High-resolution imaging shows why the union between two sets of chromosomes goes awry as least as often as not.
After a sperm fertilizes an egg, the chromosomes of both unite into a single genome, if all goes according to plan. Now, observations of developing embryos show that this all-important process often goes awry — a finding that helps to explain why at least half of newly formed human embryos have the wrong number of chromosomes.
Some 50–70% of embryos have aneuploidy, an abnormal number of chromosomes. Such embryos are often miscarried.
To find out why aneuploidy is so common, Melina Schuh at the Max Planck Institute for Biophysical Chemistry in Göttigen, Germany, and her collaborators used high-resolution microscopes to observe the early stages of human and cow development. They discovered a crucial step: maternal and paternal genomes, which start off enclosed in their own structures, cluster around the location where they will fuse.
This clustering enables rapid and error-free unification of the two genomes. But this complex process has many steps that can go wrong. Failures lead to aneuploidy and fragments of nuclei containing subsets of chromosomes — both of which impair the development of healthy embryos.