Cells in the human body need a way to interact with and receive signals from various substances, including hormones.

A receptor is a protein molecule in a cell or on the surface of a cell to which a substance can bind, causing a change in the activity of that cell.​

Here’s one way to think about it: A receptor is like a lock, while the substance binding to it is the key. Thus, only substances that fit the “lock” can bind to a particular receptor.

With that in mind . . .

Just as with females, males have estrogen receptors in the breasts.

Body fat tissue produces a kind of estrogen called estrone.

As a male’s body fat increases, so does the amount of estrone.

The estrogen receptors open the doors to the cells in his breasts.

His breasts grow.

The condition of male breasts is called Gynecomastia.