Embryology is the branch of biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth. Here's a breakdown of the major areas within embryology: Gametogenesis: This is the process by which male and female sex cells are formed. In males, it’s called spermatogenesis and involves the formation of sperm cells. In females, it’s called oogenesis and involves the formation of egg cells. Fertilization: This is the process where the sperm cell joins with an egg cell, leading to the formation of a zygote. This marks the beginning of the embryo's development. Embryonic Development: Post fertilization, the zygote begins a series of rapid divisions called cleavage, leading to the formation of a multicellular stage known as a blastocyst. The blastocyst then implants in the uterine wall, where it develops into an embryo. During this stage, the basic body plan and primary tissue layers (endoderm, mesoderm, and ectoderm) are established. Fetal Development: After the initial stages of embryonic development, the developing organism is referred to as a fetus. This period is characterized by the rapid growth and maturation of tissues and organs that were formed during the embryonic period. Teratology: This is the study of abnormalities of physiological development. It often focuses on congenital birth defects, their causes, and their prevention.

1. GENERAL EMBRYOLOGY
1. Development of embryology
2. Germ cell and fertilization
3. Blastocyst and implantation
4. Formation of the germ layer
5. Differentiation of trilaminar germ and
formation of embryo
6. Fetal membrane and placenta
7. Twins and multiple birth

2. Preembryonic period: 1st week to end of 2nd week
fertilization to formation of bilaminar germ disc
Embryonic period: 3rd week to end of 8th week
formation of embryo
Fetal period: 9th week to birth
growth
Perinatal stage: 26th week to 4 week after birth

3. Human
？
Plant
Animal

4. Development of embryology
Semen+menstration
Harvey, 1651:
All life from oocyte
Malpighi, 1675:
A micro-chicken in an egg
Leewenhoek, 1677 :
A micro-human in a sperm
Aristotle, B.C 384-322

5. Haeckel, 1868
Biogenetic law
Spemann, 1869-1941
Experimental embryology
Wilmut,1997
Dolly

6. Germ cell and fertilization
Germ cell
spermatozoon and ovum
1. Spermatozoon
Capacitation：In female reproductive tract, the sperms were
enabled to bind to the zona pellucida receptors.
removal of glycoproteins (decapacitation factor) which cover the
sperms
2. Ovum
secondary oocyte arrested at metaphase in the second meiotic
division

7. Fertilization: The process in which the spermatozoon penetrates
into the ovum to form fertilized ovum.
In the ampulla of oviduct

8. nucleus acrosome
Penetration and
release of acrosomal
enzyme
Cell
membrane
of ovum
Process of fertilization
Sperm bind to sperm receptor ZP-3 induce
Acrosome reaction：release of acrosomal enzyme

9. ovum
fertilized ovum
sperm

10.  The second meiotic division
of the secondary oocyte is
rapidly lifted and the second
polar body is released,
leaving a haploid female
nucleus.
Zona reaction: Cortical granules→perivitelline space
→degrade ZP-3, alteration of zona pellucida →barrier
for sperm penetration

11. Contac of sperm and ovum

12. Sperm penetrate into the ovum

13. Formation of female and male pronucleus

14. Fusion of pronuclei

15. ① Fusion of the
membrane of
sperm and ovum
② The nucleus
of sperm
penetrate into
ovum
③ Formation of
pronuclei
④ Fusion of
pronuclei
Process of fertilization
Alteration of ovum 24 h

16. 2.Condition of fertilization
① Normal ovum
② Normal sperm
sufficient numbers
③ Certain time
12-24h
④ Free reproductive tract

17. 3.Significance of fertilization
① A new life
② Diploid
inheritance and aberrance
③ Sex determination

18. Formation of blastocyst
and implantation
Cleavage and formation of blastocyst
1. Cleavage： early division of fertilized egg
2. Blastomere：daughter cells from cleavage
3. Morula：12 to 16- cell stage, enclosed in the zona pellucida,
like morus
4. Blastocyst：about 100 blastmeres
blastocoele、inner cell mass and trophoblast
polar trophoblast

19. Phase of cleavage
Fertilized
egg
2-cell
stage
18~36h
4-cell
stage
36~48h
8-cell
stage
48~60h

20. Morula
12 to 16-cell
stage
The third day

21. Appearance and inner structure of blastocyst
Appearance Inner structure

22. Movement of fertilized egg in the oviduct

23. Implantation (imbed)
The process by which the blastocyst settles
into endometrium. 6th day to 11th day
Fundus and body of uterus
zona pellucida disappear at the 5th day
1.Trophoblast：proliferate and differentiate into two layers
①Syncytiotrophoblast：outer layer, fused each other, cell
boundaries disappear
②Cytotrophoblast：inner layer， cell boundaries distinct,
simple cuboidal cells

24. Movement and implantation of fertilized egg

25. Implantation at 7th day
Endometrium
Polar trophoblast
Inner cell mass
Cytotrophoblast

26. 2.Decidua：
endometrium implantation decidua
decidual response
 Decidua basalis：under the implantation site
 Decidua capsularis：between the implantation
site and the uterine lumen
 Decidua parietalis：remaining endometrium

