Revision on General Embryology 1

1. Weeks of
Development
Dr.Sherif Fahmy

2. First Week
1- Fertilization.
2- Cleavage.
3- Migration.
4- Formation of morula and
blastocyst.
Dr.Sherif Fahmy

3. Second Week
1- Implantation of the blastocyst.
2- Changes in blastocyst to form
chorionic vesicle.
Dr.Sherif Fahmy

4. Third Week
1- Formation of 3 types of
chorionic velli from the
chorion.
2- Gastrulation which is
formation of trilaminar disc.
Dr.Sherif Fahmy

5. Organogenesis (4th
-8th
Week)
1- Development of ectoderm,
mesoderm and endoderm.
2- Folding.
Dr.Sherif Fahmy

6. Formation of Fetal
Membranes
1- Placenta.
2- Amniotic cavity.
3- Yolk sac.
4- Umbilical cord.
5- Chorion.
Dr.Sherif Fahmy

7. First Week of
Development
Dr.Sherif Fahmy

8. FERTILIZATION
• It is the process by which a sperm
units with the ovum.
•Site: It occurs at ampullary part
of uterine tube (outer 1/3 of
uterine tube)
Dr.Sherif Fahmy

9. Sperm & Oocyte
Dr. Sherif Fahmy
Corona
radiata
Zona pellucida
Cell
membrane
Secondary oocyte arrested in
2nd
meiotic division
Head
Neck
Middle
piece
Tail
Dr.Sherif Fahmy

10. FERTILIZATION
Dr. Sherif Fahmy
Dr.Sherif Fahmy

11. Results of Fertilization
• Formation of zygot.
• Restoration of diploid number (46
chromosomes).
• Determination of sex.
• Cleavage (segmentation) starts, during
which the zygote travels through
uterine tube by help of cilia and
contraction of uterine tube.
Dr.Sherif Fahmy

12. Dr.Sherif Fahmy

13. Cleavage &
Migration
(Page 14)
Dr.Sherif Fahmy

14. Dr.Sherif Fahmy

15. Dr.Sherif Fahmy

16. Blastocele
Inner cell mass
(Embryoblast) Outer cell mass
(Trophoblast)
Embryonic pole
Abembryonic pole Blastocyst
Dr.Sherif Fahmy

17. Second Week of
Pregnancy
Dr.Sherif Fahmy

18. Implantation
• Dif.: It is the process by which
blastocyst is embedded in the
endometrium.
• Timing: Starts at 7th
day and
completed at 11th
day.
• Site: usually at upper part of
posterior wall of uterus near fundus.
Dr.Sherif Fahmy

19. Steps of Implantation
• Blastocyst fixes its embryonic pole to
site of implantation.
• Trophoblast cells at embryonic pole
ptoliferate to form outer layer of
syncytiotrophoblast.
• Syncytiotrophoblast form proteolytic
enzymes that erodes endometrium
to form implantation cavity.
Dr.Sherif Fahmy

20. -Blastocyst enters the implantation
cavity.
-Endometrium after implantation is
called decidua.
-Site of penetration is closed by
fibrin clot (coagulum) at 9th
day.
-Surface epithelium overgrows the
fibrin clot at 11th
day.
Dr.Sherif Fahmy

21. Outer cell mass
(trophoblast)
Endometrial
arteriol
Endometrial
gland
Blastocyst
Fixation of embryonic
pole to implantation
site
Endometrium
Dr.Sherif Fahmy

22. Endometrium
Syncytiotrophoblast
Blastocyst
Cytotrophoblast
Dr.Sherif Fahmy

23. Syncytiotrophoblast
Cytotrophoblast
Trophoblast
Amniotic
cavity
Primary
yolk sac
Dr.Sherif Fahmy

24. Dr.Sherif Fahmy

25. Abnormal Sites of Implantation
• 1- Ectopic pregnany: They are tubal,
ovarian or omental (peritoneum).
• 2- Placenta praevia: Parietalis,
marginalis and centralis.
Dr.Sherif Fahmy

26. Ectopic
Pregnancy
Dr.Sherif Fahmy

27. Placenta Praevia
Placenta
praevia
parietalis
Normal
implantation
site
Placenta
praevia
marginalis
Placenta
praevia
centralis
Dr.Sherif Fahmy

28. Decidua
• It is endometrium after implantation
which is sheded after birth of fetus.
• Character of endometrium:
• Increased secretory function of uterine
glands.
• Decidual cells are stromal cells filled with
glycogen.
• Arteries become more spiral with arterio-
venous anastomosis.
Dr.Sherif Fahmy

