This document discusses the normal process of embryological development in multifetal gestations. It begins with definitions of terms like zygosity and chronicity. The two main types of twinning - monozygotic and dizygotic - are then explained in terms of their formation and placental characteristics. The stages of embryonic development are outlined, including gastrulation, formation of the three germ layers and structures like the notochord. Later stages of folding, organogenesis and differentiation of tissues from the germ layers are also summarized. Potential clinical implications for multifetal gestations are briefly mentioned.

1. PREPARED BY DR.HANSAR K(R1)
MODERATOR DR.EYOB
(GYNACOLOGIST&OBSTETRICIAN,MFMS)
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Normal process of embryologic
development of multifetal gestation

2. Outline
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 Definition
 Incidence
 Process of twining
 Clinical correlation
 Reference

3. Definition
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 A multiple pregnancy is a pregnancy in which
more than one fetus develops in the uterus
 Incidence 1-2%
 It could be twins, triplets, quadriplets …

4. Risk factors
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 Maternal age
 Race
 ART
 Contraceptive
 Family history

5. terms
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 Zigosity
referes to genetic make up twin pregnancy
 Chronicity
indicates placental composition

6. Determination of zygosity
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 Examining fetal genders
 Placenta
 Genetic testing

7. ZYGOCITY
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 Monozygotic(identical)twins
 fertilization of one egg by one sperm and
subsequent spontaneous cleavage of the
fertilized ovum
• MZ twinning rates are constant across all
variables

8. monozygotic theory
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• Is considered as a tertogenic effect when an old
ovum fertilization of an “old” ovum with a more
fragile zona pellucida or inadequate cytoplasm
and with damage to the inner cell mass that leads
to two separate points of regrowth and splitting of
the fertilized ovum

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 Monozygotic twins are produced in one of the
following ways
 Early blastomere separation
 The cells formed in the first few divisions of the zygote
are totipotent,
 In such a case, the fetuses DCDA

10. DCDA
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 Duplication of inner cell mass
 when the blastocyst is formed, two inner cell masses
form within it and each develops into a complete
fetus MCDA

12. MCDA
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 Duplication of embryonic disc
 The inner cell mass may split into two
 two separate embryonic discs MCMA

14. MCMA
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16. Dizygosity (fraternal twins)
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 Fertilization of 2 ova by two separate sperm.
 DZ twins always develop dichorionic diamniotic
placentation because each blastocyst generates
its own chorionic and amniotic sacs.

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18. summary
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19. Clinical correlation
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 TTTS
 TAPS
 TRAP
 Conjoined twins
 Discordant twins

20. Clinical correlation
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 On a rare occasions division occurs after 2 weeks
of the development of embryonic disc result in
incomplete splitting called conjoined
twins(siamese).
 Five types of fusion
 thoracophagus
 omphalophagus
 Pyopagus
 Craniophagus
 ischiopagus

21. Contd..
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22. Contd…
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 Parasitic twins
 When one member is of conjoined twin remains
rudimentary due to diminished blood supply and
grow like parasite from the well developed co-
twin.

23. Vanishing twins
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 Spontaneous disappearance of1 or more of the
pregnancy after multiple gestation
 36%twins 53%triplets

24. Fetus acardicus
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 Occurs only in monozygotic twins
 Part of one fetus remains amorphous and
become parasitic without heart.

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26. Embryonic period
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• From Third to eighth weeks
• period of organogenesis,
• It is the period when the three germ layers
gives rise to a number of specific tissues
and organs.
• cited as the time when the majority of
birth defects are induced
• The embryonic disc undergoes a process of
folding.

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Hydatidiform mole

29. Bilaminar germ disc at 9th day
Hypoblast
Epiblast
Amniotic cavity
Syncytio-
trophoblast
Primary yolk sac

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32. Phases of embryonic
development
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 Is drived in to three; to some extent it is related
1. The first phase is growth ,which involve cell division and
elaboration of cell products.
2. The second phase is morphogenesis, which invovles
development of shape,size or other features.
3. The third phase is differentiation ,maturation of physiologic
process.

33. Formation of prochordal plate:
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 At one circular area near the margin of the disc,
the cubical cells of the endoderm become
columnar.
 prochordal plate determines the central axis of
the embryo

34. FORMATION OF PRIMITIVE STREAK:
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 A transient structure that forms in the blastula
 during the early stages of embryonic
development.
 some of the epiblast cells lying along the central
axis, begin to proliferate, and form an elevation
that bulges into the
amniotic cavity. Primitive streak
 Formation of primitive streak marks the beginning
of gastrulation.

