embryology

1. •ALAQAH-2
•Trilaminar Germ Disc
•3rd week

2. • Introduction
• Formation of the primitive streak
• Genesis of the germ layers
• Genesis of the notochord
• Location of the epiblast cell target and the
development of the primitive streak
• The bilaminar membranes

3. • Evolution of the mesoblast
– The paraxial mesoderm and the differentiation
of the somites
– The intermediate mesoblast
– The lateral mesoblast
• The intraembryonic coelom
• Induction of the neural plate - neurulation

4. • During first week of development
zygote multiplies and form morula,
which contains inner cell mass in
the center.
• The morula changes into blastula
or blastocyst during the same
week and the inner cell mass has
shifted to one-side, and is now
called embryoblast.

5. • During second week of development the
cells of the embryoblast differentiate into
two layers, the epiblast, which forms the
floor of the amniotic cavity and the
hypoblast, which forms the roof of yolk
sac.
• The cells of each of the germ layers form
a flat circular disc and together they are
known as the bilaminar germ disc.

6. • In the median sagittal plane in the cephalic
region the hypoblast disc shows a slight
thickening forming an oval plate of large
vesicular cells, the prochordal plate. It is a
small circular or oval area of columnar cells in
the hypoblast, which are firmly adherent to the
overlying epiblast disc.
• Prochordal plate together with overlying
epiblast forms oropharyngeal
(buccopharyngeal) membrane.

7. • Similarly in the caudal region the
hypoblast cells show a slight
thickening forming an oval plate of
large vesicular cells. This is cloacal
plate.
• Together with overlying epiblast it is
given the name cloacal membrane.
• During 3rd week the developing
embryo has three layers.

8. Gastrulation
• It is the process by which the embryo acquires
three germ layers.
• During the third week mesoblast layer develops
between the epiblast and hypoblast.
• Day-16. The epiblast cells near primitive streak
proliferate, and loose their connection with one
another.
• They also develop foot like processes called
pseudopodia and start walking down through
primitive groove.
• This process of invagination and ingress is
called gastrulation.

9. • The bilaminar germ disk differentiates
itself further into a trilaminar embryo, in
that the cells flow in over the primitive
streak between the two already existing
germinal layers and so form the third
embryonic germinal layer
(mesoblast/derm).

10. • This phenomenon is also termed
epithelio-mesenchymal transition
(gastrulation in lower vertebrates)

11. • The germ disc now becomes trilaminar and the
three layers are now given new names.
– Ectoderm (Superior layer)
– Mesoderm (Middle layer)
– Endoderm (Inferior layer)
• The process by which the bilaminar germ disc
is converted into trilaminar germ disc is called
gastrulation and the developing structure is
now called gastrula.
• Gastrulation begins with formation of
primitive streak.

13. PRIMITIVE STREAK
• It is a thickened linear band of epiblast
cells present in the midline in the caudal
region on the superior surface of epiblast.
• It starts developing by the beginning of 3rd
week close to cloacal plate.
• The cells of the epiblast multiply and move
toward primitive streak and the streak
elongates in cephalic direction.

14. 15-day developing embryo
showing bilaminar germ disc.

15. • At this stage the embryonic disk is oval
-shaped and the ectoblast is bathed in
amniotic fluid.

16. • As the streak elongates by addition of cells, its
cranial end thickens to form a primitive knot or
node.
• The primitive knot is a slightly elevated
rounded area at the cranial end of the primitive
streak.
• Concurrently, a narrow groove and pit develop
in the primitive streak and primitive knot, called
primitive groove and primitive pit
respectively. The groove and the pit are
continuous with each other.

17. 1
2
3
4
5
6
7
8
9
NB
Primitive groove
Primitive pit
Primitive node
Oropharyngeal membrane
Cardial plate
Sectional edge of amniotic
membrane
Mesoderm
Endoderm
Future cloacal membrane
1+2+3 primitive streak
• Embryonic disk viewed dorsally.
The red arrows show schematically the migration
directions of the epiblast cells to their points of final
destination.

18. FORMATION OF ENDODERM
• Day-15. The epiblast cells near primitive streak
proliferate, and loose their connection with one
another.
• They also develop foot like processes called
pseudopodia and ingress down through primitive
groove.
• These cells invade the hypoblast and replace its
cells.
• Eventually the hypoblast is completely replaced
by a new crop of cells derived from epiblast.
Now hypoblast is given the name endoderm.

21. • Day 16. As more and more cells of the epiblast
multiply and migrate toward the primitive
groove, they detach and fall through the groove
on the underlying hypoblast as mesoblast cells.
• Here these wandering mesoblast cells spread
in lateral and cephalic direction and form a
network of cells called mesoblast.
• Gradually mesoblast cells become
organized into a layer called
intraembryonic mesoderm.

