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Anatomy of the breast

Breast anatomy infographic

The breast

The breast is a body part with major symbolic meaning and functions. Although its primary
biological role is breastfeeding infants, for some women the breast is significant in terms of
aesthetics, seduction and sexuality.

The female breast is made up of cells and structures that produce and secrete milk.

Some definitions from the :

  • Mammary glands: the cells responsible for milk production. They are made up of 1,525 small structural units, called lobules, that produce milk when stimulated by prolactin, a hormone secreted during pregnancy.
  • Excretory ducts: they carry the milk from the mammary glands to the nipple.
  • Nipple: the protruding end through which milk is expelled.
  • Areola: the brownish or pinkish part around the nipple. It contains glands that produce secretions to lubricate and disinfect the nipple and areola. The areola also has a muscle that erects the nipple.
  • Adipose tissue: it is the fatty tissue that protects the breast. The amount of adipose tissue varies from one woman to the next.
  • Connective tissue: it is a structural tissue. The breast is also supported by ligaments that cross the breast from the skin to the pectoral muscle.
  • Lymphatic system: it protects us from infections and diseases. It carries lymph, a fluid containing immune-system cells called lymphocytes that eliminate foreign bodies, bacteria and viruses. This system is made up of lymphatic vessels that collect and transport lymph away from the breast to small and bean-shaped masses of lymphatic cells, called lymph nodes. The lymph nodes filter foreign bodies and cancer cells out of the lymph.

Throughout our lives, our breasts are influenced by sex hormones, primarily estrogen and progesterone. As the concentration of these hormones varies throughout a woman’s life (puberty, menstrual cycle, pregnancy, breastfeeding and menopause), these fluctuations cause changes in breast function and size.

Main roles of sex hormones


Estrogen and progesterone are the top two female hormones and they play several essential roles in the body. During puberty, they stimulate the development of the sexual organs and prepare the body for pregnancy.

To trigger hormone synthesis, the brain secretes follicle stimulating hormone (FSH) and luteinizing hormone (LH). Travelling through the bloodstream to the ovaries, they stimulate the ovaries to produce and secrete estrogen and progesterone. The ovaries also produce a small amount of testosterone and androgen, sex hormones involved in hair growth and sex drive, among other things.

Here are the main roles of estrogen and progesterone:


  • Develops the female organs during puberty (uterus, breasts, thickening of the vagina)
  • Controls bone formation
  • Distributes fat to the hips
  • Broadens the pelvis
  • Feminizes the voice
  • Renews tissue
  • Triggers sexual desire
  • Affects the brain (memory, learning, mood), blood vessels, skin and hair


  • Prepares the body for pregnancy
  • Favours implantation of the fertilized egg into the uterus (thickens the lining of the uterus)
  • Prevents contractions and ovulation during pregnancy
  • Prepares the mammary glands during pregnancy
  • Works as a sedative on the brain

Hormones are physiological messengers.

Various organs secrete hormones into the blood to send messages to other parts of the body. When a message reaches the targeted organ, it responds by adjusting its activities and behaviour. Like a key in a lock, hormones bind to their respective receptors in a cell to trigger a specific action within that cell.

Here are a few examples:

  • Insulin secreted by the pancreas tells other organs that sugar is available in the blood.
  • Melatonin secreted by the brain tells the body it is time to sleep.
  • Cortisol secreted by the adrenal gland in the kidney tells the body to react to a stressful situation.

Estrogen and progesterone receptors are nuclear receptors.

Estrogen interacts with the estrogen receptor (ER), and progesterone with the progesterone receptor (PR), both of which are considered nuclear receptors. They are called “nuclear” because, to carry out their respective actions, these hormones travel to a cell’s nucleus and interact with DNA to trigger the production of specific proteins.


The estrogen and progesterone receptors’ target genes are involved in cell proliferation and differentiation, DNA replication and programmed cell death (apoptosis), among other functions. These processes are necessary to the hormones’ roles in the various target tissues.

Given the important role of estrogen and progesterone in breast development, altered hormone signalling can lead to breast cancer. More details are available in the following sections.

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