Placentation in Mammals

In all viviparous animals, embryonic development takes place inside the uterus of the mother, because the eggs are microlecithal and the amount of stored yolk is not sufficient for the developing embryo. Such embryos get attached to the uterine wall to draw essential substances from the maternal circulation through the placenta.

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

2. Placentation in mammals
•Inallviviparousanimalstheembryonicdevelopmenttakesplaceinsidetheuterusofthe
mother,becausetheeggsaremicrolecithalandtheamountofstoredyolkisnotsufficientto
thedevelopingembryo.Suchembryosgetattachedtotheuterinewalltodrawessential
substancesfromthematernalcirculationthroughtheplacenta.
Definitionofplacenta:
•Aplacentaisanorganbuiltupofmaternalandfoetaltissuesjointly.Itservesforthe
transportofnutrientsfromthemothertissueswiththoseoftheembryoaswellasthe
exchangeofgasesbetweenthetissuesofthetwo.Thusaplacentamaybedefinedasa
temporaryconnectionbetweenthematernalandfoetaltissuesforthepurposeofshelter,
nutrition,respiration,excretionanddefense.
1.Foetalplacenta:itisformedbytheextraembryonicmembranes(chorion,allantoisoryolk
sac)fromtissuesofembryo.Chorionistheprincipalcomponentoffoetalplacenta.
2. Maternal placenta: it is formed only by a single component i.e. The mother’s uterine
endometrium.

3. Classification of Placenta:
•1. Classification Based on the Degree of Intimacy
•2. Classification Based on the Distribution of Villi
•3. Classification Based on the Degree of Involvement of Fetal and Maternal
Tissues
•4. Classification Based on the Types of Implantation

4. Classification Based on the Degree of Intimacy:
i.Non-DeciduousPlacentaorSemiPlacenta:
•Innon-deciduousplacentatheimplantationissuperficial.Thisoccursinmost
mammalswheretheblastocystslieintheuterinecavity.Atthepointofcontact
withthewalloftheuterus,theblastocystsurfacegivesoutfingerlike
projectioncalledchorionicvillithatpenetrateintothedepressionsoftheuterine
wallandarelooselyunited.
•Atthetimeofbirth,whenparturitiontakesplace,thechorionicvilliaresimply
withdrawnfromthecavitiesoftheuterinewallwithoutcausinganydamageor
bleeding.Thistypeofplacentaformationisseeninpig,cattle,horseetc.,where
thelesshazardousbirthprocessallowsthefemalestoresumefullrunningspeed
soonafterbirth.
•Asthechorionicvillidonotfusewiththeendometrium,suchaplacentaisalso
calledsemiplacenta.

5. Classification Based on the Degree of Intimacy continue…
ii.DeciduousPlacentaorPlacentaVera:
•Incat,dog,primates,rodentsetc.,thedegreeofintimacybetweenthechorionic
villiandtheendometriumisgreatlyincreased.Theuterinewallgetseroded.The
chorionicvillifusewiththeerodeduterinemucosa.Suchaplacentaistermedas
placentavera(trueplacenta).
•Atthetimewhenparturitiontakesplacetheuterinewalldoesnotremainintact.It
tearsawayandextensivehaemorrhagetakesplaceatbirth.Suchatypeofplacenta
istermedasdeciduousplacenta.
•Thisphenomenaofshedding(tearingoff)andreplacementofmaternaltissueis
termedasdecidua(meaning,toshed).Heretheplacentaisphysiologicallymore
efficient,wherethemothersareprotectedenoughtorecoverfullyafterchildbirth.
iii. Contra-Deciduate Placenta:
•A somewhat modified type of deciduate placenta is seen in Paramelesand Talpa
(mole), where there is loss of both maternal tissue as well as foetal portion of
placenta. Such a placenta is called contra-deciduate placenta.

6. Classification Based on the Distribution of Villi:
i.DiffusedPlacenta:
•Indiffusedplacentathevilliarenumerousandarescattereduniformlyover
thewholeofchorion.Itisseeninungulates(pig,horse,mareetc.)andin
cetacea.
ii.CotyledonaryPlacenta:
•Inthistypeofplacentathevillibecomeaggregatedinspecialregionsor
patchestoformsmalltufts.Therestpartofthechorionsurfaceremains
smooth.Itisseeninruminant(cud-chewing)ungulatessuchascattle,sheep,
deeretc.Incamelandgiraffeanintermediatetypeofplacentaisseenwhere
thevilliarescatteredandarealsoarrangedincotyledons.

