3. Gastrulation and the three-layer formation
The terms "gastrula" and "gastrulation"
were coined by Ernst Haeckel, in his 1872
work "Biology of Calcareous Sponges".
4. Characters of the gastrulation stage
The gastrula phase is characterized by the following:
1. It is the first stages of cellular differentiation.
2. Embryonic cells begin to follow in specific layers through morphogenetic
movements.
3. The three embryonic layers (external Ectoderm, medium Mesoderm and
Endoderm internal layer).
4. Cellular divisions lose synchronization between them.
5. The metabolism changes within the cells and the oxidation process becomes
the dominant.
6. Nucleus at this stage has more impact than before in the process of control
of development and shows chromosomes or paternal genes and its effect.
5. Gastrulation
Gastrulation rearranges the cells of
a blastula into a three-layered
embryo, called a gastrula, which has
a primitive gut or called archenteron.
The three layers produced by
gastrulation are called embryonic
germ layers
1. The ectoderm forms the outer
layer
2. The endoderm lines the digestive
tract, and lungs
3. The mesoderm partly fills the
space between the endoderm and
ectoderm
6. • The blastula consists of numerous cells, the positions of which were established
during cleavage.
• During gastrulation, these cells are given new positions and new neighbors, and
the multilayered body plan of the organism is established.
• The cells that will form the endodermal and mesodermal organs are brought to
the inside of the embryo, while the cells that will form the skin and nervous
system are spread over its outside surface.
• Thus, the three germ layers-outer ectoderm, inner endoderm, and interstitial
mesoderm-are first produced during gastrulation.
• In addition, the stage is set for the interactions of these newly positioned tissues.
Gastrulation
7. The three layers (Endoderm, Mesoderm, and
Ectoderm) are formed and organized in their
proper locations during gastrulation.
This fate map diagram of a Xenopus blastula
shows:
cells whose fate is to become ectoderm
in blue and green,(skin and neural tube)
Cells whose fate is to become mesoderm
in red,(muscles, bones heart, blood, kidney …)
Cells whose fate is to become endoderm
in yellow.(digestive system, liver lunges…)
Notice that the cells that will become endoderm
are NOT internal, yet. It will become internal
during gastrulation stage.
Gastrulation
8.
9. • Gastrulation involves some combination of several types of movements.
• These movements involve the entire embryo, and cell migrations in one part of
the gastrulating embryo must be intimately coordinated with other movements
that are taking place simultaneously.
• Although patterns of gastrulation vary enormously throughout the animal
kingdom, there are only a few basic types of cell movements.
Type of cell movement during gastrulation
10. 1. Invagination: The in folding of a region of cells.(e.g= sea urchin gastrula)
2. Involution: The inward movement of an expanding outer layer so that it spreads
over the internal surface of the remaining external cells.(e.g frog gastrula).
3. Ingression: The migration of individual cells from the surface layer into the
interior of the embryo. The cells become mesenchymal (i,e" they separate from
one another) and migrate independently.(e.g Drsophlia and sea urchin mesoderm)
4. Delamination: The splitting of one cellular sheet into two more or less parallel
sheets. (chick embryo gastrolation)
5. Epiboly, The movement of epithelial sheets (usually of ectodermal cells) that
spread as a unit (rather than individually) to enclose the deeper layers of the
embryo.(e.g formation of ectoderm in frog, sea urchin and tunicates embryos).
Type of cell movement during gastrulation
12. The frog blastula is many cell layers with blastocoel at the end of cleavage stage.
The blastula start to form the dorsal lip were the cell in the animal pole start to
move from the outside to the inside via the dorsal lip of Blastopore.
Cells of the dorsal lip originate in the gray crescent area (in the fertilized ova)
and invaginate to the inside of the embryo.
Cells continue to move from the embryo surface to the inside of the embryo by
involution.
These invagenated cells become the endoderm and mesoderm.
The blastocoel is replaced by a new cavity inside the gastrula stage called
archenteron which will become the cavity of elementary canal later.
The surface of the embryo is now ectoderm, the innermost layer is endoderm, and
the middle layer is mesoderm.
The blastopore encircles a yolk plug when gastrulation is completed. Witch will
become the posterior area of the embryo.
Gastrulation in the frog
13. •Gastrulation begins on dorsal side Below the equator, in region of grey crescent.
• Cells invaginate to form a slender blastopore.
•Dorsal lip of blastopore will become important organizing region of embryo
(Spemann organizer)
•Cells become elongated as they contact the inner surface (Bottle cells).
Future ectoderm
Key
Future endoderm
Future mesoderm
SURFACE VIEW
Animal pole
Vegetal pole
Early
gastrula
Blastopore
Blastocoel
Dorsal lip
of blasto-
pore
CROSS SECTION
Dorsal lip
of blastopore
Bottle cell
Cell Movements in Amphibian Gastrulation
14. • A small group of cells change shape,
narrowing at the exterior edge of the
blastula.
• This change in cell shape, called
apical constriction, creates a local
invagination, which pushes more
interior cells upwards and begins to
roll a sheet of cells towards the
interior.
• The constricted cells are called bottle
cells, due to their shape (like an
upside-down bottle in these images).
How does the blastopore lip form?
16. Future ectoderm
Key
Future endoderm
Future mesoderm
SURFACE VIEW
Blastocoel
shrinking
CROSS SECTION
Archenteron
Next steps:
•Involution of the cells at the marginal zone (outer sheet spreads over inner
sheet).
Cells from Animal pole undergo epiboly
Converge at the blastopore, When reach blastopore, travel inward Bottle cells
continue to migrate, form leading edge of archenteron (primitive gut) .
