this is how a human gets made every one
of us started out this way with a pretty
good idea what happens over the nine
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
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for decades studying this crucial early
time in a human embryos 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 fertilizes
egg and one cell grows to two four eight
and so on around day five or six 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 weekend it implants into the wall of
the uterus what happens next has been a
bit of a mystery by implanting itself
into the wombs 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 day6 embryo growing in a lab
cells destined to become the actual
fetus attacked in green those that go on
to form the placenta in blue by day
eight the cells are sourcing and
arranging themselves the green fetal
cells have condensed together and in red
here are 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 ten 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 nine 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 signaling its presence to
the mother via a hormone the one that
pregnancy tests pick up HCG labeled here
in yellow after two weeks the team's
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 signaling or even the
formation of the primitive streak the
crucial thread of cells which guides a
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 analyzed high-resolution 3d atlases
of human embryos and fetuses one team
found that the left and right 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
to human embryos 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
fertilization 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
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