What’s in your Boob?

Yvette Zhu
July 25, 2020

Breasts are a pretty rad human organ. But what’s in a breast? The physiology behind them is fascinating and is often overshadowed by portrayals in the media. However, we wanted to unveil the underlying way that breasts work as an organ to provide babies with sustenance. So here’s all you need to know about what is in your boob and its function!

Anatomy of the Boob 101

Both male and female human breasts contain mammary glands that function to produce and secrete milk. The alveoli are the basic unit of mammary glands and are made up of two types of cells. The cuboidal cells make the milk and the myoepithelial cells contract to move the milk along. All the alveoli that drain into the same lactiferous duct are grouped together to form a lobule. Did you know that there is not a single hole at the nipple, but actually many microscopic holes that are all the openings of the lactiferous ducts? The rest of the breast is mostly adipose, or fatty tissue. Throughout the breast there is a framework of connective tissues and ligaments made of collagen and elastin supporting the breast structure.

The structures on the outside of the boob are much simpler. There is the nipple, as we mentioned before, where the lactiferous duct opens to the surface and milk is excreted. And then the areola is the darker region around the nipple and plays a key role in a proper breastfeeding latch. This area also contains sebaceous glands, called Montgomery glands, which produce an oily substance to protect the nipple and areola during breastfeeding.

Figure: Anatomy of the human breast [1]
Visual credit: Grace Zhang, SimpliFed

Why do my boobs get bigger during pregnancy?

During pregnancy many hormones are increasing and causing changes within your body.  In particular, high levels of estrogen and progesterone act in concert with other hormones to further develop and grow the milk duct system.  This results in the increase in breast size and sometimes sensitivity and soreness and the increased visibility of blood vessels. The nipple and areola also grow larger and darker in preparation for breastfeeding.  The breast is usually fully capable of lactation after the 5th to 6th month of pregnancy [5,6].

How does the boob know when to secrete milk?

It all happens because of the magical letdown reflex, also known as the milk ejection reflex. The reflex begins with mechanoreceptors, receptors that detect changes in pressure, in the nipples sensing a baby suckling and then sending a message to a region in the brain called the hypothalamus. Next, the hypothalamus signals the posterior pituitary gland to release the hormone oxytocin. Oxytocin causes the contraction of the myoepithelial cells, the muscle cells of the mammary gland we mentioned above, thereby moving milk down the ducts to exit the nipple where the baby is suckling [4,6,7].

But suckling not only triggers the letdown reflex, it also stimulates production of other hormones. At the same time the hypothalamus is signalling the release of oxytocin, it is also telling the posterior pituitary gland to secrete prolactin. Prolactin promotes the production of milk by the alveoli in the mammary glands, also known as lactogenesis. The levels of prolactin increase after the start of breastfeeding and peak about 30 minutes afterwards [7]. This serves to ensure milk is made for the next feed. These two processes make sure the boob secretes and ejects milk when a baby suckles.

Some fun facts

The magical letdown reflex can be triggered not only by the baby suckling the nipple, but also by just the sound of crying babies! Research has shown that information sent to the auditory center of the brain can lead to the same cascade of hormone production and the ejection of milk [4]. This “backup” mechanism allows moms in the vicinity of a crying baby to respond immediately and nourish the hungry baby!  

Did you know that each boob produces milk independently? Breast milk also contains a substance that stops secretion of milk called the feedback inhibitor of lactation (FIL). The presence of FIL prohibits cells from further making milk. So if milk is not removed from the breast, FIL builds up stopping the breast from continuing to make more milk and becoming too full. This explains why one breast can stop producing if a baby only suckles the other side [7].


  1. “Anatomy of the Breast.” Memorial Sloan Kettering Cancer Center, www.mskcc.org/cancer-care/types/breast/anatomy-breast
  2. “Breast Anatomy | Breastfeeding.” Khan Academy and Stanford School of Medicine, 2014, www.khanacademy.org/partner-content/stanford-medicine/stanford-school-of-medicine/breastfeeding/v/breast-anatomy
  3. “Breastfeeding – Letdown Reflex | Breastfeeding.” Khan Academy and Stanford School of Medicine, 2014, www.khanacademy.org/partner-content/stanford-medicine/stanford-school-of-medicine/breastfeeding/v/breastfeeding-letdown-reflex
  4. McNeilly, Alan S., et al. “Release Of Oxytocin And Prolactin In Response To Suckling.” British Medical Journal (Clinical Research Edition), vol. 286, no. 6361, 1983, pp. 257–259. JSTOR, www.jstor.org/stable/29509433. Accessed 3 June 2020
  5. “Normal Breast Development and Changes.” Normal Breast Development and Changes | Johns Hopkins Medicine, www.hopkinsmedicine.org/health/conditions-and-diseases/normal-breast-development-and-changes
  6. Maternal, Fetal, & Neonatal Physiology, by Susan Blackburn, Elsevier Health Sciences, 2014, p. 146
  7. Infant and Young Child Feeding: Model Chapter for Textbooks for Medical Students and Allied Health Professionals, World Health Organization, 2009, pp. 2.1–2.12