This is how a human gets made.
Every one of us started out this way.
We have a pretty good idea
what happens over the 9 months
it takes to produce a newborn,
but there’s this one bit
that's been missing.
Right at the beginning,
before this animation even starts -
the very first few weeks.
Now, armed with new ways of
growing human embryos in the lab,
scientists are learning
what the very beginning of
human development looks like.
For decades,
studying this crucial, early time
in a human embryo’s development
was a technical headache -
embryos are hard to get
and keep alive outside the body.
And it's an ethical challenge.
Guidelines put together in
the late 70s and 80s
prevent scientists from growing
embryos for longer than 14 days.
Despite the hurdles,
scientists do know a bit
about what happens in
those critical weeks -
often from animal studies
or rare human tissue samples.
They know that sperm
fertilises egg,
and one cell grows to two,
four, eight, and so on.
Around day 5 or 6,
the blob starts to feature
different types of cell.
Scientists call this blob
a blastocyst.
All being well, the cells
in the blastocyst
begin to differentiate.
One week in,
it implants into
the wall of the uterus.
What happens next
has been a bit of a mystery:
by implanting itself
into the womb’s wall,
the embryo basically hides,
and that makes studying it
in humans impossible.
One way of exploring
this crucial time,
is to study embryos
donated by people who
no longer need them
for fertility treatment.
In the last few years,
several labs have developed
new ways to
nurture these embryos.
Their techniques have allowed
them to start building a
picture of human development
that's more detailed
than ever before.
Here’s a day 6 embryo
growing in a lab.
Cells destined to become
the actual fetus
are tagged in green,
those that go on to form
the placenta in blue.
By day 8 the cells are
sorting and arranging themselves -
the green fetal cells have
condensed together,
and in red here are the cells
that will form the interface
between the baby
and the placenta.
It seems these ‘extra-embryonic’
structures develop a little later
in humans than expected
from studies in other animals.
Day 10. The whole thing
increases in size,
and bundles of cells start
developing into support structures.
Surprisingly at this stage,
the embryo can direct
its own development,
with no input from
the mother's tissues.
By day 12, the outskirts
of the embryo are preparing
to bind more strongly
to the wall of the uterus.
After all, if this were
a natural pregnancy,
the embryo would be there
for the next 9 months.
You can even see
little holes appearing
ready for the mother's
blood vessels to start
supplying the embryo with
essential oxygen and nutrients.
By now the embryo
is signalling its presence
to the mother via
a hormone – the one that
pregnancy tests pick up - HCG -
labelled here in yellow.
After two weeks,
the teams ended
their experiments in line with
the ethical 14-day limit.
To study what happens
after 14 days,
researchers had to turn
to different techniques.
Recently scientists have
built artificial, embryo-like
structures from stem cells.
Using these partial models,
they can study things like
cell signalling, or even
the formation of
the primitive streak -
the crucial thread of cells
which guides the process
called gastrulation.
That's the moment
the embryo decides which
end will become the head.
Here they use
human stem cells growing
in an animal embryo
to explore that process.
But even after this
early phase is over,
there is still a lot of work
involved in building a body.
To study later phases,
scientists have made and
analysed high-resolution,
3D atlases of human
embryos and fetuses.
One team found that
the left and rights hands
don't simply mirror each other
when growing their nerves.
Instead, some branches take
random paths in each limb.
Another group saw muscles that
grew in early embryos,
only to disappear as the
fetus developed - like these.
As yet, they're not sure why.
It's becoming ever clearer
just how important a
human embryo’s first few weeks are.
Many scientists hope that
more research will lead
to a better grasp of why
some pregnancies fail,
and how birth defects arise.
Maybe even make
in vitro fertilisation work better.
But some just want to
understand exactly what happens
to this tiny ball of cells,
in this short spark of time
at the beginning of all of us.