27. Decidua

28. Formation of germ layer
Bilaminar germ disc
1.Bilaminar germ disk：
Inner cell mass→2 layers of cell in disc (7th day)
Epiblast: columnar cells adjacent to trophoblast
Hypoblast: cuboidal cells adjacent to blastocoele

29. Bilaminar
germ disc
Epiblast
Hypoblast
Columnar cells
Cuboidal cells
Epiblast
Hypoblast

30. 2. Amnion：8th day
epiblast → amnioblast → aminiotic membrane
→ amniotic cavity → amniotic fluid
3.Primary yolk sac： 9th day
hypoblast →extraembyronic endoderm
→primary yolk sac

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

32. Bilaminar germ disc
Decidua
Syncytiotrophoblast
Epiblast
Hypoblast
Primary
yolk sac
Amniotic
cavity
Cytotrophoblast

33. 4. Extraembryonic mesoderm: 10th-11th day
5. Extraembryonic cavity: 12th-13th day
Extraembryonic mesoderm: visceral layer
parietal layer
6. Secondary yolk sac: 13th day
7. Body stalk: 14th day

34. Embryo at 10th day
Primary yolk sac
Extraembryonic
mesoderm
Germ disc
Amnion

35. Bilaminar germ disc
Decidua
Amniotic
cavity
Epiblast
Hypoblast
Extraembryonic
mesoderm
Primary
yolk sac
Cytotrophoblast
Syncytiotrophoblast

36. Embryo at 13th day
Extraembryonic cavity
Primary yolk sac
Secondary yolk sac
Body stalk

37. Bilaminar germ disc
Body stalk
Secondary
yolk sac
Extraembryonic
cavity
Primary stem
villus
Extraembryonic
mesoderm
Visceral layer
Extraembryonic
mesoderm
Parietal layer

38. Trilaminar germ disc
primitive streak, primitive groove 3rd week
primitive node, primitive pit 3rd week
1. Endoderm: primitive groove →hypoblast →
endoderm
2. Mesoderm： primitive groove →between
epiblast and hypoblat →mesoderm
3. Ectoderm：epiblast →ectoderm
Formation of trilaminar germ

39. Primitive
streak
Epiblast
Hypoblast
Epiblast
Hypoblast
Primitive
streak

40. Determine the
direction of the
embryo
notochord
Primitive
node
Primitive
pit
Primitive
groove
Primitive
streak
Significance of
primitive streak

41. Hypoblast
Epiblast
Primitive
streak
Endoderm
Formation of endoderm

42. Epiblast
Endoderm
Formation of mesoderm
Mesoderm

43. Ectoderm
Endoderm
Mesoderm
Trilaminar germ disc

44. Epiblast
proliferation
Primitive pit
Head
process
Notochordal
tube
Neurenteric
canal
Notochord
Significance of notochord ?

45. No mesoderm
Buccopharyngeal
membrane
Cloacal
membrane

46. Differentiation of trilaminar germ and
formation of embryo
Differentiation of trilaminar germ
1.Differentiation of ectoderm
①Neural tube：blastema of CNS
neural plate (18th-19th day)
Neuro-epithelium (neural ectoderm): pseudostratified columnar
→neural groove , neural fold
→ neural tube (closed at 22nd day)

47. Neural groove
Paraxial mesoderm Intermediate mesoderm

48. Neural groove
Paraxial mesoderm intermediate mesoderm

49. Neural groove
Neural crest
Notocord

50. 1、Differentiation of ectoderm
Anterior neuropore closed at 25th
Posterior neuropore closed at 27th
Neural tube
Neural fold
closed at 22nd day, from 4th somite

51. Unclosed Anterior neuropore Unclosed Posterior neuropore

52. ②Neural crest： blastema of PNS
chromaffin cell, parafollicular cell
some of bone, cartilage and
muscle of head
③Superficial ectoderm: after formation of neural tube
epidermis

53. 2.Differentiation of mesoderm 17th day
①Paraxial mesoderm：somite at 20th day, from the head
Dermotome→ dermis, hypodermis
Myotome → skeletal muscle,
Sclerotome → axial skeleton.
②Intermediate mesoderm：urinary and
reproductive system
③Lateral mesoderm：parietal mesoderm，
viseral mesoderm，
intraembryonic coelomic cavity

54. Somite

55. Parietal mesoderm
Intraembryonic
coelomic cavit
Visceral mesoderm
Differentiation of lateral mesoderm
Dermatome
Sclerotome

56. Extraembryonic coelomic cavity
Intraembryonic coelomic cavity
Differentiation of lateral mesoderm

57. 3. Differentiation of endoderm
Epithelium of primitive gut →
epithelium of digestive tract, digestive gland,
respiratory tract and lung
Primitive pharynx →5 pairs of pharyngeal pouch

58. Formation of primitive gut
A B

59. Parietal mesoderm
Intraembryonic coelomic cavity
Visceral mesoderm

60. Formation of embryonic body (4-8th week)
Disc Column
folds
the number of somite increase
face development, formation of branchial pouch
breach of buccopharyngeal membrane
5th week: upper and lower limb buds
6th week: hand and foot plates
7th week: formation of face, disappear of somite
8th week: breach of cloacal membrane
20-30th day

