- Cell migration is key to embryo development and formation of germ layers. The blastula possesses polarity and symmetry, with distinct cell areas that give rise to different embryo parts. - Tracking migrating cells helps map the locations of ectoderm, mesoderm and endoderm, aiding understanding of early embryo patterning. This process of showing each blastula part's fate is called a "fate map". - Fate maps are constructed using vital stains or carbon particles to label and track cells, revealing cell movements and ultimate tissues formed. This clarifies gastrulation and represents specific blastula areas' destinies.

2. » Embryogenesis is the process by
which the embryo forms and
develops.
» During the course of development,
cells divide, migrate, and specialize.
» Embryogenesis starts with
the fertilization of the egg cell by
a sperm cell, (spermatozoon). Once
fertilized, the ovum is referred to as
a zygote. The zygote
undergoes mitotic divisions with no
significant growth and cellular
differentiation, leading to
development of a multicellular
embryo.

3. » Post-fertilization events refers to all the events of sexual
reproduction taking place after fertilization, that is, after the
formation of zygote.
» The major event taking place in this stage is embryo-genesis,
i.e., development of the embryo.
» Post fertilization basically involves :
i. Early embryonic development,
ii. Establishing multi-cellularity,
iii. Formation of blastula,
iv. Formation of embryonic germ layer.

4. After fertilization, cleavage occurs. Cleavage is a series of mitotic
division which results in formation of smaller cells, called
blastomeres.
The rate of mitotic division slows down, the blastomeres then
undergoes movements wherein they change their position relative
to one another. This is known as gastrulation which involves
formation of the three germ layers, i.e., The ectoderm, the
endoderm and the mesoderm.
Once the three germ layers are established, the cells interact to
with one another and rearrange themselves to form tissues and
organs. This is known as organogenesis.

5. » Gastrulation is the process of highly
coordinated cell and tissue movements
whereby the cells of the blastula are
dramatically rearranged.
» During gastrulation, these cells are
given new positions and the
multilayered body plan of the
organism is established.
» The cells that will form the
endodermal and mesodermal organs
are brought inside the embryo, while
the cells that will form skin and
nervous system are spread over its
outside surface.
» Thus, three germ layers, i.e.,
ectoderm, endoderm and mesoderm
are formed during the gastrulation.
» Gastrulation involves some
combination of different type of
movements :

6. » At a very early stage in development of embryo, the embryo
acquires a three layered disc form called “The embryonic disc”.
» The three layers that constitute this embryonic disc are :
ectoderm, mesoderm, and endoderm.

7. » The blastocyst is a spherical cyst
that is lined by flattened
trophoblast cells, and inside it
there is a mass of cells, the inner
cell mass.
Some of the inner cell mass
differentiate into flattened cells,
that come to line its free surface.
This constitute the Endoderm.
The remaining cells of the inner
cell mass become columnar and
form the second germ layer, the
Ectoderm.

8. A space appears between the
ectoderm and trophoblast, called
amniotic cavity, filled with amniotic
fluid.
The flattened cells that arises from
the endoderm spreads and line the
inside of the blastocystic cavity. This
lining is called Heuser’s membrane.
In this a way cavity is formed called
the primary yolk sac.
The cells of the trophoblast give rise
to a mass of cells called extra
embryonic mesoderm.

9. Small cavities appear in the extra
embryonic mesoderm. Gradually, these
cavities join together to form larger
cavity, i.e., chorionic cavity (extra
embryonic coelom). This caused the
extra embryonic mesoderm to split into
two layers, i.e., splanchopleuric and
somatopleuric extra embryonic
mesoderm.
The developing embryo is now
suspended into the extra embryonic
coelom and it is attached to the
blastocyst by connecting stalk.
At this stage, two important
membranes are formed, i.e., chorion
and amnion.

10. With the appearance of extra embryonic
mesoderm and later of the extra embryonic
coelom, the yolk sac becomes smaller in size
and is called secondary yolk sac. Also there
is a change in the nature of lining cells, they
are no longer flattened but become cubical.
The cells of the endoderm then form the
prochordal plate which determines the
central axis of the embryo and enables to
distinguish between head end and tail end.
After the formation of prochordal plate, the
ectodermal cells begin to proliferate, and
form primitive streak. The cells that
proliferate in this region pass sideways and
form intra embryonic mesoderm or
secondary mesoderm.

13. » Cell migration is a central process in the development and maintenance of
multicellular organisms.
» The blastula possesses polarity and bilateral symmetry. It contains cell
areas, which becomes germ layer and give rise to different parts of the
embryo.
» Tracking of these cells in blastula is used to denote/study which specific
region will become which part of the embryo.
» Tracking the migrating cells helps to map the location of ectoderm,
mesoderm and endoderm.
» It helps to understand the patterning of the early embryo as this plays a key
role in ensuring the correct development of the embryo.
» This process showing the fate of each part of blastula is called as a “FATE
MAP”.

14. Fate map can be constructed by following methods :
A. By using vital stains
B. By using carbon particles
A. BY VITAL STAINS :
» This method was devised by Walter
Vogt in 1929.
» It consists of soaking a piece of agar
in a vital dye (Nile blue sulphate,
neural red, bismark brown), and then
applying the piece of agar to the
surface of the embryo in necessary
position,
» Dye diffuses from agar and the cells
of the embryo to which the agar was
applied takes up the sufficient dye to
produce a stain on the surface of the
embryo.
» Once the cells were effectively
labeled, the agar chip could be
removed and the embryo was
allowed to develop normally.
» With this method, it was possible to
discern movements of particular cell
populations and the ultimate organ or
tissue into which they integrated.
» Recently, cellophane has been used
as stain carrier instead of agar.

16. B. BY CARBON PARTICLES :
» The cells of the developing
embryo can be marked by
another method independent of
vital stains, i.e., by carbon
particles.
» This method was developed by
N. Spratt in 1946.
» In this method, tiny particles
of carbon are applied to the
surface of the embryo.
» These particles stick to the
surface of the embryo and
hence can be used as markers
enabling to track the
movement of the cells and
draw the fate maps.
» Radioactive labelling of the
blastomeres is the most
technique used.
» Such techniques helps in clear
understanding of gastrulation
and representation of the
destiny of the particular area of
the blastula.