Embryology

1. TWINNING
Dr.RabiaInamGandapore

2. Twinning
 Twins are two offspring produced by the
same pregnancy
 Twinscaneither be monozygotic ("identical"),
develop from just one zygote that will then split and
form two embryos
 Dizygotic ("fraternal"),develop fromtwo different
eggs; each are fertilized by separate sperm cells.

3. Frequency
 Once in about every eighty births
 Increase in frequency is due to increase in mothers
age and increase in use of fertility treatments(ART)
 Dizygotic are common then monozygotic
 T
win boys are more common then boy and girl,
which is more common then twin girls

4. Dizygotic Twins("non-identical twins",
"dissimilar twins", "biovular twins“)
Dizygotic/Fraternal twins
🞑 Approximately 90% of twinsare dizygotic
🞑 7 – 11/1000 births
🞑 By fertilization of two oocytes shed simultaneously
🞑 Showshereditary tendency
🞑 Incidenceincreaseswith maternal age & with fertility
treatments
🞑 Sex may be same or opposite
🞑 Asentirely different geneticconstitutionsoresemblance
like brothers and sisters

6. Fetal Membranes In Twins
🞑 Zygotes implant individually in
uterusand have their own
placenta & separate amnion
and chorionic sac
🞑 Placentas may fuse and
anastomosesbetween their
blood vessels result in
erythrocyte mosaicism
🞑 Similarly wallsof chorionic
sacs may also fuse

8. Development Of Dizygotic Twins

9. Monozygotic twins
🞑 Develop from single fertilized ovum
🞑 3-4/1000 births
🞑 Genetically identical
🞑 Result from splitting of zygote at an
early stage
🞑 Closely resembleinblood groups,
fingerprints, sex and external
appearance such as eye and hair
color

10. Splitting
 T
wo cell stage

11. Splitting
 Early blastocyst stage

12. Splitting
 Bilaminar germ disc
stage before appearance
of primitive streak

13. Development Of Monozygotic Twins

14. Third Type Of Twins
 This type is uniovular but
dispermatic following an
irregular meiotic cycle during
oogenesis.
 It is said that the either the
ovum, or seconday oocyte or
primary oocyte may undergo
unequal cytoplasmic division,
instead of throwing off a
polar body.

15. Fetal membranes

16. Twin Defects
 Erythrocyte mosaicism: Anastomosis b/w blood
vessels of fused placenta of DZ; R
BCsof two different
types.
 Dizygotic twins with hormonal imbalance:
testosteroneof male affect the developmentof
female, females tendsto havesquare jaw, larger
teeth and better ball skills than girls but mostly are
infertile

17.  Fetus papyraceous: Death
and resorption of one
fetus at theend of first or
beginning of 2nd trimester
(Vanishing T
win)

18. Twin-twin Transfusion Syndrome
 Shunting of arterial blood
from one twin through AV
anastomosisinto venous
circulation of other twin.
Donor twin is anemic
while therecipient twin is
polycythemic
 Occurs mostly in
monochorionic monozygotic
pregnancies
 Outcome is poor. Death of
both twins

19. CONJOINED OR SIAMESETWINS
(Partial SplittingOf Primitive Node & Streak)
 Conjoinedtwins(or the
commonly used term
"siamese")are monozygotic
twins whose bodies are joined
together during pregnancy
 This condition occurs in about
1 in50,000 human
pregnancies
 Most conjoined twins are now
evaluated for surgery to
attempt to separate them into
separate functional bodies.

20. Conjoined twins, identical
twins whose bodies are
joined
Chang and Eng Bunker
theoriginal Siamese
Twins

21.  In 1870, Chang suffered a stroke
 Despite his brother's ailing
condition, Eng remained in good
health.
 On January 17, 1874, Chang died
while the brothers were asleep
 Eng died approximately three
hours later.
 An autopsy revealed that Chang
had died of a cerebral blood clot
but Eng's due to shock

22. Thoracopagus(18.5%)
 Two bodies fused from the upper thorax to lower
belly. The heart is always involved in these cases.

23. Omphalopagus(10%)
 Bodies fused at the lower chest. Unlike
thoracopagus, the heart is never involved in these
cases; however, the twins often share a liver,
digestive system, diaphragm and other organs.

24. Thoraco-omphalopagus(28%)
 Two bodies fused from the upper chest to the lower
chest.Thesetwinsusuallysharea heart, and may
also share the liver or part of the digestive system.

25. Craniopagus(6%)
 Fused skulls, but separate bodies. These twins can
be conjoined at the back of the head, the front of
the head, or the side of the head, but not on the
face or the base of the skull.

26. Ischiopagus:
Fused lower half of the two bodies, with spines
conjoined end-to-end at a 180° angle. These twins
havefour arms;two, three or four legs; and
typically one external set of genitalia and anus.

27. Thank You
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