Hello, and welcome to this lecture on the first
week of human development, the pre-embryo stage.
We'll be looking at the events from
fertilization through to implantation.
The first stage is on the fascinating journey
of the human from egg to adult. The outcomes
we’ll look at include looking at the early
division stages, the cleavage stages as early
parts of development and then we’ll look
at implantation into the uterine wall, the
uterus of the mother. We will touch briefly
on one of the things that can go wrong, which
is an ectopic pregnancy, and we’ll also look
briefly at some reproductive technologies.
So, let’s begin at the beginning. Here,
we have a newly fertilized egg.
Fertilization is covered in a separate lecture in this
series. Now, this egg is large as human cells go,
it is about 80 microns in diameter, about
eight to ten times the size of a normal adult cell.
When division takes place at these early
stages, the cell is half in size at each division.
So the two-cell stage is half the size of the
egg itself, the four-cell stage is a quarter
of the size, and so on until the cells reach
something like the normal adult size.
It’s important to understand that there’s no growth
taking place at this time and the developing
egg and the cells are surrounded by a protein
coat called the zona pellucida. It’s an
acellular, semi-rigid material surrounding
the egg, and all of the divisions take place
at these early stages inside the zona. At these
stages, all the cells are still equivalent.
So if I took a four-cell embryo and I divided
up the cells into four individual cells, each
one of them will be capable of giving rise
to a full normal individual. They would be
clones of each other, but at this stage, all
of the cells are equivalent to each other.
This is the same process, but here, illustrated
by photographs from left to right. Here, you
can clearly see the zona pellucida round the
outside and the cells inside as division takes
place, getting smaller at each division. After
they formed a ball of cells, they become very
compact and it becomes harder to see the individual
cell boundaries, as you can see on the extreme
edge of the picture. This ball of cells is
actually becoming hollow and this stage is
called the blastocyst.
So, looking at this in the slightly larger view,
you can see the blastocyst with a hollow
space inside, but you can also see that there’s
a concentration of cells which are forming
inside the ball of cells. So the ball of cells
as a whole is called the blastocyst, and that
little group of cells on the inside is quite
sensibly called the inner cell mass.
Now, the interesting thing about the inner cell
mass is it is the inner cell mass that will
give rise to the baby that’s born. So only
a few of those cells are actually going to
give rise to the baby, and the other cells will
contribute to the extra-embryonic membranes
such as the placenta, and that’s also covered
in another lecture in this series.
The cells which form the shell of the blastocyst
are called trophoblast cells. So these form an
outer layer with the inner cell mass on the
inside. Now, these actually look different
from each other, but this difference is highly
significant. The cells are divided into two
different sorts for the first time. Now, you
can recognize trophoblast cells from inner
cell-mass cells. What will happen to these cells
is very different. So these early distinctions
between inner cell mass and trophoblast, as
we’ll see, is highly significant.
It’s a very important moment in your life as a whole.