1. Amphibian embryos undergo radial cleavage where cell divisions are slower in the vegetal hemisphere containing yolk. Gastrulation begins with the invagination of cells at the blastopore forming the archenteron.
2. Fate mapping shows superficial cells form ectoderm and endoderm, while deeper cells form mesoderm. During gastrulation, bottle cells migrate inward expanding the archenteron.
3. Spemann and Mangold's experiments showed the dorsal lip tissue of the blastopore, known as the organizer, can induce a secondary embryonic axis when transplanted, demonstrating its inductive properties.
2. Amphibian Cleavage
• Radially symmetrical, holoblastic
• but unlike sea urchin, mesolecithal
egg
• Yolk is concentrated in vegetal pole
• Cell divisions are slower in the
vegetal hemisphere
• First cleavage bisects the grey crescent
– Second cleavage begins in animal pole, while
first cleavage is not yet complete in vegetal
pole
– As in sea urchins, there are no Gap phases in
the cell cycle to allow for rapid divisions
3. • First & Second cleavage
– Meridional
– At right angle to first one and
also Meridional
• Third cleavage
– Equatorial (but not actually at the
equator)
– Divides the embryo into 4 small
– micromeres, 4 large micromeres
• As cleavage continues
_ animal pole packed with many small cells
-vegetal pole has fewer large yolk-laden
cells
4. • At 16-64 cells, embryo is called a morula
– Solid ball of cells
• At 128 cell stage, embryo is a blastula
– Open cavity called blastocoel has appeared in animal pole
• FUNCTION OF BLASTOCOEL
1. Permits cell migration during gastrulation
2. Prevents cells below from interacting with the cells
above prematurely.
5. Amphibian Gastrulation
• Different in different species
• Goals
–Bring endoderm cells to the inside of the
embryo
–Allow ectoderm cells to coat the outside of
the embryo
–Position mesoderm cells in between
6. Fate-maps
Fate-mapping of blastula stage embryos has
p provided some insight
– Using vital dyes to mark cells
– Superficial layers of embryo form ectoderm and
endoderm
– Mesoderm lie mostly in the
deeper layers of cells
– Surface of animal hemisphere
will become cells of ectoderm
– Vegetal hemisphere will form
cells of gut and associated organs
– Mesodermal cell will form
internal cytoplasm around
equator
7. Cell Movements in Amphibian
Gastrulation
• Gastrulation begins on future dorsal side
–Below the equator, in region of grey
crescent
–Cells invaginate to form a slit like 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)
8. • Bottle cell line the archenteron as it forms
• Invagination of cells initiate archenteron
formation
• Gastrulation begins at marginal zone ,not at
vegetal zone as in sea urchin
9.
10. Cell Movements in Amphibian
Gastrulation
• 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)
11. Amphibian Gastrulation
• 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
• Next yolk plug formation
12. • YOLK PLUG :
Yolk plug is the remaining patch
of endodermal cells that is created during the
formation of the dorsal lip of the blastopore
which remains exposed on the vegetal surface of
the blastula that will eventually be internalized
by epiboly.
13. Dorsal lip of the blastopore
yolk plug
ventral lip of the blastopore.
14. Amphibian Axis Formation and “The
Organizer”
• Amphibian gastrulation and axis formation
are an example of regulative development
• Inductive interactions occur between cells
• This was demonstrated by Hans Spemann
and Hilde Mangold
– Nobel Prize winners
15. Spemann and Mangold
• Performed many types of transplants at the
early gastrula and late gastrula stages in the
newt embryo
• These experiments showed that in most cases,
the cells of the embryo are not committed
until at least the late gastrula stage
• But - There is ONE tissue from the early
gastrula that is already committed. . .
16.
17. CONDITIONAL DEVELOPMENT
Early newt gastrula cells were not committed to a
specific fate .such cells are said to exhibit conditional
development
AUTONOMOUS DEVELOPMENT
Late newt gastrula cells that were committed to
specific fate such cells are said to exhibit autonomous
development
18. • There is ONE tissue from the early gastrula that
is already committed is dorsal lip of
blastophore,the tissue derived from the gray
crescent cytoplasm
• When this tissue transplanted into presumptive
belly skin region of another gastrula not only
continued to form blastopore lip
• but also initiated gastrulation and
embryogenesis in the surrounding tissue
• Two conjoined embryos were formed instead of
one