29. Parts of decidua:
• Decidua basalis: It is the part of decidua
between blastocyst and myometrium. It
forms the fetal part of placenta.
• Decidua capsularis: It covers the blastocyst
except embryonic pole and separates it from
uterine cavity.
• Decidua parietalis: It is the rest of
endometrium that lines the rest of uterine
cavity.
Dr.Sherif Fahmy

30. Decidua basalis
Decidua capsularis
Decidua parietalis
Uterine cavity
Dr.Sherif Fahmy

31. Fate of decidua:
• Decidua basalis shares in the
formation of placenta.
• Decidua capsularis and parietalis
fuse together and shedded with
placenta after delivery.
Dr.Sherif Fahmy

32. Decidua basalis
Decidua
parietalis
Decidua
capsularis
Uterine
cavity
Fused decidua
parietalis and
capsularis
Decidua
basalis
Dr.Sherif Fahmy

33. Changes of Blastocyst
in the Second Week of
Pregnancy
Dr.Sherif Fahmy

34. Blastocele
Inner cell mass
(Embryoblast) Outer cell mass
(Trophoblast)
Embryonic pole
Abembryonic pole Blastocyst
6th
day
Dr.Sherif Fahmy

35. 7th
day:
Dr.Sherif Fahmy

36. 8TH
Day of Pregnancy
Endometrium
Syncytiotrophoblast
Cytotrophoblasts Hypoblasts
Amniotic cavity
Amnioblasts
Epiblast
Dr.Sherif Fahmy

37. 9th
& 10th
days
Fibrin clot
Primary yolk sac
Heuser’s
membrane
Lacunar spacesSyncytio-
trophoblast
Endometrial
arteriol
Hypoblast
Amniotic
cavity
Epiblast
Amnioblast
Cyto-
trophoblast
Dr.Sherif Fahmy

38. 11th
& 12th
days
Blood inside lacunae
Endometrial
sinusoid
Syncytio-trophoblast
Endometrial
epithelium
Extraembryonic
mesoderm
Primary yolk sac
Large
spaces
Amniotic
cavity
Cytotrophoblast
Bilaminar
embryonic disc
Dr.Sherif Fahmy

39. 11th
& 12th
days
Chorionic Vesicle
Extra-
embryonic
mesoderm
Dr.Sherif Fahmy

40. 13th
day
Endodermal
cells
Secondary yolk
sac
Exocoelomic cyst
Somatic
mesoderm
Connecting stalk
Somatic
mesoderem
Splanchnic
mesoderm
1ry chorionic villi
Extra-
embryonic
coelom
Chorionic
cavity)
Dr.Sherif Fahmy
Cyto-
trophoblast
Amniotic
cavity
Chorionic
Vesicle
Dr.Sherif Fahmy

41. 3rd
Week of Pregnancy
A- Changes in the chorion.
B- Changes in the embryonic
disc.
Dr.Sherif Fahmy

42. 13th
day
Somatic
mesoderm
1ry chorionic villi
Dr.Sherif Fahmy
Cyto-
trophoblastChorionic
Vesicle
Intervillous space
filled with maternal
blood
Syncytiotrophoblast
Dr.Sherif Fahmy

43. Chorionic Vesicle (at the end of 3rd
week)
Dr.Sherif Fahmy

44. Changes in the chorion
Formation of chorionic velli:
1- Primary velli.
2- Secondary velli.
3- Tertiary velli
A-Chorion frondosum.
B-Chorion leave.
Dr.Sherif Fahmy

45. Dr. Sherif Fahmy
Syncytiotrophoblast
Cytotrophoblast
Primary chorionic velli
Dr.Sherif Fahmy

46. Dr. Sherif Fahmy
Secondary chorionic velli
Syncytiotrophoblas
Cytotrophoblast
Somatic mesoderm
Dr.Sherif Fahmy

47. Dr. Sherif Fahmy
Fetal blood
vessels
Tertiary velli
(chorion frondosum)
Tertiary velli (chorion leave)
Cytotrophoblas
tic shell
Tertiary chorionic velli
Dr.Sherif Fahmy

48. Decidua
basalis
Chorion frondosum
Chorionic
plate
Chorion leave
Dr.Sherif Fahmy

49. Development of
Trilaminar Embryonic
Disc
Gastrulation
Dr.Sherif Fahmy

50. Gastrulation
It begins by formation of:
1- Primitive streak.
2- Primitive node.
3- Invagination.
3- Bucco-pharyngeal membrane.
4- Cloacal membrane.
Dr.Sherif Fahmy

