Terminal duct lobular unit

The terminal duct lobular unit is the working unit of the breast. The name tells you what it is made of: a terminal duct, the last stretch of tubing; the intralobular duct, which runs inside; and the secretory vesicles, small sacs that drain into it.

Picture a small bunch of grapes hanging from the end of a hose: the hose is the duct and the grapes are the little sacs. That is what the gland looks like inside, because it ends in units of that kind, marked by clusters of acini, which are tiny sacs, and by fine ductules.

Around them is supporting tissue, which in medicine is called stroma, and it is not the same inside as outside. The tissue inside the unit is called lobular stroma, and it is loose and rich in cells; the tissue outside is interstitial stroma. That difference marks where the unit ends and where its surroundings begin.

In pregnancy these units start working in a different way: they make enormous numbers of new cells, because the body is getting ready to produce milk. And inside the unit, the hormone progesterone is needed for those cells to proliferate and to finish maturing, which in medicine is called terminal differentiation.