Women curious about what's happening inside their breasts after they're done breastfeeding might be astonished to learn about this latest finding. It turns out that as breasts transition from being full-time, milk-producing factories into more dormant organs, milk-secreting cells morph into cannibalistic cellular eaters that begin consuming each other.

It sounds considerably more apocalyptic than it actually is. This cannibalistic behavior appears to be a boon to female health, helping to safely clear the debris of dying cells and leftover milk that comes in the aftermath of weaning a child. In fact, by better understanding this process, scientists hope to glean new insights into the treatment of breast cancer.

“One of the least understood aspects of this process is how the excess milk and large numbers of dead cells are removed from the mammary gland without substantial activation of the immune system,” said Matthew Naylor, a cancer biologist at the University of Sydney in Australia, to New Scientist.

Dead or dying cells are usually cleared from the body by immune cells through a process called phagocytosis, but immune involvement often means inflammation. If phagocytosis played a large role in the breasts, this would likely result in substantial pain and swelling for women, especially considering the amount of tissue that requires removal after breastfeeding ceases. Excess inflammation could also be a risk factor for developing breast cancer.

For the study, Nasreen Akhtar and colleagues at the University of Sheffield deleted a particular gene, Rac1, in lab mice. The gene is known to play a role in both normal milk production in breast cells and in phagocytosis in immune cells, making it a curious candidate for research.

It didn't take long for dead cells and milk to flood the breasts of the mice, causing swelling and a state of chronic inflammation. This ultimately impaired the ability of the animals to regenerate their tissue, thus reducing milk production in future pregnancies.

Looking closer at the results, researchers noticed that Rac1 acted by keeping dead or dying cells attached in their milk-producing formations, perhaps to make them more accessible so surrounding cells can more easily gobble them up. This may also keep the dead cells away from immune cells patrolling in the breast ducts.

Immune cells still play a role in the process, but because breast cells clean up after themselves before the immune cells move in, the job is more manageable.

And since women have an increased risk of developing breast cancer in the first five to 10 years following pregnancy, studying the delicate balance between breast cell cannibalism and immune response could be the key to understanding breast cancer risk factors.

“Given this new role for Rac1 in the removal of excess or dead cells, thereby suppressing inflammation, the current study also identifies a potential role for Rac1 in breast cancer that is yet to be explored,” explained Naylor.