This document summarizes key events in human embryonic development during the third week. It describes how the primitive streak forms and gives rise to the three germ layers. It also discusses how the notochord, neural tube, and somites begin to develop as the embryo undergoes folding to take on a cylindrical shape. Additionally, it provides an overview of initial organ formation and the establishment of the primitive cardiovascular system during this critical period of embryogenesis.

1. Human Embryology II

2. Embryonic period – the third week of development
 The 2nd stage of gastrulation
 Germ layer initial differentiation and axial organ formation
 Primitive cardiovascular system formation
 Subsequent chorion development
 Allantois appearance
 Folding

3. The second stage of gastrulation
 results in
- trilaminar embryonic disk formation
 occurs
- on the 14th to 15th day of development

4. Only epiblast participates in the 2nd stage of gastrulation
Hypoblast does not take part in the embryo body formation
 Epiblast gives rise to embryonic
- ectoderm
- endoderm
- mesoderm
 Hypoblast is displaced to
- extraembryonic regions

5. Primitive streak is the key structure of the 2nd stage of
gastrulation
 Epiblastic cells at the disk cranial end
- proliferate
- migrate along the disk margins
- converge at the disk caudal end
- turn back to the disk cranial end
towards the midline
ß
primitive streak Þ
cranial end
caudal end

6. Primitive streak anterior portion thickens to form the
primitive knot or Hensen’s nodule
 Primitive groove
- develops in the primitive streak
- is continuous with the primitive pit
in the primitive knot
Û

7. Primitive streak is a source of the embryonic mesoderm and
embryonic endoderm
 Primitive streak cells migrate Þ
- into the primitive groove
- inwardly between the epiblast and hypoblast
early-migrating cells Þ endoderm
later-migrating cells Þ mesoderm

8. Gastrulation is completed with the trilaminar disk formation
 As soon as the primitive streak gives rise to
- embryonic endoderm
- embryonic mesoderm
Û
remaining epiblastic cells are referred to as
- embryonic ectoderm

9. Duplication of the primitive streak results in twinning
Удв
оен
ие
Ü duplication of the primitive streak
Ü monochorial
monoamniotic twins
~30% ~70% ~4%

10. Conjoint twins (~1% of monozygotic twins) result from
the primitive streak duplication
partial duplication of the primitive streak
(Y-shaped)
ß
complete duplication of the primitive streak
but incomplete duplication of the germ layers
ß
bifurcation of the spinal cord and vertebral column fusion of soft tissues (Siamese twins)

11. Siamese twins
 may be separated surgically
Þ

12. Initial germ layer differentiation and the axial organ formation
 Complex of the axial organs includes
- notochord
- neural tube
- mesodermal somites

13. Notochord is the first to appear concurrently with mesoderm
 Primitive pit
- extends into the primitive knot
- forms the notochordal canal
 Primitive knot cells
- migrate through the canal
- give rise to the notochord

14. Notochordal process looks like a cellular rod
 extends
- cranially from the primitive knot
- between the ectoderm and endoderm
wing-like mesoderm is on
each side of the notochord Ü
Þ

15. Notochord forms the embryo midline axis around which
the vertebral column develops
 Notochord
- disappears where it becomes surrounded by the vertebral bodies
- persists as the nucleus pulposus of the intervertebral disks
- induces the overlying ectoderm to form the neural plate
Þ

16. Neurulation or the neural tube formation is induced by
the notochord with the adjacent mesoderm
Stages of the neural tube development
 neural plate (15 – 16 days)
 neural groove and neural folds (18 – 21 days)
 neural tube (23 – 25 day)
Û

17. Neuroectoderm includes the neural tube and neural crest
 Neural tube
ß
Brain
Spinal cord
Retina
Olfactory epithelium
 Neural crest
ß
Neural ganglia
Pia mater and arachnoid
Skin melanocytes
Adrenal medulla
Thyroid gland C-cells

18. Surface ectoderm remains after the neural tube separation
 Gives rise to
- skin epidermis
- sweat and sebaceous glands
- nails and hair
- mammary glands
- salivary glands
- tooth enamel
- oral cavity epithelium
- corneal epithelium

19. Mesoderm subdivision
 Paraxial mesoderm Þ somites
- myotome
- dermotome
- sclerotome
 Intermediate mesoderm (somite cord) Þ nephrogonadotome
 Lateral mesoderm Þ parietal layer or somatopleure
visceral layer or splanchnopleure
Ü coelom in the lateral mesoderm

