Sea Urchins and fertilisation - The chemistry of almost everything (28/31)


Uploaded by OUlearn on 03.09.2009

Transcript:
Michael Whitaker's been studying sea urchins since he was a student.
But it's not the urchins themselves he's interested in
as much as their sperm and eggs.
He believes there's a possibility
that chemistry plays a role in the first task that faces the sperm.
The distance the sperm has to swim to find the egg is a few millimetres
but nevertheless it has to swim in the right direction.
If it swims off into infinity then it's never going to achieve its end
which is to fertilise an egg.
To do that the sperm needs to be able to sense that the egg is there.
Probably there's a chemoattractant that the egg releases
and the sperm then senses that chemoattractant,
swims up the gradient and finds the egg at the end of the road.
In many ways sea urchin eggs and sperm are similar to those of humans.
Within the head of all sperm are special chemicals.
When contact is made there's a chemical reaction.
This chemical reaction is the key to getting the sperm into the egg.
The reaction makes the outer walls of the head of the sperm rupture
and powerful enzymes break out.
These enzymes eat into the cell wall, allowing the sperm to enter.
Chemistry also has a crucial role in explaining what happens next -
the tickle.
So having found the egg, another task that confronts the sperm
is then to activate this egg and it's more difficult than one might think
because the egg is enormous.
It's 100 micrometers in diameter,
the sperm, is very small, probably about a thousandth of that size.
So the question is how can this tiny sperm activate this enormous egg?
How does the egg know that the sperm is there,
it being so large and the sperm being so small?
It's the egg which has a method for self-activation, auto-activation.
There's a calcium wave that the egg has already programmed into it.
And the egg is very sensitive
and the egg is looking for the tickle from the sperm
that tells it that the sperm is there and it then explodes.
And that explosion is fertilisation, it's an explosion of calcium.
For several years researchers have believed
that metal ions play a crucial role in communication.
Whitaker and his colleagues have been developing techniques
to understand the role of calcium.
The egg is injected with chemical dyes
which when in contact with calcium are known to fluoresce.
Using this method researchers explore the role calcium ions play
in the egg's first few minutes of life.
The sperm tickles the egg with dramatic results.
It propagates a massive release of calcium ions across the cell.
This major change in the concentration of just one element
produces crucial changes in the egg.
It does a number of things and one of the things that it does
is to raise an envelope around the egg called a fertilisation envelope
and that's a protective envelope
that stops other sperm getting at the egg once it's fertilised
and also protects the egg from knocks and bangs while it's developing.
The embryo will hatch out of that envelope later in development.
This wave of calcium is also the starting gun for life.
The wave is the chemical signal for the egg
to trigger the process of creating a life.
Indeed the calcium wave is just as important as the sperm,
perhaps it's even more important.
There are some insects, for example,
that can develop parthenogenetically, there is no sperm,
just an egg that develops spontaneously within the insect
and it's a way of compressing generations.
You can make this happen artificially with sea urchins
if you treat the sea urchins with a calcium onophore
which is something that will cause that calcium explosion
without there being a sperm around, then the egg will go on to develop
and you'll get little sea urchins that don't have a father.