Transcript PLACENTA & FETAL MEMBRANES

Amniotic cavity
Endometrium
Maternal
blood vessels
Proliferating
syncytiotrophoblast
Cytotrophoblast
Amniotic cavity
Bilayered
embryonic
disc:
• Epiblast
• Hypoblast
Endometrial
epithelium
Lacuna (intervillus
space) containing
maternal blood
Chorionic villus
Primary
germ layers:
• Ectoderm
• Mesoderm
Chorion
• Endoderm
Amnion
Forming
body stalk
Allantois
Yolk sac
Extraembryonic
mesoderm
Chorion
being formed
Extraembryonic
coelom
Lumen of uterus
(a) 71/2-day implanting
blastocyst
(b) 12-day implanted
blastocyst
(c) 16-day embryo
Changes in trophoblast( formation of chorion)
1) During implantation, trophoblast forms 2 layers:
- Outer syncytiotrophoblast.
- Inner cytotrophoblast.
The syncytio trophoblast sends finger like projections(villi)
that invade the endometrium(decidua). These villi erode the
decidual blood vessels forming lacunae in intervillous space fil
led with maternal blood(lacunar stage).
At day 12 cells of syncytiotrophoblast penetrate more deepl
y,
the lacunea becomes continous with maternal blood sinsoids
so, maternal blood enters lacunar system→ uteroplacental
circulation begins.
by the appearance of extra embryonic mesoderm the wall of choronic
vesicle is called chorion .
The chorion = sycyt. + cytotroph blast.+outer layer of
3 stages of chorionic villi appear:
1)primary villi: a core of cytotrophblast surrounded by syncytiotrophobl
ast.
2) secondary : as the 1ry but invaded with a core of mesoderm. 3rd eek
3) tertiary : as the 2ry + fetal blood capillaries. 3rd week
Types of 3ry villi:
a) stem(anchoring) villi :fix the ch. Vesicle to the decidua.
b) Free (absorbing) villi : in the intervillous space to increase surface
area
for exchange between fetal and maternal blood.
Fate of chorionic villi:
1) Villi opposite the decidua basalis enlarge forming chorion frondosu
m→
fetal part of placenta.
1) Villi opposite decidua capsularis atrophy→ chrion laeve
Cytotrophoblastic shell : the cytotrophblast at the end of anchoring villous
penetrate the sycytiotrophoblast and extend to fuse with similar cytotrophoblast
From other villous Forming a continuous shell surrounding the embryonic vesicle
to prevent further penetration of the decidua by syncytiotrophoblast .also it
Changes in the Trophoblast
By the beginning of the 2nd month, trophoblast is characterized
by a great number of 2° & 3° villi that give it a radial
appearance.
 Chorionic plate: the extreembryonic mesoderm lining the
inside of cytotrophoblast. That border the placenta from fetal
side.
 Decidual plate: decidua basalis border the placenta from
maternal side.
 Outer cytotrophoblast shell attaches the villi to maternal
decidua.
 Lacunar or intervillous space in between the villi
 Syncytial knots – the thin syncytium breaks off and drop into
intervillous blood lakes. Usually degenerates and causes no
harm
Chorion Frondosum &
Decidua Basalis
Chorion frondosum (bushy or villous chorion): well developed
villi on embryonic pole. This
Chorion laeve (smooth chorion): villi on the aembryonic pole
Decidua basalis: decidua over ch. frondosum
Decidua capsularis: over aembryonic pole
Decidua parietalis (decidua vera):
Amniochorionic membrane:fusion of amnion and chorion that
obliterates the chorionic cavity . It is the thin membrane that
ruptures during labour.
Placenta
Decidua basalis
Chorionic villi
Yolk sac
Amnion
Amniotic
cavity
Umbilical cord
Decidua
capsularis
Extraembryonic
coelom
(f) 13-week fetus
Uterus
Lumen of
uterus
Structure of the Placenta
By the beginning of the 4th month, placenta has 2
components:
 Fetal portion –: chorion frondosum
 Maternal portion – decidua basalis
 Chorionic & decidual plates: borders placenta on
fetal and maternal side
 Junctional zone: in between 2 plates , contain
intervillous space
 Decidual septa that projects into intervillous space
but not reach chorionic plate.
 cotyledons: the placenta is divided by decidual
septum into compartements
Full-term Placenta
At full term, placenta is discoid with a diameter of
15~25 cm, is approximately 3 cm thick, & weighs
about 500~600 g.
Sources: fetal part + maternal part.
Has 2 surfaces:
1)fetal surface – smooth, coverd by amnion, umb. Cord
attached to its centre.
2)Maternal surface – irregular, divided into 15 – 20 lobes
or cotyledons separted by placental septa.
Circulation of the Placenta
The intervillous spaces of a mature placenta contain
approximately 150 ml of blood, which is replenished about 3 or
4 times per minute.
Fetal blood arrives to the placenta by 2 umb. Arteries that
branch to form capillaries in the villi where gaseous exchange
occur with maternal blood in intervillous space. finally
oxygenated blood returns to fetus via lt. umbilical vein
Placental membrane / barrier (x): maternal in the intervillous
spac and fetal blood in the capillaries don’t mix. They are
separated by:
– syncytiotrophoblast
- cytotrophoblast.
– extraembryonic mesoderm
- endothelium of fetal
capillaries.
By 4th month: layer 2,3 disappears
Function of the Placenta
Exchange of metabolic & gaseous products between maternal
& fetal bloodostreams
 Exchange of gases .
 Exchange of nutrients & electrolytes .
 Transmission of maternal antibodies
Production of hormones: progestrone, estrogen, relaxin,
placental lactogen and HCG.
• Exchange ofwaste products
• Placental membrane protects the fetus as maternal and fetal
blood don’t mix.
• Substances cross that membrane:
- organisms: AIDS, geman measles
- Drugs: thialodomide→ short limbs
Chemicals: insecticides.
Maternal antibodies.
Rhesus factor: if the mother is Rh negative and the baby i
s
Rh posithive, the mother produces anti Rh antibodies which
causes hemolysis of fetal red blood cells.
- Usually 1st baby is not harmed but subsequent babies suffer
.
Anomalies:
1) Placenta previa: centralis , marginalis, lateralis.
2) Accessory placenta.
3) Bipartite , tripartite placenta.
4) Batteldore placenta: cord is inserted at periphery o
•
Velamentous insertion of the cord: the cord is inserted outsi
de
the placenta, i.e in the membranes.
Chorion epithelioma: malignant tumour of the placenta.
Placenta accreta, increta and placenta percreta: villi deeply
invading the endometrium, myometrium and whole thickness
of
the uterus respectively.
Dizygotic
-2/3 of the twins.
- results from two oocytes
fertilized by different sperms.
-Each have its own placenta,
amnion and chorionic cavity.
Twins
Monozygotic Twins
- From a single fertilized ovum.
- they result from splitting of t
he zygote at various stages of
development .
a) Spliiting at early blastocyst
stage.
a) Splitting of inner cell mass
within same blastocyst.
c) Splitting of bilaminar germ di
sc
Frequency of Types of Placentas & Fetal
Membranes in Monozygotic (MZ) & Dizygotic
(DZ) Twins
Single Chorion
Two Chorions
Zygosity
Single
Amnion
MZ
Very rare
65%
25%
10%
DZ
-
-
40%
60%
Two
Fused
Two
Amnions Placentas Placentas
Conjoind (Siamese) twins