Extraembryonic Membranes & Multiple Gestation

medicoseacademics@gmail.com
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Extraembryonic Membranes &
Multiple Gestation
Dr. Muhammad Ali Rabbani
Asst. Professor Anatomy
MBBS (NMC, Multan)
FCPS (Anatomy)
CHPE (Certificate in Health Professional Education)
DHPE (Diploma in Health Professional Education)
MBA (Masters of Business Administration)
MPH (Masters of Public Health)

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Learning Objectives
By the end of this lecture students should be able to
• Describe the development and components of the placenta.
• Explain placental circulation, exchange and major functions.
• Describe the development and functions of amnion & amniotic fluid.
• Explain the development & significance of the yolk sac and allantois.
• Describe the formation and main features of the umbilical cord.
• Relate common abnormalities of the placenta, membranes, amniotic
fluid and umbilical cord to their developmental basis.
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Early Development

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4.5 days 6 days

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7.5-day

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9-day

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12-day

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13-day

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4th
Week
(end)

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2nd
Month
(start)

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Placenta

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The placenta is the primary site of nutrient and gas exchange between
the mother and featus
• The placenta is a fetomaternal organ that has two components
• Fetal part – from the chorionic sac
• Maternal part – from the endometrium
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Fetal Component – Chorionic Plate
Villi
• Primary Villi – end of second week1
• cytotrophoblast covered by a syncytium
• Secondary villi – early in third week1
• Mesoderm added at core
• Tertiary / mature villi
• Blood vessels differentiate in mesoderm
• Make contact with vessels of chorionic plate and umbilical vessels
• Heart begin to beat in fourth week of dev.
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Fetal Component – Chorionic Plate
• Outer Cytotrophoblastic Shell
• Stem / Anchoring Villi
• Free Terminal Villi
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end of the third week
end of the fourth week

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• Embryonic pole
vs
Abembryonic pole
beginning of the second month

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Chorion- Parts
• In the early weeks (till beginning of 8th
week)1
, villi
cover the entire surface of the chorion
• By third month chorion is divided as follows
• Chorion frondosum (bushy chorion)
• villi on the embryonic pole continue to grow and expand
• Chorion laeve (smooth chorion)
• Villi on the abembryonic pole degenerate
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Delivery of Maternal Blood
• Supplied by spiral arteries
• Cytotrophoblast cell erode these by endovascular
invasion to release blood in intervillous spaces
• Cytotrophoblast cells undergo an epithelial-to-
endothelial transition
• Invade and replace the terminal ends of spiral
arteries, creating hybrid vessels
• Small-diameter, high-resistance → larger
diameter, low-resistance
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Maturation of Villi
• Layers
• Disappearance of
cytotrophoblast progresses
from the larger to smaller villi
• Villi where cytotrophoblast
exists, do not participate in
exchange
• Branching
• Syncytial knots
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Decidua – Parts
• Endometrium of the uterus in a pregnant woman
• Functional layer of the endometrium, which is shed during parturition
• Contain abundant lipids and glycogen
• Divided into following parts
• Decidua Basalis / Decidual Plate (compact layer)
• over the chorion frondosum
• Tightly connected to chorion frondosum to make placenta
• Decidua Capsularis
• over the abembryonic pole
• Becomes stretched and degenerates as it obliterate uterine cavity
• Decidua Parietalis
• Over rest of the uterus
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End of second month End of third month

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Structure of Placenta
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• Fetal Portion – chorion frondosum
• Maternal Portion – decidua basalis
• Chorionic plate
• Intervillous spaces
• Decidual Septa
• Cotyledons

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Full Term Placenta
• Covers approximately 15% to 30% of internal surface of the uterus
• Diameter of 15 to 25 cm
• Thickness 3cm
• Weighs 500-600g (1/6th
of fetal weight)
• Expelled 30 min after birth as afterbirth
• Maternal side
• Cobblestone appearance, 15-20 cotyledons, covered by thin layers of decidua basalis
• Fetal Surface
• Chorionic plate, chorionic vessels, covered by amnion (shining)
• Attachment of the umbilical cord is usually eccentric and occasionally even marginal
• velamentous insertion – insertion into chorionic membranes
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Placenta Previa
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Placental
Abnormalities
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Placental Abnormalities
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Battledore placenta
Velamentous placenta

