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1. EARLY DEVELOPMENT
IN BIRDS
Group 3
Najito, Obnamia, Penolio,
Pureza, Reyes, Sacil,
Soriano, Tarrega, Valencia,
Zara

2. CLEAVAGE

3. CLEAVAGE IN BIRD EGGS
Accessible all year
Easily raised
At any particular temperature, developmental stage
can be accurately predicted.
Large numbers of embryos can be obtained at the
same stage.
Chick embryo can be surgically manipulated
Often served as a surrogate for human embryos.

4. CLEAVAGE IN BIRD EGGS
Fertilization occur in the
oviduct before the albumen
and the shell are secreted
upon it.
The egg is telocithal (like
that of a fish)
Eggs undergo discoidal
meroblastic cleavage.
CLEAVAGE IN BIRD EGGS

5. First cleavage furrow appear centrally in the
blastodisc
Equatorial and vertical cleavages divide the
blastoderm into 5-6 cell tissue thick
Subgerminal cavity – space between
blastoderm and yolk.
It is created when blastoderm cell absorb
fluid from the albumin and secrete it between
themselves and the yolk.
At this stage, deep cells in the center of the
blastoderm shed and die, leaving the one cell
thick area pellucida
DISCOIDAL MEROBLASTIC
CLEAVAGE

6. DISCOIDAL MEROBLASTIC
CLEAVAGE
Area pellucida forms most of the
actual embryo.
Area opaca – the peripheral ring of
blastoderm cell that have not shed
their deep cells
Marginal zone – thin layer cell
between area pellucida and area oraca
Some of the marginal zone cells
become very important in determining
cell fate during early chick
development.
DISCOIDAL MEROBLASTIC
CLEAVAGE

7. DISCOIDAL MEROBLASTIC
CLEAVAGE
Figure 1. Discoidal Meroblastic
Cleavage

8. GASTRULATION IN
THE CHICK EMBRYO

9. The time a hen lay an egg, the blastoderm contains about 20, 000 cells.
Most area pellucida cell remain at the surface, froming the epiblast
Other pellucida cells delaminated and migrated individually into the subgerminal
cavity to form the polyinvagination island (primary hypoblast)
It is an archipelago of disconnected clusters containing 5-20 cells each.
A sheet of cells from the posterior margin of the blastoderm (with Koller’s
sickle) migrates anteriorly to join the polyinvagination island, later forming the
secondary hypoblast.
GASTRULATION – THE
HYPOBLAST

10. Figure 3. The primary epiblast
GASTRULATION – THE
HYPOBLAST
Figure 4. Forming of secondary
epiblast

11. GASTRULATION – THE
HYPOBLAST
Two layered blastoderm (epiblast and hypoblast) is jooned together
at the margin of the orea opaca, and the space between then forms a
blastocoel.
The embryo entirely come from the epiblast
Hypoblast cells form portion of external membranes (esp. the yolk
sac and stalk)
Yolk stalk link the yolk mass to the endodermal digestive tube.
All 3 layers are formed from the epiblastic cells.
GASTRULATION – THE
HYPOBLAST

12. GASTRULATION – THE
HYPOBLAST
Figure 5. Hypoblast cells migration from deep cells of the posterior
region

13. GASTRULATION – THE
PRIMITIVE STREAK
Primitive streak –
the major structural
characteristic of
avian, reptilian and
mammalian
gastrulation.
GASTRULATION – THE
HYPOBLAST
Figure 6. Cell migration during gastrulation

14. The streak elongates
toward the future head
region.
At the same time, the
secondary hypoblast cells
continue to migrate
anterior to the posterior
margin of the blastoderm.
The streak extends 60-
75% of the length of area
pellucida.
GASTRULATION – THE PRIMITIVE
STREAK
Figure 7. Anterior and posterior view during
gastrulation
Figure 8. Formation of the

15. The streak defines the
axes of the embryo
(extend from posterior to
anterior, migrate cell from
dorsal side to ventral side)
Those close to the streak
will be the medial
structure, and farther will
be the distal structures
GASTRULATION – THE PRIMITIVE
STREAK
Figure 9. Formation of the foregut and other
structures

16. GASTRULATION – THE
PRIMITIVE STREAK
As cell converge from the streak, a depression forms within the streak (called
primitive groove)
It serves as the opening to the migrating cell into the blastocoel (analogous to
amphibian blastopore)
Primitve knot or Hensen’s node – regional thickening of cells at the anterior
end of the primitive streak.
It is the functional equivalent of the dorsal lip of the amphibian blastopore and
the fish embryonic shield.
Primitve pit – a funnel shape depression at the center of Hensen’s node.
GASTRULATION – THE PRIMITIVE
STREAK

