Useful for students studying embryology

1. Determination of Embryonic axis
• Unfertilized egg has a polarity along the animal-vegetal axis.
Thus the germ layers can be mapped.
• This mapping is done because there are transcription factors
VegT and TGF-β family paracrine factor Vg1. the mRNA for
these proteins is located in the cortex of the vegetal
hemisphere of the egg. This factor is present at vegetal pole
during cleavage.
• If VegT is destroyed the entire embryo develops as an
epidermis. No mesoderm and endoderm component.
• If vg1 destroyed, no endoderm, no dorsal mesoderm.
• Thus allocation of germ layers depends on pre-localized
cytoplasmic determinants.

2. • Amphibian development: Regulative type.
• Inductive interactions are required for embryonic axis
development. First demonstrated by Hans Spemann in1903.
• The expts were carried out on Newt. Early blastomeres have
identical nuclei. The ligature was carried out at the axis of first
cleavage plane. Each capable of producing entire larva.
• The other ligature was done longitudinal but perpendicular to the
plane of first cleavage. Thus separating dorsal and ventral
regions. The results were different. The future dorsal side of the
embryo produced normal larva. Other region was a mass of
ventral cells.
• There are dramatic movements in the egg cytoplasm after
fertilization. In some amphibians grey crescent is exposed directly
opposite to the entry of the sperm. First cleavage separates the
grey crescent equally into two blastomeres. However, in the above
experiment the grey crescent was restricted into one blastomere
only. That blastomere developed normally. Something in the grey
crescent is required for normal development.

4. • The fate map showed that the grey crescent area gives
rise to the cells required for initiation of gastrulation.
These are the cells which forms the dorsal lip of
blastopore. The cells of the dorsal lip of blastopore are
committed to invaginate into blastula, thus initiating
gastrulation and future notochord.
• The future development is dependent on the cell
interactions.

5. • Hans Spemann and Hilde Mangold carried
out spectacular transplantation experiments
in 1924.

6. Doral lip = “Organizer” organizes
secondary DV axis induced
ventral cells to change fates

7. Fate and Axis Determination
Induction and Determination
1) Anterior posterior, dorsalventral, and left right
axes are specified by events triggered at
fertilization and realized during gastrulation.
2) Mesoderm is determined by transcription factors
and paracrine factors from the vegetal region.
3) Of all tissues in the Xenopus pregastrula, only
the dorsal lip of the blastopore has its fate
determined.
4) The dorsal lip will determine the fates of
notochord and head endomesoderm.

8. • Spemann referred dorsal lip of blastopore and its
derivatives as an organizer because
1. Host ventral tissue changed its fate to form neural tube
and dorsal mesodermal tissue.
2. They organized the development of secondary
embryonic axis.
He also said that in normal development these cells
organize the dorsal ectoderm into neural tube and
transform flanking mesoderm into anterior posterior
axis. Now it is known that it initiates a series of
inductive events. However, the induction caused by
dorsal lip of blastopore is called primary embryonic
induction

9. • Transplantation experiments established the organizing
properties of the dorsal blastopore lip, which …
1) Selfdifferentiates
(all others conditionally specified)
2) Establishes the dorsal ventral axis
3) Specifies multiple tissues, including…
• Dorsal mesoderm, which includes…head
• mesoderm (prechordal plate)
• chordamesoderm
• (notochord
4) Dorsalizes surrounding mesoderm into paraxial mesoderm
5) Induces the neural tube
6) Initiates the movements of gastrulation
• How is the dorsal lip specified?

10. • The dorsal lip cells are at right place and at right time.
There are two signals involved one signal tells them that
they are dorsal and other tells that they are mesodermal.
• These mesodermal cells reside above a special group of
vegetal cells.

11. If animal cap cells are combined with vegetal cells many of the animal
cap cells form mesoderm. It is observed that a ventral signal is
released throughout the vegetal region of the embryo. The dorsal
vegetal cells of the blastula capable of inducing the organizer have
been called the Nieukoop center. It is observed that the dorsal most
vegetal cells i.e. the Nieukoop center is so special because of β-
catenin. It is a multifunctional protein that can act as transcription
factor. Initially it is synthesized throughout the egg but with the entry of
the sperm and fertilization, it gets accumulated in the vegetal region
during cytoplasmic movements.

12. • How β-catenin becomes localized in the side
opposite to sperm entry?
During fertilization there is translocation of Disheveled protein
from vegetal pole to the dorsal side.
Dorsalization of βcatenin
accomplished by
a) protecting βcatenin
in dorsal area,
b) degrading βcatenin
everywhere else.
Mechanism cortical
rotation …

13. • Signals for the axial determination
• The dorsal signal: β-catenin
• The vegetal TGF-β-like signal
• The mesodermal signal, Nodal-relating
proteins

14. • The vegetal cells are responsible for
causing the initiation of gastrulation
• Rescue by transplantation of dorsal vegetal
blastomeres which can induce another axis
in the embryo.

15. UV irradiated embryos

17. Localization of catenin in the dorsal portion.