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Circulation in Placenta
• Receive blood through 80 to 100 high pressure spiral arteries
• Intervillous spaces contain 150mL blood
• replenished about three or four times per minute
• surface area of 4 to 14m2
• Exchange occur only in mature villi
• Microvilli on syncytium
• Reduction in layers by 4th
month (discussed)
• Placental barrier
• not a true barrier – many substances pass through it freely
• Hemochorial type – maternal blood is supplied by chorionic derivative
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Erythroblastosis Fetalis
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Functions of Placenta
• Main functions of the placenta are
(1) Metabolism (e.g., synthesis of glycogen)
(2) Transport of gases and nutrients
(3) Endocrine secretion (e.g., human chorionic gonadotropin [hCG]).
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Metabolism
• The placenta, particularly during early pregnancy, synthesizes
• Glycogen
• Cholesterol
• fatty acids
• which serve as sources of nutrients and energy for the embryo/fetus
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Transfer / Exchange
• Gases
• oxygen, carbon dioxide, and carbon monoxide
• At term, the fetus extracts 20 to 30 mL of oxygen per minute
• O2 delivery is flow limited, not diffusion limited
• Nutrients and Electrolytes
• Occurs bi-directionally via various transport mechanism
• Hormones
• Thyroxine, triiodothyronine exception for water soluble
• Unconjugated steroids pass freely
• Maternal Antibodies and proteins
• IgG begin transported at about 14 weeks
• Complement system at the end of first trimester
• IgG production reaches adult levels at 3 years of age
• Waste
• Drugs
• Infectious Agents 42

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Hormone Production
All hormones are synthesized in the syncytial trophoblast
• Progesterone – sufficient to support pregnancy by 4th
month
• Estriol – until just before the end of pregnancy
• hCG – during first two months
• Human chorionic Somatomammotropin (human placental lactogen)
• gives the fetus priority on maternal blood glucose
• makes the mother somewhat diabetogenic
• It also promotes breast development for milk production
• Human chorionic thyrotropin
• Human chorionic corticotropin
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Amnion

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Development
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3 weeks

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4 weeks

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10 weeks

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End of Third Month – Chorionic Cavity Obliterated 1
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20 weeks
By 22 to 24 weeks, the reduced blood supply to the decidua capsularis causes it to
degenerate and disappear, after this chorionic laeve and decidua parietalis fuse2

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Amniotic Bands
• Occasionally, tears in the amnion result in amniotic bands that may
encircle part of the fetus, particularly the limbs and digits
• Amputations, ring constrictions, and other abnormalities, including
craniofacial deformations, may result
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Premature Rupture of Membranes (PROM)
• Rupture of the membranes before uterine contractions (10%)
• Preterm PROM occurs before 37 completed weeks of pregnancy (3%)
• Risk factors
• previous pregnancy affected by prematurity or PROM, black race, smoking,
infections, and severe polyhydramnios
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Amniotic Fluid
• A clear, watery fluid that fills the amniotic cavity
• 7.2 and specific gravity 1.0069 to 1.008
• Increases with gestational age1
• 30 mL at 10 weeks of gestation
• 450 mL at 20 weeks
• 800 to 1,000 mL at 37 weeks
• Produced in part by amniotic cells but is derived primarily from maternal blood
• Replaced every 3 hours1,2
• Mostly through amniochorionic membrane, others umbilical cord, fetal side placenta
• From the beginning of the 5 month, the fetus swallow 400mL a day, absorbed by GIT
• Almost same amount excreted in urine
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Amniotic Fluid – Composition
• Aqua
• Undissolved material
• desquamated fetal epithelial cells
• Organic compounds
• carbohydrates, fats, enzymes, hormones, and pigment
• Inorganic salts
• As pregnancy advances, the composition of the amniotic fluid changes as
fetal excreta is added
• This is also basis of tests based on amniocentesis
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Amniotic Fluid- Functions
1) Permits symmetric external growth of the embryo and fetus
2) Acts as a barrier to infection
3) Permits normal fetal lung development
4) absorbs jolts
5) prevents adherence of the embryo to the amnion
6) Maintain relatively uniform and constant temp around fetus
7) allows for fetal movements, aiding in muscular development
8) Maintain hemostasis of fluid electrolyte
9) amniochorionic membrane forms a hydrostatic wedge that helps to dilate the
cervical canal
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Hydroamnios / Polyhydroamnios
• Excess of amniotic fluid (1,500 to 2,000 mL)
• Causes
• idiopathic causes (35%)
• maternal diabetes (25%]
• congenital malformations, that prevent the infant from swallowing the fluid
• central nervous system disorders (e.g., anencephaly)
• gastrointestinal defects (atresias, e.g., esophageal)
• Complications / Associations
• severe defects of the central nervous system, such as meroencephaly (anencephaly)
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Oligohydroamnios
• Decreased amount of amniotic fluid (< 400 mL]
• Cause
• Placental Insufficiency
• PrePROM
• Renal agenesis
• Obstructive Uropathy
• Complication
• constrict the fetus
• there may be too little fluid for the fetus to “ breathe” into its lungs resulting in lung
hypoplasia
• facial defects, and limb defects
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Yolk Sac (Umbilical Vesicle)
Umbilical vesicle preferred in humans as it does not contain yolk

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3 weeks

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4 weeks

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10 weeks

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End of Third Month – Chorionic Cavity Obliterated 1
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20 weeks

