ALL sperm are not created equal.

Some contain abnormal numbers

of chromosomes which can trigger

miscarriages or lead to conditions

such as Down’s syndrome if they

manage to fertilise an egg.

Healthy sperm usually

outcompete abnormal sperm

in the arduous race to the egg,

but an increase in the use of

IVF techniques such as

intracytoplasmic sperm injection

(ICSI), in which a sperm is injected

into an egg, has improved their

chances. “The presence of a single

sperm makes ICSI possible,” says

Ulrik Kvist at the Karolinska

Institute’s Andrology Centre in

Stockholm, Sweden. It’s a short

cut to the egg, so better methods

are needed to screen out the bad

sperm from the good, he says.

Now Myung-Geol Pang at

Chung-Ang University in

Gyeonggi-Do, South Korea, and

his colleagues may have found

one. At present, fertility clinics

rely on the way sperm look to

select healthy ones for ICSI, which

is a particularly useful method for

men with very low sperm counts.

However, these men are also at

greater risk of producing

“aneuploid” sperm with the wrong

number of chromosomes: while

around 10 per cent of sperm from

a normal male are aneuploid, that

figure is closer to 70 per cent for

men with low sperm counts.

Eggs can be assessed relatively

easily by staining and counting the

number of chromosomes in the

polar body – a by-product of egg

development. In contrast, staining

a sperm’s chromosomes usually

damages them. To get around this

problem, Pang combined

fluorescence in situ hybridisation

(FISH), in which fluorescent DNA

labels are used to identify whether

chromosomes are missing, with

the hypo-osmotic swelling test

(HOST), used to assess whether

sperm are alive. When sperm are

put into a solution that is more

dilute than the fluid inside them,

water moves into the sperm cell,

causing it to swell up. Pang

wondered whether the swelling

would look different in healthy

sperm and those containing

abnormal numbers of

chromosomes. His team analysed

more than 16,000 sperm from

three fertile men and six with low

sperm counts, and found that it

did (see Diagram, below).

When the method was used to

select sperm from men with low

sperm counts, there was a 20-fold

decrease in the frequency of

aneuploidy in the selected sperm

compared with when no selection

was used. “This is much lower

than the frequency of aneuploidy

in sperm taken from healthy men,”

says Pang, who presented his

results at the European Society

for Human Reproduction and

Embryology meeting in Lyon,

France, earlier this month. He now

hopes to test whether the method

results in healthier embryos.

“If it works it would potentially

be very beneficial,” says Alan

Handyside of the Bridge Fertility

Centre, London. Linda Geddes, Lyon ●

Swell test can weed out the duff sperm

“The method produced a 20-fold decrease in the frequency of sperm with abnormal numbers of chromosomes”

SOUNDBITES

‹ The evidence was complete and the death sentence was appropriate.›

A Chinese court justifies the execution of

Zheng Xiaoyu, ex-director of the state

food and drug administration, found

guilty of taking bribes to authorise sub-

standard medicines that led to several

deaths (Xinhua news agency, 10 July)

‹ [It] would pretty much slay any curator or librarian to have such a large error bar, but astronomy often has high numbers: 100,000 is OK.›

Alison Doane, curator of the collection

of glass photographic plates at Harvard

College Observatory that contain

100 years’ worth of images of the sky,

on how the observatory might have

500,000 plates, give or take 100,000

(The New York Times, 10 July)

‹ Pitching is incredibly hard. I think I’ll stick to physics.›

German physicist Sebastian Martin

on how difficult it is to pitch a baseball.

A team at San Francisco’s Exploratorium

that included a physicist and a former

baseball player tried to teach Martin the

art and science of pitching (The San Francisco Chronicle, 9 July)

‹ I have often imagined the streets of Tucson or Phoenix as abandoned, and it’s a bit scary.›

Climate scientist Julie Cole of the

University of Arizona, drawing parallels

between the current dry conditions in

the American Southwest and droughts

during the 12th and 13th centuries that

led to the downfall of the thriving

Anasazi civilisation in New Mexico

(National Public Radio, 9 July)

‹ This will be a period of cold turkey for him. He has become addicted to human laughter and applause.›

A staff member at Berlin zoo, home of

Knut, the 7-month-old polar bear that

has become the poster child for climate

change, on how he is getting cranky

and needs to be weaned away from

humans (The Times, London, 10 July)

www.newscientist.com 14 July 2007 | NewScientist | 13

SELECTING THE BEST SPERM

The number of chromosomes a sperm contains affects how it responds to the “hypo-osmotic

swelling test”

DEAD SPERM HEALTHY SPERM

correct number of chromosomes

ABNORMAL SPERM

wrong number of chromosomes

–Appearances can be deceptive–

CNR

I/SP

L

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