lifelines: Sperm marks the spot
JOHN WHITFIELD
A sperm can enter an egg almost
anywhere on its surface. And yet, in mice,its point of
entry seems to determine how the cells of the early
embryo divide and arrange themselves, researchers at
the University of Cambridge in England now report1. It
may even influence the layout of the finished human
body.
Sperm are known to affect the
pattern of development in many invertebrates,including
worms and molluscs, and also frogs. But it had been
thought that mammalian embryos did not begin to
organize themselves until later.
"This role for sperm was
unexpected, even though it is true for other systems,"
says Magdalena Zernicka-Goetz, who carried out the
research with Karolina Piotrowska.
A fertilized mouse egg's first
division occurs along a plane that includes the
sperm's point of entry. And at the second cell
division, the cell that inherits this point usually
divides before its sister cell. The plane of division
and the order of division may decide where these
cells' descendants end up and what jobs they do, says
Zernicka-Goetz.
Slightly later in development,
the embryo is a ball of cells — a blastocyst. Some of
the blastocyst's cells will form the fetus while
others will form tissues to nourish and protect the
fetus and will be lost later.
In most blastocysts, the sperm's
point of entry lies along the equator that separates
these two cell types, Piotrowska and Zernicka-Goetz
have found.
Some researchers also believe
that this axis also demarcates the front and back of
the finished animal.
The sperm's point of entry is
visible only briefly on the egg's membrane. The
researchers marked the spot with microscopic
fluorescent beads.
Richard Gardner, a developmental
biologist at the University of Oxford, UK, who has
found similar results in his own experiments2 points
out that the sperm entry point does not lie on the
equator between the two cell types insome blastocysts,
describing Plotrowska and Zernicka-Goetz's results as
"suggestive, but not conclusive".
It is not clear how the sperm's
entry point might influence development — it may have
effects on the cell's molecular skeleton — or when,
where and what genes are activated. "It'll take the
next few years to identify the molecular mechanisms
behind these changes," Zernicka-Goetz says.
But work such as this at least
shows that not everything is written in the genes.
"The genome is meaningful only in context of a living
cell," says Roger Pedersen, a developmental biologist
at the University of California in San Francisco.
"Classical developmental biologists knew this, and we
have been distracted from it by our focus on DNA for
the past few decades.
"Now, we can bring the awesome
power of genomics to bear on the issue. I think this
is a wonderful example of post-genomic biology."
1.Piotrowska, K. &
Zernicka-Goetz, M. Role for sperm in spatial
patterning
of the early mouse embryo.
Nature 409, 517–521 (2001).
2.Gardner, R. L. Specification
of embryonic axes begins before cleavage in
normal mouse development.
Development [in the press].
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This article made me think about cloning.
Another can of worms?
Noemi
London, UK
http://www.noemi.freeserve.co.uk
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