Fat, ligaments and connective tissue
Spaces around the lobules and ducts are filled with fat, ligaments and connective tissue. The amount of fat in your breasts largely determines their size. The actual milk-producing structures are nearly the same in all women. Female breast tissue is sensitive to cyclic changes in hormone levels. Most women's breast tissue changes as they age, with more fat relative to the amount of dense tissue.
Lymph nodes and lymph ducts
The lymphatic system is a network of lymph nodes and lymph ducts that helps fight infection. Lymph nodes — found under the armpit, above the collarbone, behind the breastbone and in other parts of the body — trap harmful substances that might be in the lymphatic system and safely drain them from the body.