Building a baby: The first two weeks

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


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