51. Amniotic
cavity
Buccopharyngeal
membrane
Primitive
node & pit
Primitive
streak
Cloacal
membrane
Yolk sac
Hypoblast
Epiblast
Dr.Sherif Fahmy

52. Invagination
- Epiblast cells migrate to primitive streak.
-Then they pass beneath epiblast to become
flask-shaped and separated from the epiblast and
form:
1- Endoderm that replaces the hypoblast.
2- Third layer between epiblast and endoderm
which consists of intra-embryonic mesoderm
with notochord in the median region..
3- Remaining epiblast cells after formation of
notchord and intraembryonic mesoderm will be
named ectoderm.
Dr.Sherif Fahmy

53. Primitive streak
Primitive node and pit
Epiblast
Hypoblast
Invaginating cells from epiblast lyaer
Dr.Sherif Fahmy

54. Formation of
Notochord
Dr.Sherif Fahmy

55. Notochordal
process
Extension of primitive
pit into prenotochordal
process
Primitive streak
Connecting
stalk
Allantois
LS
Dr.Sherif Fahmy

56. 2-Notochordal
canal
Cloacal membrane
Allantois
Buccopharyngea
l membrane
LS
Dr.Sherif Fahmy

57. Extraembryonic
mesoderm
Notochordal
canal
Intraembryonic
mesoderm
Epiblast
(Ectoderm)
Endoderm
TS
Dr.Sherif Fahmy

58. Amniotic cavity
Yolk
sac
Degenerating endoderm and floor of notochordal canal
Roof of notochordal canal
LS
Dr.Sherif Fahmy

59. 3-Neurenteric canal
Buccopharyngea
l membrane
Cloacal
membrane
LS
Dr.Sherif Fahmy

60. 4-Notochordal
plate
Notochordal plate (roof of the
canal) which intercalate (fused) to
endodermal layer.
TS
Dr.Sherif Fahmy

61. TS
Dr.Sherif Fahmy

62. 5-Definitive Notochord
TS
Dr.Sherif Fahmy

63. Development of Notochord
• 1- Pre-notochordal process: Proliferation of cells from
primitive pit forms a cord of cells in median plane till
prochordal plate.
• 2- Notochordal canal: Canalization of the process forms
notochordal canal.
• 3- Notochordal-ectodermal fusion: fusion between floor of
the canal and endoderm.
• 4- Neur-enteric canal: Temporary communication between
amniotic cavity and yolk sac due to degeneration of floor of
notochordal canal and underlying endoderm.
• 5- Notochordal plate Persistence roof of notochordal
canal.
• 6- Defenitive notochord: Regeneration of endoderm only.
Persistent roof of notochordal canal becomes folded uponDr.Sherif Fahmy

64. Importance of
Notochord
1- Induction of vertebral column
development.
2- Temporary axial skeleton.
Dr.Sherif Fahmy

65. Fate of Notochord
• It is the primitive axial skeleton around
which the vertebral column is formed.
• It remains in intervertebral disc as
nucleus pulposus.
Dr.Sherif Fahmy

66. Dr.Sherif Fahmy

67. Annulus
fibrosus
Nucleus
pulposus
Parts of Intervertebral disc
Dr.Sherif Fahmy

68. Intra-embryonic Mesoderm
• It is formed from proliferating cells from sides
of primitive node and streak.
• It fills the space between ectoderm and
endoderm except at buccopharyngeal
membrane, cloacal membrane (fusion
between ectoderm and endoderm caudal to
primitive streak) and median region which is
occupied by notochord.
Dr.Sherif Fahmy

69. Primitive
streak
Primitive
node and
pit Buccopharyngeal membrane
Epiblast (ectoderm)
Endoderm
Intra-embryonic
mesoderm
Dr.Sherif Fahmy

70. Epiblast (ectoderm)
Hypoblast Endoderm
Flask-shaped
cells
Dr.Sherif Fahmy

71. Amniotic cavity
Amnio-
ectodermal
junction
Intra-emryonic
mesoderm
Secondary
yolk sac
Endoder
m
Epiblast
(ectoderm)
Bucco-
pharyngeal
membrane
Cloacal
membrane
Primitive
streak
Primitive
node
Dr.Sherif Fahmy

72. 17 th Day
Dr.Sherif Fahmy

73. Differentiation of Intra-embryonic
Mesoderm
• Intra-embryonic mesoderm on each side
of notochord, divides into:
1- Paraxial Mesoderm: on both sides of
notochord.
2- Intermediate Mesoderm: Middle part of
the mesoderm.
3- Lateral plate Mesoderm: Lateral part
which communicates with that of the
opposite side infront prochordal plate.
Dr.Sherif Fahmy

