National Geographic: The Incredible Human Body Page #3

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and now it continues

in your offspring.

Narrator:
Overnight, DNA from Inez's eggs

and Darryl's sperm unite,

and 13 of the 27 eggs

show the telltale dimple

that indicates success.

It's working.

With exquisite grace,

one cell becomes two,

two become four; each duplicates

the original, unique DNA.

The enchanted progression

of cell division continues.

For five days,

the embryos are monitored.

Finally, the division

creates masses of cells,

known as blastocysts,

and any one

of these blastocysts

may become a part

of the Pearson family.

Prosser:
This embryo here...

If you look at the outer shell

on the center embryo,

it's very thin.

The embryo is getting ready

to hatch out of it's shell.

It's a very nice blastocyst.

The inner cell mass is going

to become the embryo itself -

what you normally think of

when you think of a baby...

Arms, head, legs, toes, fingers.

And, actually,

this inner cell mass

is where you find

the embryonic stem cells,

which are very much in the

center of the genetic revolution

that's going on right now.

Narrator:
Embryonic stem cells stand

in the vanguard of human life.

These magical

all-purpose cells

will eventually transform into

every cell type in the human body.

This extraordinary potential

of stem cells

has made isolating them one

of the holy grails of science,

Although a controversial one.

Dr. John Gearhart is at the

forefront of that achievement.

Gearhart:
These cells

have two properties.

One is that if you

keep them in the dish,

under certain

culture conditions,

they will continue to form

more cells like themselves.

So you can grow a room full

of these embryonic stem cells

and they are undifferentiated

cells; they all look alike.

If you take some

of these cells, though,

and you put them out

in different kinds

of growth conditions,

these cells are capable

of forming all the cell types

that are present

in the human body.

What we are looking at here

are heart muscle cells

that are beating as a tissue.

Narrator:
Once these were stem cells

with uncharted destinies.

Dr. Gearhart has directed their

development into heart cells,

now able to beat

in perfect synchrony.

Gearhart:
It's always been

the dream of humankind

that someday we'd be able

to replace tissues in the body

that were either damaged

or diseased or simply worn out.

But we really never had the

starting material to do this.

Now we have in the laboratory,

in our dishes, growing nicely,

virtually all the cell types

that are present

in the human body.

Coffey:
They make a "you."

A stem cell can make you.

That's pretty powerful!

And I can

control this stem cell

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