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• Primary and Secondary Yolk sac (discussed)
• Can be observed sonographically early in the fifth week
• By 10 weeks, the umbilical vesicle has shrunk to a pear-shaped
remnant approximately 5 mm in diameter
• By 20 weeks, the umbilical vesicle is very small
• Thereafter, it is usually not visible (very rarely persists, not sig.)
• Omphaloenteric duct usually detaches from the midgut loop by the
end of the sixth week
• Failure – Meckel diverticulum
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Significance
• Transfer of nutrients to the embryo during the second and third
weeks
• Blood development first occurs in the well-vascularized
extraembryonic mesoderm covering the wall of the umbilical vesicle
beginning in the third week
• During the fourth week, the endoderm of the umbilical vesicle is
incorporated into the embryo as the primordial gut
• Primordial germ cells appear in the endodermal lining of the wall of
the umbilical vesicle in the third week and subsequently migrate
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Allantois

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• Appears on day 16 as a small diverticulum of umbilical vesicle
• During the second month, the extraembryonic part of the allantois
degenerates
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Function
• Other animals (reptiles, birds, and most mammals)
• respiratory function and/or
• acts as a reservoir for urine during embryonic life
• In Humans
• Surrounding mesenchyme forms umbilical blood vessels
• Blood formation occurs in its wall during the third to fifth weeks.
• Proximal part persist throughout dev. as urachus
Urachus in adults is represented as median umbilical ligament
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Allantoic Cysts
• Allantoic cysts, remnants of the extraembryonic portion of the
allantois, are usually found between the fetal umbilical vessels
• can be detected by ultrasonography
• They are most commonly detected in the proximal part of the
umbilical cord, near its attachment to the anterior abdominal wall
• The cysts are generally asymptomatic until childhood or adolescence,
when they can become infected and inflamed.
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Umbilical Cord

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• Primitive Umbilical Ring
• reflection between the amnion and ectoderm
(amnio-ectodermal junction)
• At 5th
week, following structure pass through
• Connecting stalk – containing allantois and
umbilical vessels
• Yolk stalk (vitelline duct) accompanied by
vitelline vessels
• Canal connecting the intraembryonic and
extraembryonic cavities
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• By the end of third month amniotic cavity enlarges rapidly
• Amnion fuses with chorion obliterating chorionic cavity
• Amnion begins to envelop the connecting and yolk sac stalks,
crowding them together and giving rise to the primitive umbilical
cord
• Distally – yolk sac and umbilical vesicles
• Proximally – intestinal loops and remnants of allantois
• Cavity in the cord is obliterated after retraction of physiological
umbilical hernia
• Later allantois and the vitelline duct and its vessels are also
obliterated, all that remains in the cord are the umbilical vessels
surrounded by Wharton jelly
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Umbilical Cord Abnormalities
• At birth 2 cm in diameter and 50 to 60 cm long
• Length of cord reflects intrauterine movements
• Long cord – false knots, sometime true knots, around neck, prolapse
• Short cord – pulling on placenta
• Single umbilical artery
• 1 in 200
• 20% chance of cardiovascular anomaly
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Stages of Labour
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Multiple Pregnancies

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Learning Objectives
• Differentiate between monozygotic and dizygotic twins.
• Explain different types of monozygotic twins.
• Define conjoint twins
• State the different anomalies associated with twin pregnancies?
(vanishing twin, fetus papyraceus, twin-twin transfusion syndrome)

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Dr Muhammad Ali Rabbani

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Dizygotic Twins
• Two separate ova a released. Both fertilized independently and implant
independently.
• Thus there will be two separate sets of placenta, chorion and amnion.
• Both will have different genomes. Twins born will have differences as there are
between any two non twin siblings. Both may be even of different sex.
• Sometimes the implantation may be so close that placenta of both will be fused.
• Twins will be dichorionic and diamniotic (DCDA)

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Monozygotic Twins
• One ovum is released and is fertilized.
• Fertilized ovum (zygote) may then split and the two fragments grow
independently to form twins.
• The time when the split occurs, determines which type of twins will
form.

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Dichorionic Diamniotic (DCDA)
• When separation occurs within 3 days of conception. Earliest
separation occur at two cell stage and later in the morula stage.
• Thus two blastocysts form. Grow independently.
• Each Zygote will have its own placenta and its own amniotic cavity.

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Monochorionic Diamniotic (MCDA)
• When the split occurs between day 4-8 (early blastocyst stage), inner
cell mass split into two, outer cell mass will not split.
• Thus only one trophoblast grows, leading to only One Placenta
• Two separate embryoblast grows, leading to two different embryos
with two different amniotic cavity.

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Monochorionic Monoamniotic (MCMA)
• When split occurs between day 8-12 at bilaminar germ disk, just
before appearance of primitive streak.
• Separate Primitive Streaks appear on each disk after the split, thus
both will enter in gastrulation phase and continue to grow into
separate fetus.
• But the Amniotic cavity above the epiblast will not split, thus both
twins will share the same amniotic cavity in addition to the placenta.

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Conjoined or Siamese Twins
• If splitting is delayed beyond day 12, the embryonic disk has formed
and conjoined or Siamese Twins are formed.
• Different types of Siamese Twins are named according to the body
parts from which they are fused.

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Twin-Twin Transfusion , Vanishing Twin
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Thank You
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Extraembryonic Membranes & Multiple Gestation