74. Bucco-pharyngeal
membrane
Cloacal
membrane
Notochord
Intermediate mesoderme
Paraxial mesoderm
Lateral plate mesoderm
Dr.Sherif Fahmy

75. 1-Paraxial Mesoderm
• It is the most medial mesoderm.
• It is divided into cubical masses called
somites (42 – 44 pairs).
• Formation of somites starts At the 20th
day by formation of one pair at cranial
region.
• Somites are classified into: 4 occipital, 8
cervical, 12 thoracic, 5 lumbar, 5 sacral
and 8 – 10 coccygeal. Dr.Sherif Fahmy

76. Fate of Somites
• 1- Sclerotome: It is ventro-medial part
that form vertebral column and
intervertebral discs.
• 2- Dermo-myotome: It is the dorso-
lateral part which subdivided into:
A- Dermatome: Forms dermis of skin.
B- Myotome: Forms skeletal muscles of
trunk and limbs.
Dr.Sherif Fahmy

77. NotochordSomites of Paraxial
mesoderm
Intermediate mesoderm
Intraembryonic
coelom
Pericardium
Pleura
Peritoneal canal
Dr.Sherif Fahmy

78. Dr.Sherif Fahmy

79. Notochord
Neural tube
Somite
Sclerotom
e
Myotom
e
Dermatome
Neural tube
Vertebra
Skeletal muscles
Dermis
Dr.Sherif Fahmy

80. Muscles of
back
Muscles of
anterolateral
aspect of body
Muscles of
limb
Dorsal ramus
of spinal nerve
Ventral ramus
of spinal
nerve
Dermo-myotomes
of brachial plexus
Dr.Sherif Fahmy

81. 2-Intermediate Mesoderm
• Narrow strip between paraxial
and lateral plate mesoderm.
• It is divided into many
segments.
• It forms uro-genital system.
Dr.Sherif Fahmy

82. Lateral Plate Mesoderm
• Flat plate of mesoderm between intermediate
mesoderm and margin of embryonic disc.
• It is continuous with that of other side infront
prochordal plate.
• Intra-embryonic coelom: It is formed from fused
small cavities. This coelom forms serous
membranes of the body (pericardium, pleura and
peritoneum).
• Lateral plate mesoderm is split by the coelom into:
-Somatic mesoderm & Splanchnic mesoderm.
Dr.Sherif Fahmy

83. Bucco-pharyngeal
membrane
Cloacal
membrane
Notochord
Intermediate mesoderme
Paraxial mesoderm
Lateral plate mesoderm
Dr.Sherif Fahmy

84. NotochordSomites of Paraxial
mesoderm
Intermediate mesoderm
Intraembryonic
coelom
Pericardium
Pleura
Peritoneal
canal
Cardiogeni
c area
Septum
transversum
Dr.Sherif Fahmy

85. Neural groove
Amnion
Somatic
mesoderm
Intraembryonic
coelom
Notochor
d
Splanchnic mesoderm
Paraaxial
mesodermIntermediate
mesoderm
Lateral plate
mesoderm
T.S.
Dr.Sherif Fahmy

86. Amnion
Somatic
mesoderm
Intraembryonic
coelom
Splanchnic mesoderm
Lateral plate
mesoderm
T.S.
Dr.Sherif Fahmy

87. Dr.Sherif Fahmy

88. Organogenesis
Embryonic Period (4th
–
8th
week)
Dr.Sherif Fahmy

89. Fate of Ectoderm
Dr.Sherif Fahmy

90. DEVELOPMENT OF NEURAL TUBE
• Neural plate is median thickened area between
primitive node and prochordal membrane. Two
strips separate neural plate from the rest of
ectoderm which are called neural crest.
• Neural folds are raised margins of neural plate
while depressed median region is called neural
groove.
• Neural tube is formed by fusion between two
neural folds in its middle and extends cranio-
caudally. Cranial and caudal ends (neuropores) are
the last to be closed. Dr.Sherif Fahmy

91. Neural groove
Neural fold
Notochord
Fusing neural folds to
form neural tube
Neural crest
Ectoderm
Endoderm
Dr.Sherif Fahmy

