Week 1 notes BSN

1. EMBRYOLOGY
HNS 2126
ANNASTACIA MBISI

2. 2nd to 3rd week of development
• Learning objectives
Week 2: Bilaminar germ disc
Week 3: Trilaminar germ disc
Fetal membrane

3. Second week of development
• The second week of development occurs within the
uterine cavity.
• It is the period when the implantation largely
takes place.
• Additionally, during this period the placental and
the embryonic structures differentiate

4. Second week of development cont’d
• Things tend to happen in ‘twos’, hence it is
commonly referred to as the “week of twos”.
• These events occur concurrently as the
implantation process continues.

5. The events that occur in “twos”
• The cells of the blastocyst initially exist in two
masses: the inner and outer cell masses
The inner mass contains compacted cells
grouped on one side.
These cells will later form the embryo and so is
termed the embryoblast

6. The events that occur in “twos”
cont’d
The outer cell mass flatten to form a ring
enclosing the inner cell mass and the
blastocyst cavity.
These cells will later form the placenta, hence
termed the trophoblast.

7. The events that occur in “twos”
cont’d
• The blastocyst can be described to have two poles:
the embryonic and abembryonic poles.
The embryonic pole is the side with the inner cell
mass.
This is the side that will implant first.
The abembryonic pole is the side without the
inner cell mass

8. Fig 1: the two cell masses and poles

9. The events that occur in “twos”
cont’d
• The cells of the embryoblast reorganize into two
layers: the epiblast and the hypoblast.
The epiblast contains a layer of tall cells on the
upper part, while the hypoblast contain a layer
of flat cells on the lower side.

10. Fig 2: the bilaminar disc

11. The events that occur in “twos”
cont’d
• Two cavities develop: the amniotic and the
umbilical vesicle (yolk sac) cavities.
The amniotic cavity appears within the inner
cell mass as it differentiates.
This cavity is surrounded by a layer of flattened
cells called amnioblast cells which constitute
the amnion.

12. The events that occur in “twos”
cont’d
These cells secret the amniotic fluid into the
cavity.
The umbilical vesicle is lined by the hypoblast
cells.
Accordingly, the bilaminar disc lies between the
amniotic cavity and the umbilical vesicle

13. Fig 3: the two cavities

14. The events that occur in “twos”
cont’d
• The outer cell mass (trophoblast) differentiate into
two layers: the cytotrophoblast and the
syncitiotrophoblast.
The syncitiotrophoblast is the outer zone of
rapidly expanding, multinucleated mass in which
no cell boundaries are discernible.

15. The events that occur in “twos”
cont’d
This zone is erosive and invasive, useful during
implantation.
It also secrets the human chorionic gonadotropic
(hCG) hormone.
hCG maintains the hormonal activity of the corpus
luteum in the ovary during pregnancy, and forms the
basis of early pregnancy test.

16. Fig 4: two trophoblastic layers

17. The events that occur in “twos”
cont’d
• The extraembryonic mesoderm forms two layers:
the visceral (splanchnic) and parietal (somatic)
layers.
The extraembryonic mesoderm forms from
cells of the umbilical vesicle.

18. The events that occur in “twos”
cont’d
As it increases, spaces appear within it.
These spaces later fuse to form the
extraembryonic coelom, which surrounds the
amnion and umbilical vesicle.

19. Fig 5: layers of the extraembryonic mesoderm

20. Third week of development
• The most characteristic event occurring during the
3rd week of gestation is gastrulation, the process
that establishes all three germ layers (ectoderm,
mesoderm, and endoderm) in the embryo.

21. Gastrulation
• Gastrulation is the process of formation of the
trilaminar disc from the bilaminar disc.
• It begins by formations of the primitive streak, a
midline thickening of the epiblast cells

22. Gastrulation cont’d
• The cells of the primitive streak then lose their
contacts and migrate downwards and outwards
• These distant cells displace the hypoblast to form
the endoderm layer

23. Gastrulation cont’d
• More migrating cells sandwich between the
remaining epiblast and the developing
endoderm.
• These form the intra-embryonic mesoderm layer
• The remaining epiblast cells constitute the
ectoderm

25. Gastrulation cont’d
• The mesodermal cells immediately beneath the
early primitive streak quickly aggregate, forming a
rod of mesodermal cells called notochord
• The notochord serve as first axial support of the
embryo

26. Gastrulation cont’d
• The region of the intra-embryonic mesoderm
near the notochord differentiates into paraxial
mesoderm.
• The region furthest becomes the lateral plate
mesoderm, and the intermediate mesoderm lies
between the two masses

27. Fig 6: the trilaminar disc

28. Derivatives of the ectoderm layer
• The ectoderm is a protecting and a communicating
layer.
• The appearance of the notochord induces the
overlying ectoderm to thicken and form the neural
plate
• Cells of the plate make up the neuroectoderm that
later forms the nervous system.