61. At the end of 8th week—
a small
human

62. Fetal membrane and placenta
Fetal membrane
 Chorion
 Amnion
 Yolk sac
 Allantois
 Umbilical cord

63. Amnion
Yolk sac
Umbilical
cord
Placenta

64. 1.Chorion：
Secondary stem villus +chorinic plate
Primary stem villus：2nd week
Outer syncytiotrophoblast+
Inner cytotrophoblast
Chorinic plate : extraembryonic mesoderm +
trophoblast

65. Secondary stem villus：3rd week
Outer syncytiotrophoblast+
Intermedial extraembryonic mesoderm+
Inner cytotrophoblast
Tertiary stem villus：end of 3rd week, Blood vessels
Anchoring villus, Free villus
cytotrophoblastic cell column →
cytotrophoblastic shell

66. Primary stem villus：at 2nd week，cytotrophoblast
Secondary stem villus：at 3rd week，extraembryonic mesoderm
Tertiary stem villus：at the end of 3rd week，blood vessels

67. 


Anchoring villus
Free villus
Cytotrophoblastic shell
Cytotrophoblastic column
Stem villus

68. Evolvement of chorion
Chorion laeve Chorion frondosum
6th weeks
Chorion laeve：adjacent to decidua capsularis
Chorion frondosum： adjacent to decidua basalis

69. Abnormility of chorion：
Hydatidiform
• Chorion carcinoma
Picture

70. Hydatidiform

71. Hydatidiform

72. Hydatidiform

73. Chorion carcinoma

74. Chorion carcinoma

75. Chorion carcinoma

76. 2. Yolk sac：5th-6th week, atresia
Abnormality:
① Meckel’s diverticulum
② Umbilical fistula
① Primitive blood cells
② Primordial germ cells

77. 3. Amnion：
amniotic membrane: 0.2-0.5mm
amniotic epithelium + extraembryonic mesoderm
amniotic fluid: 500-1000ml
abnormality: ＜500 ml, oligohydramnios
＞2000 ml, polyhydramnios
protection

78. 4. Allantois：degeneration
Abnormality:
Urachal fistula
2 pairs of blood vessel→
1 umbilical vein
1 pair of umbilical artery

79. 5. Umbilical cord：50 cm
(CT, umbilical vein， umbilical artery,
degenerated yolk sac and allantois)
covered with amniotic membrane
Abnormality: ＞80 cm,
＜35 cm
connect fetus with placenta

80. Fetal membrane

81. Placenta
1.Placenta: Chorion frondosum + Decidua basalis
disk: 15-20cm
Sandwich: chorinic plate (fetus)
villi
basal plate (mother)
basal plate: decidua + cytotrophoblastic shell
placental septa → cotyledon (incomplete)

82. Umbilical artery
Umbilical vein
Spiral artery
Uterus vein
Decidua bsalis
Placenta septa

83. 2.Placental membrane：
Structure between the blood of mother and fetus
①Syncytiotrophoblast；
②Cytotrophoblast and its basal lamina；
③Connective tissue；
④Endothelium and its basal lamina
protection

84. 胎
儿
血
Syncytiotrophoblast
Cytotrophoblast
Basal lamina
Connective tissue
Basal lamina
Endothelium
Nutriment
Metabolic product
Fetal membrane
Blood of
mother

85. 3.Function：
① Substance exchange
O2, nutriment, CO2, waste
② Hormone
Human chorionic gonadotropin, HCG;
Human placental lactogen, HPL;
Human placental progesterone, HPP,
Human placental estrogen, HPE

86. Function
① Substance exchange
Placenta
Capillary of fetus Blood of mother
Ｏ2
Nutriment
water、salt、others water、salt、others
ＣＯ2
Metabolic waste
Placenta

87. Function
② Barrier
Placenta
Capillary of fetus Blood of mother
s
Placenta barrier
No way
！
Most of
microorganism

88. Function
② Barrier
Placenta
Capillary of fetus Blood of mother
Most of drugs
Placenta barrier
Most of
microorganism
Some of
microorganism

89. Function
③ Endocrine
Placenta
Hormone Function Appearance Fastigium
HCG
Corpus luteum
Pregnancy
3rd week 8th week
HCS
Mammary
gland
2nd month 8th month
HPE Corpus luteum 4th month 8th month
HPP Corpus luteum 4th month 8th month

90. Twins and multiple birth
1.Twins
monozygotic twins
dizygotic twins

91. 2.Multiple birth
monozygotic multiple birth
polyzygotic multiple birth
mixed multiple birth

92. Dizygotic twins
Blastocyst Blastocyst
2 Amnion 2 Amnion
2 Chorion 2 Chorion
2 Placenta 2 Placenta

93. Monozygotic twins
separation of blastomere, of ICM, of primitive streak
Blastocyst 2 ICM 2 primitive streak
2 Amnion 2 Amnion 1 Amnion
2 Chorion 1 Chorion 1 Chorion
2 Placenta 1 Placenta 1 Placenta