7. Classification Based on the Distribution of Villi continue..
iii. Zonary Placenta:
•In a zonary placenta the villi are confined to an annular or girdle-like zone on the
chorion (chorion is more or less elliptical in shape). Such a placenta occurs in
carnivores and may be of either incomplete zonary (e.g. raccoon) or complete
zonary (e.g. dog, cat, seal etc.) type.
iv. Discoidal Placenta:
•Here the villi become restricted to a circular disc or plate area on the dorsal
surface of blastocyst. Such a placenta is seen in insectivores, bats, rodents (rat,
mouse), rabbit and bear.

8. Classification Based on the Distribution of Villi continue..
v. Meta-Discoidal Placenta:
•Inprimatesaspecialtypeofdiscoidalplacentaisseenwherethevilliareatfirst
scatteredalloverthechorionbutlaterbecomesrestrictedtooneortwodiscs.The
mono-discoidaltypewithasinglediscisseeninman,whilethebi-discoidaltype
withtwodiscshapedvillousareasisseeninmonkeys.

9. Classification Based on the Degree of Involvement of Foetal and
Maternal Tissues:
i. Epitheliochorial Placenta:
•This type of placenta involves the contact of the chorionic epithelium with that of uterine
epithelium and thus the term epitheliochorial. The epitheliochorial placenta involves six tissue
barriers between the foetal and maternal circulation.
•It is the primitive type from which others have been derived and is seen in pig, sow, mare,
horse, cattle etc.
ii. Syndesmochorial Placenta:
•In the ruminant ungulates (cattle, sheep, deer, giraffe etc.), varying amounts of the uterine
epithelium may be absent. As a result the chorion is brought into direct contact with the
connective tissue of the uterus. Only five barriers therefore, lie between the two blood streams.
This type of placenta is termed as syndesmochorial.
iii. Endotheliochorial Placenta:
•In endotheliochorial placenta the uterine mucosa is reduced and the chorionic epithelium comes
in contact with the endothelial wall of maternal blood vessels.
•This type of placenta is characteristic of carnivores like dog, cat, bear etc. Here, only four
barriers remain between the two blood streams.

10. Classification Based on the Degree of Involvement of Foetal and Maternal
Tissues continue..
iv.HemochorialPlacenta:
•Areductionofthebarriersintothreeisobservedinhemochorialplacenta,
whichisfoundinprimates,insectivores(mole,shrew)andchiropterans(bat).
Here,theendothelialwallofthematernalbloodvesseldisappearsandthe
chorionicepitheliumisbatheddirectlyinthematernalblood.
v.HaemoendothelialPlacenta:
•Inthistypeofplacentathechorionicvilliloosestheirepitheliumand
mesenchymallayerstosuchadegreethattheendothelialwallofthefoetal
bloodvesselsremainincontactwiththematernalblood.
•Thehaemoendothelialplacentaisobservedinmouse,rat,rabbit,guinea-pig
etc.andonlyasinglebarrierseparatesthetwobloodstreams.

11. Classification Based on the Degree of Involvement of Foetal and
Maternal Tissues continue..

12. Classification Based on the Types of Implantation
i.ChoriovitellinePlacenta:
•Thechoriovitellineoryolksacplacentaisformedwhentheyolksacwall
(splanchnopleure)combineswiththechorionandthencontactsthe
endometrium.Itmaybefullyorpartiallyvascularizedbythevitellineplexus,
whichconnectswiththeomphalomesentericveinleadingtothedeveloping
heart.Bloodreturnsfromtheembryotothevitellinecirculationintheyolk
sacviatheomphalomesentericartery,whichbranchesfromthedorsalaorta.
ii.ChorioallantoisPlacenta:
•Whentheallantoisfuseswiththechorion,anallantochorionisformed.The
allantochorioncontactstheendometrium,resultinginachorioallantoic
placenta.Thisorganisthemostefficientformediatingphysiologicexchange
betweenmotherandoffspring.Itisverywellvascularizedbyumbilical
arteriesandvein.

13. Classification Based on the Types of Implantation continue..

14. Functions of Placenta
•Enables the transport of oxygen, water, electrolytes and nutrition
(carbohydrates, lipids, polypeptides, amino acids & vitamins) from maternal
to foetal blood.
•Excretion of carbon dioxide, urea etc. by the foetus into the maternal blood.
•Acts as a barrier (bacteria, viruses, drugs).
•Maternal antibodies (IgG) gives immunity to the foetus.
•Keeps the maternal & foetal blood streams separate, thereby preventing
antigenic reactions.
Placental Hormones
•Progesterone:
For maintenance of pregnancy after 4th month (when the corpus luteum
degenerates).
•Oestrogen (mainly estriol):
–Uterine growth
–Development of mammary gland

15. Thank you