29. Derivatives of the ectoderm layer
cont’d
• The rest of the ectoderm form the surface
ectoderm which later form the epidermis of the
skin.

30. Fig 7. Differentiation of the ectoderm

31. Derivatives of the mesoderm layer
• Once the intra-embryonic mesoderm has
differentiated into the paraxial, intermediate
and lateral plate mesoderm, the latter then
divides again into
somatic/parietal layer that is next to the
ectoderm,

32. Derivatives of the mesoderm layer cont’d
the splanchnic/visceral layer that is next to the
endoderm
• These two layers line a newly formed cavity called
called the intra-embryonic cavity

33. Figure 8: differentiation of the intra-embryonic
mesoderm

34. Derivatives of the mesoderm layer
cont’d
• The paraxial mesoderm forms the vertebral
column, dermis of the skin and the musculature.
• The intermediate mesoderm differentiates into
urogenital structures

35. Derivatives of the mesoderm layer
cont’d
• Somatic mesoderm gives rise to the lateral and
ventral body wall (together with the surface
ectoderm), and bones of the appendicular skeleton.
• The splanchnic mesoderm will form the muscles
and connective tissues of the gut.

36. Derivatives of the mesoderm layer
cont’d
• The intra-embryonic cavity becomes the
peritoneal, pericardial and pleural cavities

37. Derivatives of the endoderm layer
• The endoderm is a nourishing layer.
• It gives rise to the epithelial lining of digestive
system, respiratory system and urinary bladder.
• This germ layer surrounds the umbilical vesicle

38. Derivatives of the endoderm layer
cont’d
• With embryonic folding, the dorsal portion of
the vesicle is incorporated into the embryo proper
to form the primordial gut, which has a foregut,
midgut and hindgut portions

39. Figure 9: incorporation of the endoderm – longitudinal
view

40. Figure 10: incorporation of the endoderm – transverse
view

41. Formation and the role of embryonic
membrane
• The embryonic membrane that forms during the
first two to three weeks of development includes:
Amnion, York sac, Allointois and Chorion

42. Figure 11: foetal membranes

43. Amnion-formation
• Amnion is formed within the inner cell mass and
later appear above the embryo by the amnioblast
cells during the second week.
• It encloses the amniotic cavity that contains the
amniotic fluid.

44. Amnion-formation cont’d
• The embryo is suspended into the amniotic cavity
by the umbilical cord.
• The amniotic fluid comes from maternal tissue
fluid, secretions of the amnioblast cells and the
fetal urine.

45. Amnion-formation cont’d
• This fluid increases in quantity from approximately
30 ml at 10 weeks of gestation to 450 ml at 20
weeks to 800 to 1000 ml at 37 weeks
• This causes the amnion to expand and ultimately to
adhere to the inner surface of the chorion.

46. Amnion-formation cont’d
• Fluid is highly dynamic, being replenished every 3
hours!
• From the beginning of the fifth month, the fetus
swallows its own amniotic fluid and it is estimated
that it drinks about 400 ml a day, about half of the
total amount

47. Function
• The amniotic fluid prevents adherence of the fetus
to the amnion and provides shock absorbing effect
hence reducing risk of physical injury.
• It also helps in maintaining constant temperature
and pressure around the fetus

48. Function cont’d
• It protects against infections and allows free
movements of the fetus, allows symmetrical growth,
is important for lung and musculoskeletal
development and regulates fetal body temperature.

49. Function cont’d
• During childbirth, the amnio-chorionic membrane
forms a hydrostatic wedge that helps to dilate the
cervical canal, and the also lubricates the birth canal.

50. Fate
• The amniotic membrane is usually ruptured around
the time of labor.
• Some of the fluid gushes out before the baby is
delivered and some comes out after the delivery of
the baby.

51. Fate cont’d
• The membrane itself comes out with the placenta as
“after birth” since by this time it is attached to the
placenta (amnio-chorionic membrane).

52. Clinical correlates
• The amniotic fluid contains some foetal cells.
• Accordingly, this is utilized in amniocentesis for
genetic studies.

