Anatomy: milk line, extra mammary tissue

March 29, 2021

The original plan of this first-ever semester of The Lactation College was that it would run from January 1 - March 31, 2021 - completing a maiden-voyage three-month semester. However, with 1,300 students, about a month to go until the exam, your extremely positive feedback, and many more topics to cover, we can’t stop now.

This next month will continue to touch on topics that are highly suspicious of being on the exam, in a random fashion.

Again, The Lactation College has absolutely no inside knowledge of what will be on the test. We are taking our best guess to help you all out. As always, your feedback is much appreciated. Everyone of you has got this.

Touching on Testable Topics

It is wise to understand details about milk lines and extra mammary tissue.

In the developing embryo, the milk lines appear at about 4-5 weeks. While most of it eventually regresses, milk tissue can persist anywhere along the milk lines. These parallel lines run from the axilla to the groin on both sides.

The extra milk tissue can include: just one nipple or multiple extra nipples, breast tissue with no nipple, and breast tissue with a nipple. Extra tissue is estimated to occur in 1-6% of women; it can also occur in men. Enlargement of the extra milk tissue is often seen as the mother’s milk is coming in; the extra tissue can also leak milk.

An extra nipple (also called a supernumerary nipple) can look like a mole. If a test question asks about a picture that looks like a mole, think extra nipple. In practice, remember location, location, location – is the “mole” along the milk line? If so, think extra nipple. (The two pictures below are from DermNet NZ. The extra nipple in the first picture is located at 12 o’clock above the person’s left breast. The extra nipple in the second picture is located at 6 o’clock straight below the primary nipple.)

The Tail of Spence (see first picture above) is mammary glandular tissue that projects into the axillary region. The Tail of Spence is not extra milk tissue, it is tissue from the main, primary breast. It is differentiated from extra or supernumerary tissue in that it connects to the primary breast ductal system.

I received this picture from a new mother who was wondering what the bumps were in her armpit. Her baby was 3 days old and her milk was “coming in” nicely. I suspect this is Tail of Spence tissue. The plan was for her to apply cool packs if it became troublesome.


Polythelia – the presence of an extra nipple (without other breast tissue)

Polymastia – the presence of extra mammary (glandular) tissue (with or without a nipple)

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