Extraits de : « The first two weeks » puis de : « The embryo takes shape »

http://catholiquesdu.free.fr/CORPUS/VIDEO/nova2816_04_56[1].mov

“ 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 self-

defense, 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.”

“A heart begins to beat”

“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.”

“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 ?”