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Sacrococcy
geal
teratoma

36. TRILAMINAR GERM DISC:
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 An embryonic disc made up of three layers.
 the ectoderm (outer),
 Endoderm (inner)
 and mesoderm (middle).

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38. ECTODERM CELL LAYERS
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 Pluripotent ectodermal cells form the following:
– Definitive endoderm
– Intraembryonic mesoderm
Definitive ectoderm
– Notochord
– Primordial germ cells (PGC)

39. FORMATION OF NOTOCHORD
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 a midline structure that develops in the
region lying between the cranial end of the
primitive streak and the caudal end of the
prochordal plate
 primitive node ….primitive pit …notochordal
process… notochordal canal …notochordal
plate…notochord.
 nucleus pulposus

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 As the embryo enlarges, the notochord elongates
 considerably and lies in the midline, in the
position
 to be later occupied by the vertebral column.
 However, the notochord does not give rise to the
vertebral column

43. FORMATION OF THE NEURAL TUBE
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 The neural tube is formed from the ectoderm
overlying the notochord and, therefore, extends
from the prochordal plate to the primitive
 The neural tube gives rise to the brain and the
spinal cord

44. Neural groove
Neural crest
Notocor
d
Dorsal view of
a somite embryo

45. Neurulation
Anterior neuropore closed
at 25th
Posterior neuropore closed
at 27th
Neural
tube
Neural fold
closed at 22nd day, from 4th
somite
NTD

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47. Differentiation of mesoderm
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 Paraxial mesoderm：somite
 Dermotome→ dermis, hypodermis
 Myotome → skeletal muscle,
 Sclerotome → axial skeleton.

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 Intermediate mesoderm：
urogenitial system
 Lateral mesoderm：
parietal mesoderm ,
viseral mesoderm
intraembryonic coelomic cavity

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50. Differentiation of endoderm
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 Epithelium of primitive gut epithelium of digestive
tract, digestive gland
 The tube is divided into three regions:
foregut
 midgut
 hindgut
 Respiratory tract and lung
 Primitive pharynx →5 pairs of pharyngeal pouch

51. Folding of the embryo
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 Occurs in both median and horizontal planes
 Folding in median plane
 head (cranial) and tail(caudal) folds.

52. Folding of the embryo
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53. Head and tail folding
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54. Horizontal(lateral folding)
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 Folding of the side of embryo produces the right
and left lateral folds.
 Lateral folding is by growing of spinal cord and
somites.
 Initially, there is a wide connection b/n the midgut
and yolk sac.
 After lateral folding the connection is reduced.

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59. Major derivatives of germ cell layers
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60. Days
Somite
s
Length
(mm) Characteristic Features
14–15 0 0.2 Appearance of primitive streak
16–18 0 0.4 Notochordal process appears; hemopoietic cells in yolk sac
19–20 0 1.0–2.0 Intraembryonic mesoderm spread under cranial ectoderm; primitive
streak continues; umbilical vessels and cranial neural folds beginning
to form
20–21 1–4 2.0–3.0 Cranial neural folds elevated and deep neural groove established;
embryo beginning to bend
22–23 5–12 3.0–3.5 Fusion of neural folds begins in cervical region; cranial and caudal
neuropores open widely;
visceral arches 1 and 2 present; heart tube beginning to fold
24–25 13–20 3.0–4.5 Cephalocaudal folding under way; cranial neuropore closing or closed;
optic vesicles formed; otic placodes appear
26–27 21–29 3.5–5.0 Caudal neuropore closing or closed; upper limb buds appear;
three pairs of visceral arches
28–30 30–35 4.0–6.0 Fourth visceral arch formed; hindlimb buds appear;
otic vesicle and lens placode
31–35 7.0–10.0 Forelimbs paddle-shaped; nasal pits formed; embryo tightly C-shaped
36–42 9.0–14.0 Digital rays in hand and foot plates; brain vesicles prominent; external
auricle forming from auricular hillocks; umbilical herniation initiated
43–49 13.0–22.0Pigmentation of retina visible; digital rays separating;
nipples and eyelids formed; maxillary swellings fuse with medial nasal
swellings as upper lip forms; prominent umbilical herniation
50–56 21.0–31.0Limbs long, bent at elbows, knees; fingers, toes free; face more
Summary of Key Events during the Embryonic Period
h

61. Reference
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 Langman medical embryology 14th edition
 Williams gynacology 26th edition
 Uptodate 21th edition
 Gabbe obsterric 7th edition
 Inderbir Singh’s Human Embryology 11th edition

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Thank you