Normal Breasts | Anatomic Pathology Guide

Men, women, and those in transition have this pair unless surgically removed. I’ll review a few essential structures in a normal breast before we delve deeper into the pathology.

First off, the breast is mainly fat, but with a duct system embedded into it (imagine the duct system as) like the leaves of the tree catching the rain – it trickles down the branches into the trunk, then the main trunk and finally the soil. For our purposes, this tree doesn’t have roots.

Duct Systems

So duct systems, usually, there are 6-10 major duct systems (for now count the subsegmental ducts in the illustration below). The trunk and branches of the upside-down tree are all ducts; they have different names. The petals or leaves of the tree are called ductules, and the ducts directly extending from them are called terminal ducts.

Normal Breasts | Anatomic Pathology Guide

These terminal ducts converge into the extensive duct system (and given different names)and drain into the lactiferous sinus (the bulb thing just beneath the nipple). The part of the illustration where there seem to flower at the tip is the terminal duct-lobular unit or TDLU.

In reproductive-aged females, the yellow part of the “flowers” or the ductules (or acini) increase and become a lobule (which is just a collection of acini that drains to a single terminal duct) – hence the terminal duct lobular unit (terminal duct-ductules-lobule). This is where the milk comes from. The acini make the milk, and the milk drains into the terminal ducts and the nipple.


The TDLU are the structures involved in diseases like fibrocystic change, fibroadenoma, and most ductal carcinomas of the breast. At the same time, the large duct system is the structure involved in papillomas and Paget’s disease.

When there is a group of elephants, we call them a herd. In the breast, we call many lobules a lobe (blue circle above). The red circle is the ducts.

See the yellow circle inside the blue one? That’s a lobule among other lobules lumped into a single lobe. Compare the illustration with the picture beside it.

The red circle is the ducts. Inside the yellow circle is a collection of acini. This is an acinus (plural acini). The brown circles are the epithelium (with the large, kind of oval, violet nuclei, these cells produce the milk), and directly beneath them are the myoepithelium (green circles with the long-ish nuclei which are oriented perpendicular to the epithelium). Pregnant while breastfeeding? Read this!

Myoepithelium Orientation

The myoepithelium is oriented this way because when a baby sucks the nipple, there is an oxytocin release for the letdown reflex. The myoepithelium are the cells that respond to the oxytocin (for the letdown reflex), and they respond by contracting.

Since they are arranged one end to the other around the acinus, they make the lumen (hole inside the acinus) smaller, squeezing the milk into the terminal duct and eventually to the larger ducts and into the baby’s mouth via the nipple.

Again, this is a lobule made up of a group of acini. Let’s concentrate on the tissue or the stroma around it. The paler tissue directly around the acini with pink lines is called the intralobular stroma. The denser, kind if orange tissue around the lobule, is the extralobular stroma.

In breast diseases such as fibroadenoma or Phyllodes tumor, the stroma that is affected is the intralobular stroma. Read our article on Pregnancy safe acne treatments. Here’s an awesome article on breast-lift surgery.