23. 1. Primitive groove
2. Epiblast
3. Extraembryonic mesoblast
4. Definitive endoblast
5. Invading epiblastic cells forming
6. the intraembryonic mesoblast
7. Hypoblast
8. Transverse section at the level of the
primitive groove with the immigration of
epiblast cells, which form the future
mesoblast, as well as the endoblast,
which replaces the hypoblast.

25. • This intra-embryonic layer of
mesoderm gradually expands
beyond the margins of the disc
and establishes contact with the
extraembryonic mesoderm
covering the yolk sac and amnion.
• Now the mesoblast cells are often
called mesenchymal cells.

26. • At the same time, due to the migration of
cells through the primitive node in the
cranial direction, two further structures
are formed:
• the prechordal plate, which is located
cranial to the primitive node
• the notochordal process

28. • In the cephalic direction, mesenchymal
cells pass on each side of the prochordal
plate to meet each other in front of this
plate where they form cardiogenic (heart
forming) plate.
• Similarly the mesoblast or mesenchymal
cells migrate around cloacal plate and
reach posterior to cloacal plate and join
connecting stalk (the extraembryonic
mesoderm).

30. FORMATION OF NOTOCHORD
• The cells detaching and falling from
primitive pit move straight forward and
form a cord-like process, known as
notochordal or head process (day-17).
• Remember that notochordal process
contains a canal continuous with primitive
node.
• The notochordal process extends cranially
from the primitive node toward prochordal
plate.

34. • Later on the floor of the notochordal process
fuses with the underlying endoderm (day 18).
• The fused regions degenerate and openings
appear in the floor of the notochordal process,
bringing the notochordal canal into
communication with the yolk sac.
• These openings rapidly join each other and the
floor of the notochordal process disappears.
• The remaining part of notochordal process
forms a dome-shaped plate grooved from
below, known as the notochordal plate.

42. • A small passage, the neurenteric canal
temporarily communicates the amniotic cavity
with the yolk sac (day-18).
• The neurenteric canal appears due to the
degeneration and disappearance of the floor of
the notochordal process. It runs obliquely
downward and forward from the primitive pit.
• Beginning at the cranial end, the notochordal
plate enfolds and separates from the endoderm
to form a solid cord of cells, known as the
notochord (day-19).

46. • Initially the embryonic disc is flat and
essentially circular, but it soon becomes pear-
shaped and than elongated slipper-shaped as
the notochordal process grows and the
notochord forms.
• Expansion of the embryonic disc occurs mainly
in the cranial region; the caudal end remains
more or less unchanged at this stage of
development.

47. • Initially the intraembryonic mesoderm
forms a thin flattened sheet on both
sides of midline.
• By about day 17 the mesenchymal
cells close to the midline proliferate
and form thick bilateral bars of
mesoderm along the sides of
notochord, called paraxial
mesoderm.

50. • Laterally toward the edges of the disc intraembryonic
mesoderm remains thin flattened and is known as
lateral plate mesoderm.
• A number of small spaces appear in lateral plate
mesoderm. These spaces soon coalesce and divide
the lateral plate mesoderm into two layers:
1. Somatic or parietal mesoderm layer continuous
with the somatic mesoderm covering the amnion.
2. Splanchnic or visceral mesoderm layer continuous
with the splanchnic mesoderm covering the yolk
sac.
• The space between the two layers is known as
intraembryonic coelom, which on each side of the
embryonic disc, is continuous with extraembryonic
coelom.

51. Between paraxial mesoderm
and lateral plate mesoderm, the
mesenchymal cells proliferate
(day 20) and form another set
of thick bilateral bars of
mesoderm known as
intermediate mesoderm.

52. ALLANTOIS
• (Gr. Allantos, sausage)
• The allantois, also called allanto-enteric
diverticulum, appears on about day 16 as
a relatively tiny, finger-like diverticulum.
• It is an endodermal outgrowth from the
posterior wall of the secondary yolk sac
and grows into the mesoderm of the
connecting stalk.

53. Day 18 developing embryo showing
the formation of allantois.

54. • It serves as a storage place for the excretion products
of renal system. (It has a respiratory function in
embryos of reptiles, birds and some mammals).
• Contribute in development of urinary bladder.
• As the bladder enlarges, the allantois becomes the
urachus connecting the apex of bladder with the
umbilicus.
• The allantois also induces the formation of the blood
vascular system in the connecting stalk. If allantois
does not develop, the blood vessels fail to develop in
the connecting stalk. So the intraembryonic blood
vascular system cannot establish communication with
the extraembryonic blood vascular system or placenta
or with maternal blood. Thus the embryo cannot
survive.