Cell Movements in Amphibian Gastrulation
17. Once the padded or gastrophila is formed until the Pericardial plug
begins to decrease as a result of the crawl of the animal pole cells
around it from each direction, then the lips eventually become a small
longitudinal mischief known as the primitive line has two fine holes
that close the abdomen and the dorsal remains open for a while and
then close later.
After the process of lining, the fetus transforms from a hollow ball with
a single layer, into a three-layer outer body known as ectoderm,
internal known as endoderm, and cells that migrate from the outside
to the inside are the middle class, which is known as the Mesoderm.
From these three embryonic layers, all organs of the fetus arise later.
Gastrulation in Amphibian
18. SURFACE VIEW
Blastopore
Late
gastrula
Blastopore
Blastocoel
remnant
Yolk plug
CROSS SECTION
Ectoderm
Mesoderm
Endoderm
Archenteron
Future ectoderm
Key
Future endoderm
Future mesoderm
•Mass of yolk left by surrounding blastopore = yolk.
• plug Mass of yolk left by surrounding blastopore = yolk plug.
•Cells from the dorsal lip (the first cells that migrated inward)
become prechordal plate (will form head mesoderm).
•Next cells that involute form chordamesoderm (will become
notochord) Important for patterning the nervous system.
Gastrulation in Amphibian
19. Future ectoderm
Key
Future endoderm
Future mesoderm
SURFACE VIEW
Animal pole
Vegetal pole
Early
gastrula
Blastopore
Blastocoel
Dorsal lip
of blasto-
pore
CROSS SECTION
Dorsal lip
of blastopore
Late
gastrula
Blastocoel
shrinking Archenteron
Blastocoel
remnant
Archenteron
Blastopore
Blastopore Yolk plug
Ectoderm
Mesoderm
Endoderm
Gastrulation in Amphibian
20. What causes gastrulation?
What factors are responsible for all of the complex and
orchestrated movements that occur during gastrulation?
There are at least 2 hypotheses and they are not
mutually exclusive.
1. Cellular behaviors observed in each gastrula region
are caused by local accumulations of gene products
that are already present in the fertilized oocyte.
2. One region of the blastula could be determined to
organize the behavior of all other regions. The
dorsal lip of the blastopore will become the
spemann organizer that appears to be particularly
important in regulating the fate of other areas. 2
genes, goosecoid and noggin from the embryonic
organizer area seem to cause the cell invagination in
the blastopore area to mediate this effect.
22. Gastrulation in the birds
The epiblast and the hypoblast:
• By the time a hen has laid an egg, the blastoderm contains
some 20,000 cells.
• At this time, most of the cells of the area pellucida remain
at the surface, forming an "upper layer,“ called the epiblast.
• While other area pellucida cells have delaminated and
migrated individually into the sub germinal cavity to form
hypoblast islands.
• The resulting two-layered blastoderm (epiblast and
hypoblast) is joined at the marginal zone of the area opaca,
and the space between the layers forms a blastocoel.
23. • The avian embryo comes entirely from the epiblast.
• The hypoblast does not contribute any cells to the developing embryo.
• Rather, the hypoblast cells form portions of the external membranes,
especially the yolk sac and the stalk linking the yolk mass to the endodermal
digestive tube.
• Hypoblast cells also provide chemical signals that specify the migration of
epiblast cells.
• The three germ layers of the embryo proper (plus the amnion) are formed
solely from the epiblast.
Gastrulation in the birds
24. The primitive streak
• The major structural characteristic of avian, reptilian
and mammalian gastrulation is the primitive streak.
• The primitive streak becomes the blastopore lips of
amniote embryos.
• The primitive streak first arises from a local
thickening of the epiblast at the posterior edge of the
area pellucida, called Koller's sickle, and the epiblast
above it.
Gastrulation in the birds occur via
26. The Formation of primitive streak
1. The streak is First visible as cells accumulate in the middle layer, Followed by a
thickening of the epiblast at the posterior marginal zone.
2. This thickening is initiated by an increase in tile height (thickness) of the cells
forming the center of the primitive streak.
3. The presumptive streak cells around them become globular and motile, and they
appear to digest away the extracellular matrix underlying them.
4. This process allows these cells to undergo intercalation and convergent extension.
5. Convergent extension is responsible for the progression of the streak doubling in
streak length is accompanied by a concomitant halving of its width.
6. As cells converge to form the primitive streak, a depression forms within the
streak. This depression is called the primitive groove, and it serves as an opening
through which migrating cells pass into the deep layers of the embryo.
28. • As soon as the primitive streak has
formed, epiblast cells begin to
migrate through it and into the
blastocoel.
• The streak thus has a continually
changing cell population. Cells
migrating through the anterior end
pass down into the blastocoel and
migrate anteriorly, forming the
endoderm, head mesoderm, and
notochord.
• cells passing through the more
posterior portions of the primitive
streak give rise to the majority of
mesodermal tissues.
Formation Of Endoderm And Mesoderm
29. A cross-section through the embryo
allows us to observe the three germ
layers that form during gastrulation:
ectoderm, mesoderm, and endoderm.
30. Mammalian Gastrulation
• Birds and mammals are both descendants of reptilian species (albeit
different reptilian species).
• It is not surprising, that mammalian development parallels that of reptiles
and birds.
• the gastrulation movements of reptilian and avian embryos, which evolved
as an adaptation to yolky eggs, are retained in the mammalian embryo
even in the absence of large amounts of yolk.
31. Images of human embryos during gastrulation,13 - 19 days post ovulation.
Notice the primitive streak, which is analogous to the blastopore of Xenopus.
