Breast – Tissue and Systems

Breast Cancer is the most common cancer diagnosed in women in the United States.  The purpose of this article is to describe the basic anatomy and function of the human breast.  By having a better understanding of the normal breast function, you will be able to better understand different types of breast disease including symptoms that can occur.

The breast is not an organ per se, it is a specialized gland that plays a role in normal hormone function.  The breast is an important part of a woman’s identity as it relates to womanhood, motherhood, femininity, a sign of maturity, virility, and sex appeal.    The breast is basically a specialized type of gland that is normally found on top of the muscle layers of the chest.  The average breast is about ½ pound but there is significant variation.  About 80% of the breast is made of fat and 20% glands.

As a woman ages the percentage of fat increases until it is almost complete replaced with fat.  The basic structure of the breast can be thought of as a bunch of grapes.  The central stem is near the nipple and a series of ducts extend out like tree branches out to the glands (or grapes in this analogy).  Each breast has about 15-20 lobes that drain into a duct that subsequently join centrally located ducts and exit the nipple.  Within the breast there are ligaments or strands of firm tissue that help hold it in place called Cooper’s ligaments.  Excess fluid in the breast is drained by lymphatics, as are most other areas of the body.   About 75% of the breast lymphatics drain to the armpit.  Lymphatic fluid collects in lymph nodes and is sent along further eventually to return to the blood stream.

There is a significant variability in normal growth and development in a healthy breast.  It is very common for breast asymmetry that is one breast will be slightly smaller than the other or the shape may be slightly different.

As a woman undergoes hormone changes during the menstrual cycle, the breast glands enlarge and change in response to hormones.  This occurs quite dramatically during pregnancy.  Hormones such as estrogen cause breast tissue to grow.  There are receptors in the breast for these hormones on both the breast ducts and breast glands (or lobules).  This is an important concept for understanding changes that occur during different phases of life such as adolescence, pregnancy, older age and with breast cancer.  Hormones from the pituitary gland cause milk production from the breast glands.

Some variations in breast development can occur and are considered normal and not a disease.  There is an imaginary line from the collar bones down through the nipples to the groin.  This “milk line” is where extra nipples (also known as polythelia) might occur.  It is also possible to have extra breast tissue occur on this line, this most commonly occurs in the armpit.  This is known as an axillary breast.  They are usually small but bothersome and most women want to have them removed.

Early in life, a woman’s breasts are firm and are made mostly of glandular tissue and a bit of fat.  As a woman’s age, the breast is replaced mostly with fat until after menopause the breast is entirely fat.  This explains why a breast seems to “sag” as a woman ages.  The nipple normally lies around the level of the crease made by the breast against the chest.  This crease is known as the inframammary fold.  A young normal breast has a teardrop shape appearance and hangs with the nipple at the level of the inframammary fold.  The density of the breast is important to understand as it explains the benefits and limitations of different types of ways to get x-rays or imaging of the breast.  A younger breast really can’t undergo an x-ray (also known as mammogram) because it is so dense the picture will be almost all white and not provide much useful information.  On the other hand, a little lump or piece of calcification would be easy to spot in an elderly woman’s mammogram because her breast is almost entirely fat which is very dark on x-ray making a good background for imaging.

Many conditions that affect the rest of the body can also affect the breast.  The breast can be subject to injury, infection, skin disease, tumors and cancer.  Like most areas of the body, when a breast is injured it can cause scaring.  The breast undergoes a somewhat unique type of injury reaction in that with severe trauma parts of the breast scar and become calcified.  This can create problems with future imaging as these calcium deposits can be confused with other abnormalities such as cancer.  The breast can have infection which might be something as simple as a rash or severe as a big abscess (pus pocket).  The breast is more commonly infected during pregnancy and lactation.

There are different kinds of lumps that can occur in the breast.  The breast might form cysts of simple clear fluid, milk, blood or some combination of these.  The breast can also form lumps (or tumors).  Most breast lumps that occur in younger women (i.e. pre-menopausal) are not cancer but benign lumps such as fibroadenomas.  As a woman ages, the likelihood that a breast lump will be cancer increases.


  1. Katz, VL; Lentz GM; Lobo RA; Gershenson DM. (2007) Katz: Comprehensive Gynecology, 5th ed. Chapter 15. Philadelphia, PA: Mosby Elsevier
  3. US Female College Students’ Breast Health Knowledge, Attitudes, and Determinants of Screening Practices: New Implications for Health Education.Early J, Armstrong SN, Burke S, Thompson DL.J Am Coll Health. 2011 Aug-Oct;59(7):640-7.PMID: 21823959
  4. This article was originally published on September 3, 2012 and last revision and update was 9/4/2015.