Nipple anatomy

What is a nipple?

The nipple is cylinder-shaped and attached to the front of the breast. A small number of women have nipples with other shapes that may interfere with breastfeeding. Nipples come in a range of widths and lengths and may be soft or firm. The nipple is covered in skin and contains milk ducts, muscle fibres, nerves, blood vessels, and fibrous tissue. Nipple skin can be damaged by a breastfeeding baby and become infected with bacteria, viruses, and yeast. Pain originating in the nipple is sometimes felt in a different part of the body.

A) Nipple parts

The four parts of a nipple.

A nipple has several parts:

  • A face: the flat part on the top of the cylinder
  • Sides: the round part or walls
  • A bottom: where the nipple attaches to the breast

The nipple also has a root, which is made up of the tissue just behind the nipple.  

B) Nipple sides

Each nipple has four sides:

  • Inner
  • Outer
  • Top
  • Under

C) Nipple skin

The skin of the nipple is like any other skin in that it can:

  • Be damaged to varying degrees.
  • Be painful.
  • Become infected with bacteria, viruses, and yeast.
  • Heal itself.

E) Nipple firmness

Some nipples are soft and others are very firm. Some can go from soft to firm when they are stimulated or cold and from firm to soft with warmth and massage. Soft nipples are more likely to become tender and damaged in the first few days of breastfeeding. 

F) Tissues under the nipple skin

Nipples contain:

  • Milk ducts
  • Blood vessels
  • Muscles
  • Nerves
  • Fibrous tissue

1) Milk ducts  

Mothers will sometimes express and see milk coming out from only a small number of ducts. They worry that this means they do not have enough milk. However, this is normal and not a good test of a milk supply. A better way of assessing the milk supply is by ensuring that the baby is growing well or if expressing, that the amounts of milk expressed are normal. 

2) Blood vessels

The blood vessels bring blood to and from the nipple. If the nipple is irritated, the blood vessels can clamp down and prevent blood from entering the nipple. This causes the nipple to turn white and is known as nipple vasospasm. This can happen with stimulation of the skin by:

  • Painful breastfeeding
  • Cold

3) Nipple muscles

There is a net of muscle fibres travelling in two main directions under the skin of the nipple that can contract (spasm) with stimulation or cold (Zucca-Matthes 2016). The nipple will then become hard. Rarely, this can be painful.

4) Nerves of the nipple and breast

a) The nerves of the nipple and breast

Nerves are a part of the signaling system that informs the brain about goings-on in the body. Nerves send information in the form of sensations like warmth, touch, and pain.

The breast sends signals to the brain and receive signals from the brain through the chest (thoracic [T]) spinal nerves known as T3, T4, and T5. There are nerves for the right side of the body and others for the left side. They are called spinal nerves as they end in the spinal column.

The T4 nerve is the main nerve for the nipple and areola although this can vary between individuals and include the T3 and T5 nerves (Jaspars 1997; Schlenz 2000). Each of the right and left thoracic spinal nerves curve slightly upwards as they travel from the front of the body to the back. 

The T4 nerve generally goes:

  1. From the middle of your chest
  2. Over the nipple and areola
  3. Around the outside of the breast
  4. Across the lower shoulder blade
  5. To the spinal column in the middle of back

It then joins the main nerve of the body (spinal cord), which relays the information to the brain.

b) Referred pain

When the body is damaged, nerves don’t always convey the site of the injury correctly to the brain. This means that a painful injury can be “felt” anywhere along the path of the nerve. This is called referred pain.

The nipple is in the area of the T4 nerve and when injured, can refer pain toward the outside of the breast and even around to the lower shoulder blade. Sometimes this pain is described as shooting into the breast.

References

Jaspars JJ, Posma AN, van Immerseel AA, et al. The cutaneous innervation of the female breast and nipple-areola complex: implications for surgery. Br J Plast Surg. 1997 Jun;50(4):249-59
 
Schlenz I, Kuzbari R, Gruber H, et al. The sensitivity of the nipple-areola complex: an anatomic study. Plast Reconstr Surg. 2000 Mar;105(3):905-9

Zucca-Matthes G, Urban C, Vallejo A. Anatomy of the nipple and breast ducts. Gland Surg. 2016 Feb;5(1):32-6