53. Clinical correlates cont’d
• Common disorders of the amnion/amniotic fluid are
• Oligohydramnios (inadequate amniotic fluid
volume),
• Polyhydramnios (excess amniotic fluid volume)
and
• amniotic bands.

54. Umbilical vesicle (yolk sac)-formation
• It is a membranous sac situated on the ventral aspect
of the embryo and is formed by cells of the
hypoblast layer.
• In human beings it contains fluid but no yolk.
• Following embryonic folding, the size of this
membrane reduces as the pregnancy advances

55. Fig 12: embryonic folding

56. Functions
• It functions as a site of hemopoiesis (formation of
blood cells) until the 6th week of gestation when
the foetal liver takes over.
• It is also important for the transfer of nutrients
during early development before the placenta takes
over.

57. Function cont’d
• The wall of the yolk sac is known to give rise to the
primordial germ cells.
• It is incorporated into the primordial gut during
the fourth week of development

58. Fig 12: incorporation of the umbilical vesicle into
the folding embryo to form the gut

59. Fate
• The umbilical vesicles progressively disappear as the
pregnancy advances.
• Its dorsal part, however, is incorporated into the
embryo during folding to form the primordial gut.

60. Fate cont’d
• The connection between the primordial gut and
the rest of the yolk sac is called the vitelline duct.
• This duct also degenerates

61. Clinical correlate
• While the vitelline duct is mean to disappear,
sometimes it may persist, giving rise to a Merkel’s
diverticulum (Fig 13), a slight bulge in the ilium
present as a congenital malformation.

62. Figure 13: merkel’s diverticulum

63. Allantois -Formation
• This is a small tubular diverticulum that arises
from the posterior part of the yolk sac and grows
towards the connecting stalk around the third
week of development

64. Allantois –Formation cont’d
• When the hind gut (caudal part of the incorporated
yolk sac) is developed the allantois is carried
backward with it and then opens into the cloaca, a
common opening of both the urogenital and
digestive tracts.
• It shrinks gradually and gets enclosed in the
umbilical cord.

65. Fig 14: yolk sac and allantois

66. Functions
• Allantois helps the embryo exchange gases and
handles liquid waste.
• Later, it helps in the formation of the umbilical
vessels and hemopoiesis.
• It also contributes to the formation of the urinary
bladder.

67. Fate
• Between the 5th and 7th week of development, it
becomes the urachus, a duct between the bladder
and the yolk sac.
• This duct becomes obliterated to form the median
umbilical ligament

68. Clinical correlate
• Failure of the urachus to obliterate may lead to a
urachal fistula, an abnormal connection between
urinary bladder and the umbilicus

69. Chorion-Formation
• The chorion is formed by the cytotrophoblast,
syncitiotrophoblast and the somatic layer of the
extraembryonic mesoderm.
• The extraembryonic mesoderm lining the inside
of the cytotrophoblast is then known as the
chorionic plate

70. Chorion-Formation cont’d
• The chorion completely surrounds the embryo and
develops villous projections.
• In the early weeks of development, villi cover the
entire surface of the chorion.

71. Chorion-Formation cont’d
• However, as pregnancy advances, villi on the
embryonic pole continue to grow and expand, giving
rise to the chorion frondosum (bushy chorion).

72. Chorion-Formation cont’d
• Villi on the abembryonic pole degenerate and by the
third month this side of the chorion, now known as
the chorion laeve, is smooth.
• The chorion frondosum invade and destroy the
uterine decidua and at the same time absorb from
it nutritive materials for the growth of the embryo.

73. Figure 15: chorion formation and development

74. Functions
• The chorion has a protective function and also
contributes to the formation of the placenta.

75. The placenta-formation
• The placenta is a feto-maternal organ that is made
up of a larger fetal part derived from the chorion
frondosum, and a smaller maternal part developed
from the decidua basalis (endometrium)

76. Figure 16: components of the placenta

77. The placenta-formation cont’d
• It begins to develop upon implantation of the
blastocyst into the maternal endometrium, and
grows throughout pregnancy.
• The fetal circulation is separated from the maternal
circulation by a thin layer of extra-fetal tissues
known as placental membrane.

78. The placenta-formation cont’d
• The membrane is permeable and allows water,
oxygen, nutritive substances, hormones, wastes and
drugs to pass in their respective directions

79. Functions of the placenta
• Metabolism – such as the synthesis of glycogen,
cholesterol and fatty acids
• Exchange functions – gas exchange (oxygen and
carbon dioxide), nutrients, waste products and
antibodies.
• Endocrine secretion (e.g. hCG) for maintenance of
pregnancy

80. THANK YOU