After age 35, it is well-established that fertility tends to dip for most women, and now UK scientists think they have pinpointed the reason why. The key to so-called "reproductive aging," according to a study published September 2, 2010, in the journal Current Biology, is naturally declining levels of cohesins, a type of protein that acts like a glue to hold chromosomes together.
As a article on the study in the Independent newspaper explains, natural loss of these proteins over time causes the chromosomes to split unevenly when eggs are being formed. (To create eggs suitable for reproduction, the chromosomes must be halved equally.) The problem appears further compounded by the fact that the physical attachments holding the chromosomes together are established before a woman's own birth and must be maintained by cohesins until the egg is created by cell division just before ovulation.
To demonstrate this theory, researchers used mice that were the age equivalent to a woman in her early 40s. "Cohesin levels were very much reduced in eggs from older mice and the chromosomes underwent a messy division, resulting in the wrong number of chromosomes being retained in the egg," Mary Herbert, lead researcher, says in an interview with the Independent.
The findings may also help explain why older women are also at greater risk to have a baby with Down syndrome. Cohesin levels also drop in men, but sperm production appears to compensate for lowered levels of the protein.
Can you do anything to boost cohesin levels? Not yet. According to Herbert (via the Independent), "We are at the stage of saying how the engine works and what is broken. The next stage is answering the question: can it be fixed? Could you add cohesin that would do the job [of holding the chromosomes together]? A lot of effort is going into answering that."