Anatomy of The Breast
The breast is composed mainly of fat (adipose tissue) and breast tissue, along with connective tissue, nerves, veins, and arteries. Breast tissue is a complex network known as the mammary gland. Within the mammary gland, there are 15-20 lobes or compartments separated by adipose tissue. Within each lobe are several smaller compartments called lobules.
Lobules are composed of grapelike clusters of milk-secreting glands termed alveoli, which are found embedded in connective tissue. Spindle-shaped cells called myoepithelial cells, whose contractions help propel milk toward the nipple, surround the alveoli. There are about one million lobules contained within each breast (Spratt et al. 1995). The lobules are connected by tiny ducts that are joined together (much like a grape stem) into increasingly larger ducts. Within each breast there are between five and ten ductal systems, each with its own opening at the nipple.
Surrounding the nipple is a darkly shaded circle of skin called the areola. The areola appears rough because it contains modified sebaceous (oil) glands. These glands secrete small amounts of fluid to lubricate the nipple during breast-feeding.
Of all breast cancers, about 80% originate in the mammary (lactiferous) ducts, while about 20% arise in the lobules (IOM 1997). One of the most important distinctions to understand is the difference between invasive breast cancer and carcinoma in situ.