Overcrowding could damage the ovaries, new work on mice shows. Jessa Netting reports.
Whenpeople live cheek by jowl, moods sour and tempers flare. Mice react to cramped quarters in a similar way. But new research has uncovered a more sinister side effect of overpopulation on these masters of high-speed reproduction.
Female mice reacting to the stress of chronic overcrowding develop perplexing lesions on their ovaries, report researchers from Binghamton University in New York. Stress can disrupt or even halt ovulation in humans, but the state of the ovaries in these mice was a complete surprise.
When wild house mice were allowed to reproduce freely in the lab, the ovaries of two out of seven strains became covered with shapeless white masses of 'amyloid protein' fibres.
Similar amyloid fibrils create debilitating plaques in the brains of Alzheimer's patients. The study's originator, the late John J. Christian, had not seen such lesions in nearly fifty years of population research on mice.
This discovery is "probably highly applicable to stress and women's health," says Esther Sternberg, who studies the human hormonal stress response and disease susceptibility at the National Institute of Mental Health. She cautions that it is always difficult to generalize such research to humans but thinks this important finding should be investigated further.
The masses on the ovaries of the mice seemed to be leftovers of an incomplete clean-up where egg follicles had burst open to release an egg. Normally the ovulation site, or corpus luteum, produces hormones after ovulation. Then the immune cells swoop in from the arteries to clean up the broken cells and shrink the tissue.
But blood supply to the ovaries of overcrowded mice was choked off by amyloid fibrils. "There was absolutely no [blood] circulation, so we think the immune cells couldn't get to them to do their job," says biochemist John Chapman. He and his colleagues, who took over the experiment after Christian's death, report their findings in the Proceedings of the Society for Experimental Biology and Medicine1.
The researchers think that the stress hormone corticosterone may be to blame for initially suppressing the action of the immune cells. The hormone is known to depress other parts of the immune system and its levels had soared in the female mice. In fact, all of their hormone levels were seriously out of kilter, creating a chilling social atmosphere in the colonies.
Testosterone levels in the females were six to seven times higher than normal. Females stopped mating and became extremely aggressive, severely wounding the males, and in one colony, killing them all. Despite the breakdown of social structure, most of the females continued to ovulate and their uteruses bore the scars of repeated pregnancies that had been resorbed or aborted.
As for the amyloid deposits, their origin is still mysterious. The authors suggest that chronic social strife and repeated pregnancies were an ingredient in their development and would next like to test whether the mice are left infertile. They think that the lesioned ovaries could at least serve as a model of amyloid formation in diseases like Alzheimer's.
Chapman,J. C., Christian, J. J., Pawlikowski, M., Yasukawa, N. & Michael, S. Female house mice develop a unique ovarian lesion in colonies that are at maximum population density. Proceedings of the Society for Experimental Biology and Medicine 225, 80 - 89 2000.
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Netting, J. Cramped conditions create infertility. Nature (2000). https://doi.org/10.1038/news001102-6