17. GASTRULATION – THE
PRIMITIVE STREAK
As the streak form, epiblast cell begin to migrate through it and into the
blastocoel.
In the blastocoel, they migrate anteriorly, forming the foregut, head
mesoderm, and notochord.
Cell passing laterally of the streak forms the majority of endodermal and
mesodermal tissues.
Scatter factor – a 190-kDA protein thought to decompose the basal
lamina and release cells into the embryo as cell enter the streak. Can
convert epithelial sheets into mesenchymal cells in several ways. Involve
in downregulation of E-cadherin expression and prevention of E-cadherin
to function.
GASTRULATION – THE PRIMITIVE
STREAK

18. GASTRULATION – THE PRIMITIVE
STREAK
Figure 10. The primitive
streak

19. First to migrate through Hensen’s node are destined to
become the pharyngeal endoderm of the foregut.
Inside the blastocoel, endodermal cell migrate anteriorly
and displace hypoblast cell to the confined region of area
pellicda anterior portion
Germinal crescent – contain precursors of the germ cells
which later migrate through the blood vessel to the gonads
GASTRULATION – ENDODERM AND
MESODERM FORMATION

20. Next to enter through Hensen’s node move anteriorly but don’t
move far ventrally as the destined foregut endodermal cells,
rather they remain between the endoderm and epiblast to form the
head mesenchyme and prechordal plate mesoderm.
These cells all move anteriorly, pushing the epiblast to form the
head process.
The head of the avian embryo forms anterior (rostral) to Hensen’s
node.
Next cell to migrate through Hensen’s node become
chordamesoderm (notochord) cells
GASTRULATION – ENDODERM AND
MESODERM FORMATION

21. Cells migrating inwardly through the lateral portion of the
primitive streak.
In the blastocoel, these cell separate into 2 layers.
Layer 1 – deep layer join hypoblast along its midline and displace
hypoblast cell to the sides. They give rise to all endodermal organs
and most of the extraembryonic membranes (hypoblast forms the
rest)
Layer 2 – cell spread between the endoderm and epiblast, forming a
loose layer. This generate the mesodermal portion of the embryo
and extraembryonic membranes.
GASTRULATION – ENDODERM AND
MESODERM FORMATION

22. GASTRULATION – ENDODERM AND
MESODERM FORMATION
Figure 11. Close view of
gastrulation

23. The primitive streak start to regress (Hensen’s node move near the center
of area pellucida)
As the node moves posteriorly the notochord is laid down, starting at the
level of the future midbrain.
The posterior notochord (after somite 17 in the chick) forms from the
condensation of mesodermal tissue that has ingressed through the streak
(not through Hensen’s node).
This portion of the notochord extends posteriorly to form the tail of the
embryo.
GASTRULATION – REGRESSION OF
THE PRIMITIVE STREAK

24. Hensen’s node regresses to its most posterior position, forming the anal
region
At this time, all presumptive endodermal and mesodermal cells have
entered the embryo, and the epiblast is composed entirely of presumptive
ectodermal cells.
Avian (and mammalian) embryos exhibit a distinct anterior-to-posterior
gradient development maturity.
While cell of posterior portion of the embryo undergoes gastrulation, cell at
anterior end starts to form organs.
GASTRULATION – REGRESSION OF
THE PRIMITIVE STREAK

25. GASTRULATION – EPIBOLY OF THE
ECTODERM
Figure 12. Epiboly of ectoderm Figure 13. Notochord length vs. time

26. Ectodermal precursors proliferate while the presumptive mesodermal
and endodermal cells are moving inwardly.
Ectodermal cell migrate to surround the yolk by epiboly. (took 4 days
to complete)
It involves the continuous production of new cellular material and the
migration of the presumptive ectodermal cells along the underside of
the vitelline envelope
Only the cells of the outer edge of the orea opaca attach firmly to the
vitelline envelope.
GASTRULATION – EPIBOLY OF THE
ECTODERM

27. These cells are inherently different from the other blastoderm cells, as they can extend
enormous (500 μm) cytoplasmic processes onto the vitelline envelope.
These elongated filopodia are believed to be the locomotor apparatus of these marginal
cells, by which they pull the other ectodermal cells around the yolk
The filopodia appear to bind to fibronectin, a laminar protein that is a component of the chick
vitelline envelope.
If the contact between the marginal cells and the fibronectin is experimentally broken (by adding
a soluble polypeptide similar to fibronectin), the filopodia retract, and epidermal migration ceases
the ectoderm has surrounded the yolk, the endoderm has replaced the hypoblast, and the
mesoderm has positioned itself between these two regions.
GASTRULATION – EPIBOLY OF THE
ECTODERM

