If you've come to one of my talks or read my Breast Book, you are likely to know that I think the human mammary gland is a magical organ. Why do I believe this? Well, for starters, it is the only organ that we are not born with. (Try to think of another one. It's true.)
We are not born with a fully formed mammary gland. What we have instead are stem cells sitting behind each nipple. Those stem cells don't do anything until puberty begins. Then, much like those children's sponge animals that grow out of a capsule when put in water, the stem cells absorb hormones, which lead them to blossom and form ducts (which may one day carry milk) suspended in a neighborhood of fat and connective tissue, or stroma. What makes this magic happen? Thanks to an elegant piece of research published Jan. 10, 2017, in Scientific Reports we now have an answer.
To learn how the breast stem cells and the environment interact to grow the breast ducts, the researchers realized they had to figure out a way to study the effect of the mammary neighborhood/connective tissue that supports the ducts. They landed on a brilliant solution: put stem cells from a different organ into the breast tissue and see what happens. The organ they chose was the testicle. The testicular stem cells, like the mammary tissue, came from adult mice and rats.
Amazingly, the researchers found that when they put the stem cells from the testicles into the mammary tissue, the stem cells responded to and were shaped by their new neighborhood. They grew into mammary ductal cells that were capable, with the right hormonal stimulation, to make milk! Equally incredible, it didn't matter if the mammary tissue (the extracellular matrix around the stem cells) came from a rat or a mouse or whether the animal had ever been pregnant; it was still able to transform the stem cells into breast ducts.
It's not yet known whether the breast tissue/extracellular matrix directed the stem cells to become breast duct cells or if it activated signals in the fat pad that then created an environment that made it possible for the testicle stem cells to become breast cells. More research is needed to answer that question.
For now, though, we can say that, for the first time, researchers have shown that inside the breast the role of the neighborhood or extracellular matrix is to organize the breast cells into fully functional ducts capable of supporting all aspects of breast development and function. In other words, a breast duct is formed from a stem cell, and that formation is not based on the origin of the stem cell but by neighborhood of other cells around it.
How do we know that it was the breast's environment and not the stem cells that mattered? The researchers also studied what happened when the testicular stem cells were placed in different types of tissue, such as lung or abdominal fat. In these experiments, the stem cells did not change the way they did in the breast; that tissue did not have the same power and influence as the breast tissue.
This research greatly expands our understanding of the magical human breast. We knew that the neighborhood that surrounds the breast cells changes during different stages of development: before pregnancy, after finishing breastfeeding, and after menopause. Scientists have been studying how these changes might spur cancer growth. This new study clearly demonstrates that the mammary neighborhood itself can change stem cells. What does the neighborhood do to cells that are thinking about becoming cancer or already are cancer cells? And can we change the neighborhood so that these cancer cells go back to being normal breast cells? Now, that would be magic!