20. Subsequent mesoderm differentiation
 Myotome Þ skeletal muscles
 Dermatome Þ skin dermis
 Sclerotome Þ bones and cartilages
 Nephrogonadotome Þ kidney and gonads
 Coelom Þ - pericardial
- pleural
- peritoneal cavities
 Somatopleure Þ mesothelium
 Splanchnopleure Þ - mesothelium
- myocardium
- epicardium
- adrenal cortex
Þ

21. Some mesodermal cells migrate and become mesenchyme
 Mesenchyme gives rise to
- blood
- blood and lymphatic vessels
- all types of connective tissue
- smooth muscle cells
- microglial cells
- endocardium

22. Embryonic endoderm differentiation
 Gastrointestinal tract epithelium
 Pancreas parenchyma
 Liver parenchyma
 Gallbladder epithelium
 Lung epithelial parts

23. Primitive cardiovascular system formation
 Angiogenesis begins in the provisory organs
- yolk sac
- connecting stalk
- chorion

24. Embryonic vessels begin to develop about two days later
 embryonic vessels and primitive heart
arise from the mesenchyme

25. Angiogenesis and hemopoiesis occur concurrently
 Primitive blood cells
- differentiate from mesenchyme
- inside the embryonic vessels
ß
intravascular hemopoiesis

26. Cardiovascular system is the first system to attain
a functional state - by the end of the 3rd week
 сhorionic and embryonic vessels
become connected via the connecting stalk
Û

27. Chorionic villi become tertiary villi
 Composition of tertiary villi
- syncytiotrophoblast
- cytotrophoblast
- extraembryonic mesoderm
- chorionic blood vessels ß

28. Chorionic villi provide maternal-fetal blood exchange
 are bathed by maternal blood from lacunae
 Chorionic villi are
- stem or anchoring villi
- branch villi

29. Allantois appears on the 16th day of embryonic development
 is a finger-like projection
- of an embryo endoderm
- into the connecting stalk

30. Allantois exists for two months
Its remnant will be a part of the umbilical cord
 Allantois is involved in
- blood formation
- angiogenesis
- the urinary bladder development

31. Folding – the body fold formation
 begins on the 21st day of development
 There are two pairs of folds
- longitudinal folds
- transversal folds

32. Longitudinal – cranial and caudal - folds
 convert the embryonic disk into C-shaped structure
ß

33. Transversal folds
 include
- surface ectoderm
- somatopleure
- splanchnopleure
- endoderm
 move down to meet each other
 converge below the embryo body

34. Transversal fold results
 embryonic disk is converted to
a cylinder-like structure
Û

35. Folding consequences
 Embryo acquires
- cylindrical C-like body shape
- primitive gut
 . . . and separates
- from the yolk sac

36. Embryonic period from the 4th to the 8th weeks
 All tissues and organs differentiate, develop,
and begin to function
7th week embryo
Û
 The period is the most critical period of embryogenesis because
any disturbances may give rise to congenital malformations

37. Placentation or placenta development
 begins after implantation
 is completed by the 8th week
Û

38. Embryo by the 8th week – the end of the embryonic period
 is disposed in the amnion and bathed by amniotic fluid
 amnion fills the chorionic cavity
Þ
 amniotic wall underlies the chorion
Ü
 chorion fills the uterine cavity
 uterine cavity obliterates
 umbilical cord connects the embryo and chorion

39. Umbilical cord arises from the connecting stalk
 contains
- two arteries
- a vein
- mucoid connective tissue
- remnants of the yolk sac and allantois
 is covered by amniotic epithelium

40. Umbilical cord functions to connect
 fetal cardiovascular system with chorionic vessels

41. Endometrium in pregnancy is called the decidua graviditas
 Decidua basalis
- underlies the implantation site
 Decidua capsularis
- covers the implantation site
 Decidua parietalis
- remaining endometrium

42. Endometrium by the 8th week of development
 Decidua basalis
- takes part in placenta formation
 Decidua capsularis
- fuses with decidua parietalis
when the uterine cavity obliterates

43. Chorion by the 8th week of development is subdivided into
 Smooth chorion
- almost lacks villi
- is associated with the decidua capsularis
 Villous chorion
- possesses large and branched villi
- is associated with the decidua basalis

44. Smooth chorion is one of the amniochorionic membranes
 that form the fetal bladder wall
- amnion wall
- smooth chorion
- decidua capsularis

45. Villous chorion or chorion frondosum
 takes part in placenta formation

46. Placenta is a combined organ
 is formed by
- maternal body – the decidua basalis
- fetal body – the villous chorion
 Two placental parts are involved in
the maternal-fetal circulation exchange Þ

