Fate maps are the bases for experimental embryology since they provide researchers with information on which portions of the embryo normally become which larval or adult structures.

1. Cell fate (Fate maps)
Fate maps are the bases for experimental
embryology, since they provide researchers with
information on which portions of the embryo
normally become which larval or adult structures.

2.  By the late 1800s, the cell had been conclusively
demonstrated to be the basis for anatomy and physiology.
 Embryologists, too, began to base their field on the cell.
 One of the most important programs of descriptive
embryology
became the tracing of cell lineages: following individual
cells to see what they become.
 In many organisms, this fine a resolution is not possible,
but one can label groups of cells to see what that area of
the embryo will become.
 By bringing such studies together, one can construct a fate
map.
 These diagrams "map" the larval or adult structure onto
the region of the embryo from which it arose.

3. Fate maps of
some embryos
at the early
gastrula stage
are shown in
Figure 1.6.
Fate maps
have been
generated in
several ways

4. OBSERVING LIVING EMBRYOS
 The embryos of certain invertebrates are transparent, have
relatively few cells, and the daughter cells remain close to
one another.
 In such cases, it is actually possible to look through the
microscope and trace the descendants of a particular cell into
the organs they generate.
 This type of study was performed about a century ago by
Edwin G. Conklin.
 In one of these studies, he took eggs of the tunicate Styela
partita, a sea squirt that resides in the waters off the coast of
Massachusetts, and patiently followed the fates of every cell
in the embryo until each differentiated into particular
structures
Figure 1 . 7; Conklin 1905).

7.  He was helped in this endeavor by a
peculiarity of the StyeIa egg, wherein the
different cells contain different pigments.
 For example, the muscle-forming cells
always had a yellow color.
 Conklin's fate map was confirmed by cell
removal experiments.
 Removal of the 54.1 cell (which according to
the map should produce all the tail
musculature), for example, resulted in a larva
with no tail muscles (Reverberi and Minganti
1946).

8. VITAL DYE MARKING
 Most embryos are not so accommodating as to have cells
of different colors.
 Nor do all embryos have as few celIs as tunicates.
 In the early years of the twentieth century, Vogt (1929)
traced the fates of different areas of amphibian eggs by
applying
vital dyes to the region of interest.
 Vital dyes will stain cells but not kill them.
 Vogt mixed the dye with agar and spread the agar on a
microscope
slide to dry.
 The ends of the dyed agar were very thin.
 He cut chips from these ends and placed them onto a frog
embryo.
 After the dye stained the cells, the agar chip was removed
and
cell movements within the embryo could be followed
(Figure 1 .8)

10. RADIOACTlVE LABEL1NG AND
FLUORESCENT DYES
 � A variant of the dye-marking technique is
to make one area of the embryo radioactive.
 To do this, a donor embryo is usually grown
in a
solution containing radioactive thymidine.
 This base becomes incorporated into the
DNA of the dividing embryo.
 A second embryo (the host embryo) is grown
under normal conditions.
 The region of interest is cut out from the
host embryo and is replaced by a radioactive
graft from the donor

11.  One of the problems wiith both vital dyes and radioactive
labels is that, as they become more diluted with each cell
division, they become difficult to detect.
 One way around this problem is the use of fluorescent
dyes that are so intense that Once injected into individual
cells, they can still be detected in the progeny of these
cells many divisions later.
 Fluorescein-conjugated dextran, for exampIe, can be
injected into a single cell of an early embryo, and the
descendants of that cell can be seen by examining the
embryo under ultraviolet light (Figure 1 .9).
 More recently, diI, a powerfully fluorescent molecule that
becomes incorporated into lipid membranes, has also
been used to follow the fates of cells and their progeny

13. GENETIC MARKING
One permanent way of marking cells and
following their fates is to create "mosaic"
embryos in which the same organism
contains cells with different genetic
constitutions.

14.  One of the best examples of tl1is technique is the
construction of chimeric embryos/ consisting, for
example, of a graft of quail cells inside a chick
embryo.
 Chicks and quail develop in a very similar manner
(especially during early embryonic development),
and the grafted quail cells become integrated into
the chick embryo and participate in the construction
of the various organs.
 The substitution of quail cells for chick cells can be
performed on an embryo while it is still inside the
egg, and the chick that hatches will have quail cells
in particular sites, depending upon where the graft
was placed.


15. Quail cells differ from chick cells in two important
ways.
 First, the quail nucleus has condensed DNA
(heterochromatin) concentrated around the
nucleoli, making quail nuclei easily distinguishable
from chick nuclei.
 Second, cell-specfic antigens that are quail-
specific can be used to find individual quail cells,
even if they are "hidden" within a large population
of chick cells.
 In this way, fine-structure maps of the chick brain
and skeletal system have been produced (Figure
1.10; Le Douarin 1969; i.e Douarin and Teillet
1973).

17. Reference
 Developmental Biology by Scott F
Gilbert and Susan R Singer
 (Eighth Edition)
 Page no: 10-13