92. Dr.Sherif Fahmy

93. Dr.Sherif Fahmy

94. Dr.Sherif Fahmy

95. Dr.Sherif Fahmy

96. Fate of the neural tube
• The tube grows in the median region leading to
elongation of the embryonic disc in cranio-caudal
direction.
• The cranial part of the tube dilates to form the
brain vesicle while the caudal part forms the spinal
cord.
• The brain vesicle divides by 2 constrictions into:
– Forebrain: forms cerebral hemispheres and
diencephalone.
– Midbrain: forms the midbrain (upper part of brain
stem).
– Hindbrain: forms medulla, pones and cerebellum.Dr.Sherif Fahmy

97. Dr.Sherif Fahmy

98. Fate of neural crest
• Ganglia: Sensory (of cranial and spinal
nerves), sympathetic and
parasympathetic.
• Cells: Chromaffin cells of supra-renal
medulla, Schwann cells and
melanoblasts.
• Others: Pia mater, arachnoid mater,
enamel of teeth, septa of the heart and
some bones of the skull. Dr.Sherif Fahmy

99. Other derivatives of ectoderm
- Otic placodes form internal ear.
- Lens placodes form lens of the eye.
- Peripheral nerves.
- Sensory epithelium in ear, nose, eye and
epidermis of skin.
- Pituitary gland.
- Anterior part of oral cavity and lower ½
of anal canal.
Dr.Sherif Fahmy

100. Development of Endoderm
(Page 30)
-Epithelium of digestive system,
respiratory tract, most of urinary
bladder and urethera, tympanic
cavity and Eustachian tube.
-Parenchyma of liver, pancreas,
thymus, thyroid, parathyroid and
palatine tonsils. Dr.Sherif Fahmy

101. FoldingDr.Sherif Fahmy

102. FOLDING OF THE EMBRYO
• It is the process by which the embryo becomes folded upon
itself.
Time of folding:
• At the end of 3rd
week and completed at the end of 4th
week.
Causes of folding:
• Rapid increase of cranio-caudal length due to rapid growth
of neural tube and somites.
• Rapid expansion of amniotic cavity.
Types of folding:
• Head and tail folds are folding of cranial and caudal parts
of the disc.
• Lateral folds are folding of lateral parts of the disc.
Dr.Sherif Fahmy

103. Dr.Sherif Fahmy

104. Dr.Sherif Fahmy

105. Results of Folding
Dr.Sherif Fahmy

106. Dr.Sherif Fahmy

107. Dr.Sherif Fahmy

108. Embryonic disc with removed
ectoderm
Cloacal
membrane
Notochord
Paraxial
mesoderm
(somites)
Bucco-
pharyngeal
membrane
Cardiogenic
area
Septum
transversum
Peritoneal canal
Pericardium
Dr.Sherif Fahmy

109. Ectoderm
Mesoderm
Endoderm
Buccopharyngeal membrane
Cloacal
membrane
Hindgut
Midgut
Foregut
Forebrain
Forebrain bulge
Pericardial bulge
Vitelline duct Allantois
Definitive yolk sac
Stomodeum
L.S. in folded embryo
Heart
Dr.Sherif Fahmy

110. Peritoneal
canals
Gut
Ventral mersentry
Dorsal mesentry
Dr.Sherif Fahmy

111. RESULTS OF FOLDING
1-Cylindrical appearance: Transformation of
emryonic disc to cylindrical shape.
2- Amniotic cavity: Before folding it lies dorsal to
embryonic disc, after folding, it surrounds all aspects of
the embryo.
3- Formation of definitive yolk sac: It is the part
of yolk sac outside the embryo in the umbilical cord.
4- Formation of primitive umbilical ring: It is a
ventral defect in anterior abdominal wall that contains
connecting stalk, allantois and vitello-intestinal duct

112. 5-Formation of the gut:
•It is formed from endodermal layer together with part of
yolk sac.
Foregut is formed in head fold with bucco-pharyngeal
membrane closing its cranial end.
Hindgut: is formed in tail fold and closed caudally by cloacal
membrane. The caudal part is dilated and called cloaca
which is connected ventrally to allantois.
Midgut: is formed by lateral folds and present between
foregut and hindgut. It is connected with defenitive yolk sac
by vitelline duct.
6- Formation of stomodeum: Ectodermal
depression between forebrain bulge and cardiac bulge.
Dr.Sherif Fahmy

113. 7- Formation of mesenteries: Ventral and
dorsal mesenteries are formed around gut.
8- Reversal of positions:
-Heart and pericardium become cranial to septum
transversum (before folding septum transversum is
most cranial).
-Connecting stalk becomes ventral and more cranial
inspite of being most caudal.
Dr.Sherif Fahmy