47. Human placenta is discoid in shape
 It is determined by the circular form of the villous chorion
fetal part Þ
Ü maternal
part
placenta after parturition

48. Fetal part of placenta
 Chorionic plate
 Tertiary villi
 Umbilical cord is attached to the fetal surface
 Amniotic epithelium surrounds the umbilical cord
and covers the fetal placenta part

49. Chorionic plate
 is a layer of extraembryonic connective tissue
 contains blood vessels coming from the umbilical cord
 is covered with amniotic epithelium
 gives rise to chorionic villi
Ü

50. Chorionic villi arise from the chorionic plate
 project into intervillous spaces or lacunae
 are bathed by maternal blood
Û

51. Chorion attaches itself to the decidua basalis
 Stem or anchoring villi
- are attached to the endometrium
- form cytotrophoblastic shell at the site
of attachment
 Branch or floating villi
- arise from the stem villi
- float in lacunae with maternal blood
- provide the main fetoplacental exchange

52. Chorionic villi are tertiary villi
 syncytiotrophoblast
 cytotrophoblast
 extraembryonic mesoderm
 villous blood vessels
villi are bathed by the maternal blood
Ü

53. Fibrinoid material arises from the decidua basalis necrosis
 results from the syncytiotrophoblast enzyme activity
 contains fibrin and immunoglobulins
 covers villi and the endometrium
 separates the fetal tissues from maternal tissues
 takes part in immune defence

54. Maternal part of placenta
 Decidua basalis with decidual cells
 Placental septa
 Lacunae with maternal blood

55. Decidua basalis
 is a layer of the lamina propria connective tissue
 contains ruptured blood vessels and gland remnants
Û
 is underlined by the decidual plate
- remains after parturition
- is involved in the endometrium regeneration

56. Decidual cells result from decidual reaction
 arise from the endometrial stromal cells
 are rich in glycogen and lipids
 Functions
Þ
- restrict the trophoblast invasion
- provide some nourishment for the embryo and fetus
- create a layer of the placenta separation in parturition
- secrete hormone relaxin responsible for the cervix softening Ý

57. Placental septa are wedge-like areas of the endometrium
 project from the decidua basalis to the chorionic plate
(never attach themselves)
 divide placenta into 15 to 20 lobules – cotyledons
Cotyledon includes
- two or more stem villi
- numerous branch villi

58. Cotyledons are visible on the placenta maternal surface
maternal part
Ü
fetal part
placenta after parturition

59. Lacunae or intervillous spaces
 result from syncytiotrophoblast enzymatic activity
 contain maternal blood
 surround chorionic villi
 communicate with each other
Û

60. Uteroplacental circulatory system
 Maternal blood
- enters the placenta through ruptured spiral arteries
- flows into the intervillous spaces
- circulates in the intervillous spaces
- leaves the placenta through the endometrial veins
- brings O2 and nutrients
- carries away CO2 and waste products

61. Fetal blood enters the placenta through
 paired umbilical arteries
ß
 chorionic plate vessels
ß
 chorionic villus vessels
ß
 a single umbilical vein
Ü
capillary network in the small branch villi
- is in close association with maternal blood-filled lacunae
- takes part in fetal-maternal blood exchange

62. Placental barrier separates fetal blood and maternal blood
 includes only fetal tissues
- syncytiotrophoblast
- cytotrophoblast
- trophoblastic basal membrane
- villous connective tissue
- villous capillary wall
Þ
Ü fetal blood never mixes with maternal blood

63. Placental barrier ultrastructure
 Syncytiotrophoblast
 Cytotrophoblast
 Trophoblastic basal membrane
 Endothelium basal membrane
 Endothelial cells
ß
Þ ß
ß
Ý

64. Placental barrier by the last trimester of pregnancy
 becomes very thin, facilitating maternal-fetal exchange
 lacks
- cytotrophoblast (degenerates)
- connective tissue (disappears)
 includes Ý
- syncytiotrophoblast
- villous capillary wall
Þ

65. Placenta functions
 Selective fetal-maternal blood exchange
- gases, water, electrolytes
- nutrients, hormones, antibodies
- medicine, drugs, infection agents
 Synthesis of some nutrients
- glycogen, cholesterol, fatty acids
 Release of enzymes to erode the endometrium
 Hormone production
- progesterone, estrogens,
- human chorionic gonadotropin (hCG)
- human placental lactogen (hPL)
Ý
- relaxin hCG in the villus syncytiotriophoblast

66. The End
Thank you for attention!