28. AXIS FORMATION IN
THE CHICK
EMBRYO

29. Axes are specified early in the cleavage stage.
Formation of these axes are later formed during gastrulation.
AXIS FORMATION IN CHICKS

30. DV axis is established when the
dividing cells of blastoderm form a
barrier between the basic albumin (pH
9.5) above the blastodisc and acidic
subgeminal space below it (6.5).
H2O and Na+ ions are transported
from the albumin to the subgeminal
space and causes a membrane
potential difference of 25 mV.
THE ROLE OF PH IN FORMING THE
DORSAL-VENTRAL AXIS (DV)
 The difference in membrane potentials
distinguishes two sides of the epiblast:
1. The side facing the negative and
basic albumin becomes the dorsal
side.
2. The side facing the positive and
acidic subgeminal space fluid
becomes the ventral side.

31. The bilateral symmetry of the chick blastoderm is determined by gravity.
The ovum spins at rate of 10-20 revolutions per hour for about 20 hours through
the hen’s reproductive tract.
The shifting of yolk makes the lighter components to aggregate beneath one
side of the blastoderm.
Lighter components tips up the end of the blastoderm and this end becomes the
posterior portion of the embryo-the part where the primitive streak formation
begins.
THE ROLE OF GRAVITY IN FORMING THE
ANTERIOR-POSTERIOR AXIS (AP)

32. There is still no known interactions that explain how the posterior margin forms
and why it is the site of gastrulation.
The ability to form primitive streak can be seen throughout the marginal zone
and if the blastoderm is separated into parts, each part will form their own
primitive streak.
However, once a posterior marginal zone (PMZ) has formed, it controls the
marginal regions and prevent the other regions to form their own primitive streaks.
Also PMZ cells initiate gastrulation and is regarded as the equivalent of the
amphibian Nieuwkoop center.
PRIMITIVE STREAK FORMATION

33. The PMZ region like the Nieuwkoop center is thought to be the place where the
localization of β-catenin in the nucleus and TGF- β family signal coincide.
Only the PMZ regions secrete VG1 and if the PMZ tissues are grafted to the anterior
marginal zone, that region will able to form primitive streak.
Koller’s sickle
 Anterior portion
 Forms the Hensen’s node from the epiblast and middle layer cells.
 Posterior portion
 Contributes to the posterior portion of the primitive streak.
Transplantation of Koller’s sickle can cause formation of new axes and middle layer
cells in the Koller’s sickle express Goosecoid.
PRIMITIVE STREAK FORMATION

34. Regarded as the avian equivalent of the amphibian
dorsal blastopore lip since
 it is the site of gastrulation
its cells become the chordamesoderm
it act like the amphibian organizer which can organize a
second embryonic axis when its cells are transplated into
other locations
THE HENSEN’S NODE

35. THE HENSEN’S NODE
Cells of the Hensen’s node secrete chordin, noggin and nodal
proteins which antagonize the BMPs and dorsalize the ectoderm and
mesoderm.
The antagonism of BMPs does not appear to be sufficient for neural
induction.
In chick embryos, fibroblast growth factors (FGFs) generate
neuronal phenotype in epiblast cells rather than BMPs or ectopic
expression of chordin.
FGFs from the Hensen’s node and primitive streaks and beads
induce trunk and hindbrain neuronal expression in the epiblast cells.
THE HENSEN’S NODE

36. This formation is regulated by paracrine factor, Nodal and transcription factor, Pitx2. However,
there is a different mechanism of regulation in chick embryos.
As primitive streak reaches maximum length, the:
 transcription of sonic hedgehog genes ceases on the right side of the embryo due to the
expression of activin
 activin activated expression of FGF8 and FGF8 prevents the transcription of caronte gene.
The absence of caronte genes ables the bone morphogentic proteins (BMPs) to block the
expression of nodal and lefty 2 which also activates the snail gene (cSNR) that is a characteristic
of the right side of avian embryonic organs.
THE LEFT-RIGHT AXIS FORMATION
(LR)

37. On the left side of the body:
 Lefty-1 blocks the FGF8 expression
 Hedgehog activates caronte
 Caronte genes prevents BMPs to repress nodal and lefty-2 and to inhibit the blocking of
lefty-1 expression on ventral midline structures
On the left side of the body:
 Nodal and Lefty-2 activate pitx2 and repress snail (cSNR).
 Lefty-1 in the ventral midline prevents the caronte signals from passing to the right side
of the embryo.
THE LEFT-RIGHT AXIS FORMATION
(LR)

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