Researchers looked at breast fat tissue in humans and mice. They found that in obesity, the web-like network of fibers (extracellular matrix, ECM) holding fat cells in place is more rigid than normal via myofibroblasts and adipose stem cells (ASCs)

 Curious about whether obesity-related genetics might play a role, the researchers compared tissues of mice that were genetically obese and mice that became obese from overfeeding. Regardless of how they became obese, their tissues still were stiffened, the investigators found.

The researchers also ruled out a common inflammation-related molecule as playing a role. They concluded that the stiff structure itself seemed to be the relevant factor.
 
To assess that idea further, the study authors also exposed precancerous breast cells to these stiff fibrous networks from obesity-related tissue samples. Cells growing under these conditions were more likely to become completely cancerous compared to those grown in non-obesity-related samples.
 
When the researchers placed obese mice on a diet, the fibrous network thinned out a little, "hinting that weight loss can potentially reverse this tissue stiffening," the study authors explained.
 
Another question is how obesity leads to the stiffer tissues. Authors said that low oxygen in the denser tissues might be one explanation. And, they noted, it's still possible that an inflammatory process plays a role