The mystery behind the continuously ticking human biological clock seems to have been solved by biologists from the University of Pennsylvania.
Richard Schultz and Michael Lampson showed that this basic fact of reproductive life is most likely caused by weakened chromosome cohesion. It causes an increase in aneuploidy, the term for an abnormal number of chromosomes during reproductive cell division.
With age, egg cells experience decreasing amounts of a protein, REC8 that is essential for chromosomes to segregate correctly during the process that forms an egg.
Because cohesion in these cells is established during foetal development, and must remain functional up to 50 years or so in humans, defective cohesion is a good candidate for a process that might fail with increasing maternal age.
During these studies, the scientists noticed that sister kinetochores - the protein structures that mark the site where a chromosome pair is split during cell division - are farther apart in metaphase II eggs from older mice at 16 to 19 months of age compared to eggs from young mice of 6 to 14 weeks of age, a finding that drew their attention to explore reduced cohesion as a primary source for age-related aneuploidy.
The study appeared in the journal Current Biology.