The Breast and How It Works

The Breast and How It Works

how the breast works
The breast—or mammary gland—is a complex and efficient organ. Development of the breast begins before birth during the early weeks of gestation, but the mammary gland doesn’t become fully functional until the onset of lactation. The breast continues to change under the influence of normal physical processes—such as menstrual cycles, pregnancies, birth, and weaning—through menopause, when the gland begins to atrophy. Knowledge of breast anatomy and function will be helpful in understanding how normal bodily functions and environmental influences affect breastfeeding.


Composition of the Breast

The breast is a secretory gland composed of the following parts:

  • glandular tissue, which makes and transports milk,
  • connective tissue, which supports the breast,
  • blood, which nourishes breast tissue and provides the nutrients needed to make milk,
  • lymph, which removes waste,
  • nerves, which make the breast sensitive to touch and allow the baby’s suck to stimulate the release of hormones that trigger the let-down, or milk-ejection, reflex and the production of milk,
  • adipose (fatty) tissue, which offers protection from injury.

The size of the breasts is determined to a great extent by the amount of fatty tissue present, which has no effect on milk production or the quality of the milk produced.

The enlargement of the breasts during pregnancy and lactation indicates that the mammary gland is becoming functional.


Alveoli are grape-like clusters of glandular tissue in which milk is synthesized from blood.

Alveoli cells secrete milk. They are surrounded by a network of band-like myoepithelial cells, which cause the alveoli to contract when stimulated by the oxytocin released during the let-down, or milk-ejection, reflex. This action expels the milk into the ductules and down into the ducts.

Ductules and lactiferous (mammary) ducts

Ductules are branch-like tubules extending from clusters of alveoli.

Each ductule empties into larger ducts called lactiferous or mammary ducts.

Milk or lactiferous sinuses

The lactiferous ducts widen underneath the nipple and areola to become lactiferous sinuses, where milk collects.

The baby’s gums need to be well behind the end of the nipple in order for the lactiferous sinuses to be properly compressed and emptied.

Lobes and lobules

Each mammary gland forms a lobe of the breast, which consists of a single major branch of alveoli, milk ducts, and one lactiferous sinus that narrows to an opening in the nipple (nipple pore). There are fifteen to twenty-five lobes in a breast, and each lobe consists of twenty to forty lobules (a smaller milk duct with its supporting alveoli). Each lobule consists of ten to 100 supporting alveoli.

Nipple tissue

After widening into a lactiferous sinus, the duct narrows again and leads to the nipple opening or pore, although some ducts may merge near the tip of the nipple.
The nipple protrudes and becomes firmer with stimulation. It is flexible and graspable so it will conform to the baby’s palate, tongue, and gums during breastfeeding.



The nipple protrudes from the center of the darker pigmented area of the breast called the areola.

Since the baby’s sight is not fully developed at birth, it is thought that the darkened area may serve as a target to help the baby locate the center of the breast.

Montgomery glands

The areola is also the site of the Montgomery glands—small oil-producing glands that provide lubrication and alter the pH of the skin, thus discouraging the growth of bacteria on the skin of the nipple and areola (Williams 1992).

Montgomery glands enlarge during pregnancy and have a pimply appearance.

Washing the nipples with soap is unnecessary and removes the beneficial oils that are secreted by the Montgomery glands. It may cause drying and cracking, making the nipple and areola more prone to soreness.

Daily rinsing with warm water while bathing is sufficient to keep the nipples clean and preserve the lubricating and anti-bacterial properties of the oil secreted by the glands.



The increase in estrogen during pregnancy stimulates the ductule system to grow and become specific. Estrogen levels drop at delivery and remain low for the first several months of breastfeeding.


The increase in progesterone during pregnancy influences the increase in size of alveoli and lobes.


The increase in prolactin and other hormones contributes to the accelerated growth of the breast tissue during pregnancy (Rillema 1994).

Prolactin levels rise with nipple stimulation during feedings.

The pattern of pressure and release caused by the baby’s mouth on tire mother’s areola stimulates the nerve pathways from the nipple to the brain to release prolactin into her bloodstream.
Alveolar cells make milk in response to the release of prolactin when the baby sucks at the breast.

The use of nipple shields, a baby with a weak suck, or improper positioning can alter prolactin levels and hamper the mother’s milk supply (Woolridge 1980). Frequent feedings and effective sucking enhance the production of milk.

Prolactin has been referred to as the “mothering” hormone. Together with oxytocin, it may be responsible in part for the intense feeling of needing to be with the baby that many mothers experience.


Oxytocin contracts the smooth muscle of the uterus during childbirth, after birth, and during orgasm.

After birth, oxytocin contracts the smooth muscle layer of band-like cells surrounding the alveoli to squeeze the newly produced milk into the duct system.
Oxytocin is necessary for a let-down, or milk-ejection, reflex to occur.

Let-down, or milk-ejection, reflex

The let-down, or milk-ejection, reflex is responsible for making the milk available to the baby, particularly the fatty hindmilk. The baby needs this richer, creamier hindmilk for adequate weight gain.

When the baby sucks effectively at the breast, it stimulates the nerve endings in the nipple and areola, sending signals to the mother’s pituitary gland by way of her hypothalamus and initiating the secretion of prolactin and oxytocin into the bloodstream. Oxytocin quickly stimulates the band-like myoepithelial cells surrounding the alveoli to contract, squeezing milk into the ducts. The let-down may also occur without the stimulation of the baby’s suck (Cobo 1993). Prolactin stimulates the alveoli to produce more milk.

For signs of a healthy let-down, or milk-ejection, reflex and ways to encourage a delayed or inhibited let-down, see “The Let-Down, or Milk-Ejection, Reflex” in the chapter “Breastfeeding Basics.”