Breasts are a part of a humans sexual anatomy. Breasts have both functional (breastfeeding) and sexual (pleasure) functions. A breast consists of various different tissues, milk ducts, nipples (at the centre of the areola), areolae (the dark colored area of the skin surrounding the nipple), blood vessels and nerves. Hormones play a major role in the development and prevention for breasts. Certain hormones like estrogen, progesterone and prolactin help in the development of breasts while other hormones like testosterone prevents the growth of breasts.
Sources:
https://my.clevelandclinic.org/health/articles/8330-breast-anatomy
https://www.hopkinsmedicine.org/health/wellness-and-prevention/anatomy-of-the-breasts
https://www.mayoclinic.org/healthy-lifestyle/womens-health/multimedia/breast-cancer-early-stage/sls-20076628?s=1