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Extraits de : « The first two weeks »
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http://catholiquesdu.free.fr/CORPUS/VIDEO/nova2816_04_56[1].mov
“Once sperm and egg get together ...
to create a viable embryo”
“If it’s going to survive, the egg has a lot
of work to do
“First, it orders the zona to lock out all
other sperm”
“The single sperm already inside
releases its precious cargo”
“And then the egg must finish meiosis
expelling half of its chromosomes into
this tiny pouch, called a polar body”
“The sperm’s 23 chromosomes
stretch out in the roomy, welcoming egg”
“ The chromosomes of sperm and egg approach each other,
and then…”
“… the cell divides”
“Since the moment the sperm entered the egg, 24 hours have passed.
All this time, the fertilized egg is moving down the Fallopian tube toward
the uterus. Every few hours, the cells divide: 4, 8, 16… gradually
creating the building blocks needed to construct an embryo.”
“On rare occasions, the tiny
cluster of cells splits into two
groups and creates two
embryos, identical twins. But
most of the time, the cells stick
together.”
“They must complete just the
right number of cell divisions
before they arrive in the uterus,
about five days after
fertilization.”
“What started as a large, single cell has divided into just over a
hundred much smaller cells. But they are still trapped within the
hard shell of the zona.”
“Now called a blastocyst,
the bundle of cells must do two things to survive:
break out of the zona and find a source of nourishment.”
“At the beginning of the sixth
day, it orchestrates an
escape. It releases an
enzyme that eats through the
zona, and the ball of cells
squeezes out.”
“Free at last, the blastocyst
lands on the blood-rich lining
of the mother’s uterus.”
“It has just passed one hurdle but is immediately presented with
another. For, in fact, it is now in very grave danger.
Stripped of its protective coating, the blastocyst could be
attacked by the mother’s immune system as a foreign invader.
White blood cells would swarm in to devour it. In its own selfdefense, the ball of cells produces several chemicals that
suppress the mother’s immune system inside the uterus, in
effect, convincing the mother to treat it like a welcome guest.”
“Then it is free to get to work.
Searching for food and
oxygen, cells from the
blastocyst reach down and
burrow into the surrounding
tissue. Eventually, they pull
the entire bundle down into
the uterine lining…
“One milestone event takes place just two weeks after conception,
when the blastocyst is about the size of a poppy seed. This is the
moment when the cells start to organize themselves into an embryo.”
“The process is called gastrulation.”
“Gastrulation happens deep inside the mother’s uterine
lining, so it can’t be photographed.”
“This is a human embryo three
weeks after fertilization. Less
than a tenth of an inch long, its
neural tube, the beginning of
the nervous system, is already
in place.”
“A couple of days later, the top
of the tube is bulging outwards,
on its way to becoming a
brain.”
“With the primitive brain cells
exposed, we can see some are
sending feelers, making
connections to their neighbors.”
“As the days pass, changes proceed at a rapid-fire pace
throughout the embryo. Everywhere, cells are multiplying. And
they’re on the move.”
“Some reach out to one another,
forming blood vessels.”
“A heart begins to beat”
“As the embryo lengthens, the
precursor to the backbone forms.”
“Groups of cells bulge out on the sides:
the beginning of arms and legs.”
“This is the embryo 4-1/2 weeks
after fertilization. It is only about a
fifth of an inch long.”
“A large brain is developing, and on
the side of the head, an eye.”
“How does this happen ?
“How does the embryo
transform itself from a
blob of cells in different
tissues and organs and
finally into a fully
functional baby ?”