Gastrulation is the process by which the three primary germ layers (endoderm, mesoderm, ectoderm) are formed in frog embryos through coordinated cell movements. In Xenopus embryos, gastrulation begins with bottle-shaped cells invaginating at the equatorial marginal zone to form the blastopore slit. These cells become the dorsal blastopore lip. Additional mesodermal and endodermal precursor cells involute through the expanding blastopore, eventually forming a ring around the yolky endoderm and completing germ layer formation. Fate mapping studies track the origins and locations of precursor cells during gastrulation.

1. Gastrulation in Frog
Dr. Gauri Haval
Assistant professor
Abasaheb Garware College

2. • "It is not birth, marriage, or death, but gastrulation,
which is truly the most important time in your life."
Lewis Wolpert (1986)
• During gastrulation, cell movements result in a massive
reorganization of the embryo from a simple spherical ball
of cells, the blastula, into a multi-layered organism.
During gastrulation, many of the cells at or near the
surface of the embryo move to a new, more interior
location.
• The primary germ layers (endoderm, mesoderm, and
ectoderm) are formed and organized in their proper
locations during gastrulation.

3. Preparation of Fate Maps
• Tracing cell lineages. Groups of cells
studied by
1. Vital dyes
2. Radioactive labeling and fluorescent dyes.
3. Genetic marking.

4. Fate mapping tells us where things come
from in normal development.

5. Fate Map
In Xenopus mesodermal precursors
lie in deep layers of cells.
Ectoderm and endoderm arise from
superficial layers.
Precursors for the notochord and
other mesodermal tissues are
located beneath the surface in the
equatorial (marginal) region of
the embryo..

6. Process of Gastrulation
• The primary germ layers (endoderm, mesoderm, and
ectoderm) are formed and organized in their proper
locations during gastrulation. Endoderm, the most internal
germ layer, forms the lining of the gut and other internal
organs. Ectoderm, the most exterior germ layer, forms skin,
brain, the nervous system, and other external tissues.
Mesoderm, the the middle germ layer, forms muscle, the
skeletal system, and the circulatory system.
• Although the details of gastrulation differ between various
groups of animals, the cellular mechanisms involved in
gastrulation are common to all animals. Gastrulation
involves changes in cell motility, cell shape, and cell
adhesion.

7. Major types of cell movements
• Invagination: a sheet of cells (called an epithelial sheet)
bends inward.
• Ingression: individual cells leave an epithelial sheet and
become freely migrating mesenchyme cells.
• Involution: an epithelial sheet rolls inward to form an
underlying layer.

8. • Epiboly: a sheet of cells spreads by thinning.
• Intercalation: rows of cells move between one another,
creating an array of cells that is longer (in one or more
dimensions) but thinner.
• Convergent Extension: rows of cells intercalate, but the
intercalation is highly directional.

9. • Gastrulation
• Process of highly coordinated cell and
tissue movements
• Cells of blastula are dramatically arranged
• Cells take up new positions and new
neighbours
• Three germ layers are formed by
movements of the cells

10. • Gastrulation in frog embryos is initiated on
the future dorsal side of the embryo, just
below the equator in the region of the gray
crescent.
• In this region cells invaginate to form a slit
of blastopore
• In Xenopus laevis the change in shape of
cells is very essential for initiation of
gastrulation
• The cells in this region become bottle
shaped and invaginate inwards to form a
slit like blastopore.

11. Formation of blastopore lip
.

12. • The gastrulation in Xenopus begins at the
marginal zone. In this region the cells are
not yolky.
• The next phase is involution of marginal
zone cells and animal cells undergo
epiboly and converge at blastopore.
• When the migrating marginal cells reach
the dorsal lip of the blastopore, they turn
inward and travel alongthe inner surface of
the outer animal hemisphere cells. Thus,
the cells constituting the lip of the
blastopore are constantly changing.

13. • Gastrulation in Xenopus

14. • The first cells to compose the dorsal blastopore
lip are the bottle cells that invaginated to form
the leading edge of the archenteron. These cells
later become the pharyngeal cells of the foregut.
• The dorsal blastopore lip becomes composed of
cells that involute into the embryo to become the
prechordal plate (the precursor of the head
mesoderm).
• The next cells involuting into the embryo through
the dorsal blastopore lip are called the
chordamesoderm cells. These cells will form
the notochord.
• Blastocoel is displaced opposite to dorsal lip of
blastopore

15. • The blastopore expands laterally and ventrally.
• Involution continue through blastopore
• The lateral and ventral lip develop through which
additional mesodermal and endodermal
precursor cells pass.
• Blastopore forms a ring around the large
endodermal cells showing yolk plug stage

16. • At that point, all the endodermal
precursors have been brought into the
interior of the embryo, the ectoderm has
encircled the surface, and the mesoderm
has been brought between them.
• Thus formation of three germ layers
occurs during gastrulation process.
• Reference: Developmental Biology by
Scott Gilbert.