenta,
the villi are developed in the form of a belt
or girdle-like band around the middle of
their blastocyst or chorionic sac, which is
more or less elliptical in shape. Such a
placenta occurs in carnivores (e.g., cats,
dogs, etc.). Raccoon has incomplete
zonary placenta.

17. 2. Deciduous Placenta or Placenta Vera:
B.DiscoidalPlacenta:-Ininsectivores,
bats,rodents(mouse,rat,rabbit,etc.)
andbear,thevilliarerestrictedtoa
circulardiscorplateonthedorsal
surfaceofblastocyst.

18. 2. Deciduous Placenta or Placenta Vera:
C.Metadiscoidal Placenta:-Inprimatesalso
discoidalplacentaisfoundbutofspecialtype,
i.e.,chorionicvilliareatfirstscatteredbutlater
onbecomerestrictedtooneortwodiscs.
a-MonodiscoidalThus,inmantheplacentahas
asingledisc-shapedvillousareaandiscalled
monodiscoidalplacenta.
b-Bidiscoidal-Inthemonkeys,theplacenta
consistsoftwodisc-shapedvillousareasand
suchaplacentaiscalledbidiscoidalplacenta.

19. C. Classification of Placenta
According to Histology:
On histological basis, following
types of mammalian placentae
have been recognised:

20. In placenta formation six tissue or
membranes participated :
(i)The endothelium of the maternal blood vessels;
(ii) Endometrial connective tissue (mesenchyme);
(iii) Uterine epithelium;
(iv) The ectoderm of the chorion or chorionic epithelium;
(v) Chorionic connective tissue (foetal mesenchyme)
(vi) The endothelium of foetal blood vessels.

21. 1.Epithelio-Chorial Placenta:
2.Syndesmo-Chorial Placenta:
3.Endothelio-Chorial Placenta:
4.Haemo-Chorial Placenta:
5.Haemo-Endothelial Placenta:

22. There are three layers of fetal extraembryonic
membranes in the chorio-allantoicplacenta of
all mammals, all of which are components of
the mature placenta:
1.Endothelium lining allantoiccapillaries
2.Connective tissue in the form of
chorioallantoicmesoderm
3.Chorionic epithelium, the outermost layer of
fetal membranes derived from trophoblast
There are also three layers on the maternal
side, but the number of these layers which are
retained -that is, not destroyed in the process
of placentation -varies greatly among species.
The three potential maternal layers in a
placenta are:
1.Endothelium lining endometrial blood vessels
2.Connective tissue of the endometrium
3.Endometrial epithelial cells

23. Placenta Foetal
Layer
Uterus
layer
Example
Epithelio-
Chorial
3 3
Marsupials, Ungulates
(pig, horse, cow, cattle,
etc.) and lemurs
Syndesmo-
Chorial
3 2
Ruminant ungulates
(cattle, sheep)
Endothelio-
Chorial
3 1
Carnivores (dogs, cats,
bears, etc.)
Haemo-
Chorial
3 0
Primates, insectivores
(moles, shrews), and
chiropterans (bats),
Haemo-
Endothelial
2 0
Higher rodents (rat,
guinea pig, rabbit),

25. http://www.notesonzoology.com/wp-
content/uploads/2017/07/clip_image006-
82.jpg

26. 1.Epithelio-ChorialPlacenta:
The epithelio-chorialtype placenta is most primitive type
and it is found in marsupials, ungulates (pig, horse, cow,
cattle, etc.) and lemurs.
In this case, placenta is formed of six tissue or
membranes:
I.The endothelium of the maternal blood vessels;
II.Endometrial connective tissue (mesenchyme);
III.Uterine epithelium;
IV.The ectoderm of the chorion or chorionic epithelium;
V.Chorionic connective tissue (foetalmesenchyme) and
VI.The endothelium of foetalblood vessels.
Because,theimmediatecontactofthetwohalvesofthe
placentainvolveschorionicepitheliumanduterine
epithelium,thistypeofplacentaiscalledepithelio-
chorialplacenta.Thevilliofanepithelio-chorial
placenta,pushinthewallofuterusandlieinpocket-like
depressionsoftheuterinewall.

27. The foetal chorion is in contact with epithelium of the uterus
hence it is called epithelio-chorial placenta. In between foetal,
maternal parts six layers are present. If all the six layers are
present the placenta is called epithelio-chorial placenta.

28. 2.Syndesmo-Chorial Placenta:
Intheruminantungulates(cattle,sheep),the
foetalandmaternalcomponents arefusedso
intimatelyastoresultinadestructionofthe
uterineepithelium,thus,bringingthechorion
intocontactwiththeconnectivetissueofthe
uterinemucosa.Onlyfivebarriersortissues,
therefore,liebetweenthetwo(viz.,foetaland
uterine)bloodstreams.Thistypeofplacentais
calledsyndesmo-chorialplacenta.Theallanto-
chorianicvithwillpierceintotheuterusofthe
mother,thechorionwillcomeincontactwith
syndesmose ofmother’suterus.Henceitis
calledsyndesmose chorial.

29. 3.Endothelio-Chorial Placenta:-In
carnivores (dogs, cats, bears,etc.),
the uterine mucosa is also reduced
and the chorionic epithelium comes
in contact with endoethelial walls of
the maternal (uterine) blood
vessels. In such a case, therefore,
there lies only four barriers between
the foetal and maternal blood
streams. This type of placenta is
called endothelio-chorial placenta.

30. Thechorionofthe
foetuswillcomein
contactwiththe
endothelim of
mother‘suterus,
henceitiscalled
endothelio-chorial
placenta.

31. 4.Haemo-ChorialPlacenta:-Inthe
haemo-chorialplacenta ofprimates,
insectivores (moles, shrews), and
chiropterans(bats),areductionofthe
barrierstothreeoccurs,i.e.,the
endothelialwallsofmaternal(uterine)
bloodvesselsalsodisappearandthe
chorionicepitheliumisbatheddirectlyin
maternalbloodsinuses.Actually,the
chorionicvilliaresurroundedbyspaces
(sinuses)devoidofendotheliallining,
intowhich maternal bloodenters
throughtheuterinearteriesflowsout
throughtheuterineveins.

32. The placental
connections are
moreintimate.The
chorionoffoetus
willfloatinthe
blood pools of
mother’s uterus.
Henceitiscalled
haemochorial
placenta

33. 5.Haemo-EndothelialPlacenta:-In
haemo-endothelialplacentaofhigher
rodents(rat,guineapig,rabbit),the
number ofbarriersbetween the
maternalandfoetalbloodstreamsis
furtherreducedtotwo.Inthem,the
chorionicvillilosetheirepithelialand
connectivetissuelayerstosucha
degreethat,inmostplaces,thebare
endothelialliningoftheirbloodvessels
aloneseparatesthefoetalbloodfrom
thematernalbloodsinuses.