So let us go back to your early stages of development.
Cell division takes place after fertilization
and in these early stages, there was no growth.
So every time the cells divide, they get smaller.
So from one cell to two cells, they are half
in size, at four cells they are a quarter
of the size and they go on dividing until
they've reached the normal size for a human cell.
As they do so, they form a ball and this is
called the morula and then that ball would
become hollow and this stage is called the
blastocyst. And the blastocyst is not a simple
hollow ball like a tennis ball. There is a
little cluster of cells when it is inside
and that little cluster is called the inner cell mass.
So the inner cell mass is marked in
the diagram. The outer cells are trophoblast
cells and this is not just the difference
in naming. Something extremely profound has
happened here because the baby that will be
born, will come from just the few of the inner
cell mass cells and all of the other cells
that are present including especially the
trophoblast cells will contribute to the extraembryonic
membranes such as the placenta. So this first
division into two different kinds of cells
is actually really important and therefore
understanding how this takes place might be
a valuable clue to understanding the process
of differentiation of one cell from another.
So looking at these stages again, two cells
to four cells, eight cells and an increasing
numbers all the way to the hollow ball of
cells, the blastocysts with the inner cell
mass pairs to one ball within the hollow
mass of the blastocyst. And an extremely clever
experiment by Prof. Chris Graham explained
how this may happen. What to do? Just to take
some labeled cells from an embryo which are most
[inaudible 00:10:46] and you took a morula and placed
the labeled cells in different positions within
the morula and what he found was that the
cells he placed in the center of the mass
will subsequently contribute to the inner
cell mass. In other words, they will be the one
that contributed to the baby. Cells that he
placed at the edge of the ball cells, will
contribute to the trophoblast. In other words,
they would contribute to the placenta.
And what this means is, the cells that responded
to their position by changing what they were
turned into. So our question of what informs the
DNA, in this case, turns out to be a positional
signal reflecting where our cell lies with
respect to all the other cells round about it.
So that key distinction, that important decision
about inner cell mass versus trophoblast
whether it's going to be part of the baby or part
of the placenta came from a positional signal
and that is a valuable clue to understanding the
processes that underlie all differentiations.