Anatomy & Physiology: Internal Breast Structures

The photo below shows a lateral view of the breast with the underlying anatomical structures superimposed.

The lactiferous sinuses are located under the areola of the breast. The lactiferous sinuses drain out through the nipple. Lactiferous sinuses may open into Montgomery's tubercles or adjacent to them.

Milk is produced in the alveolus. The alveolus is made up of gland cells around a central duct. The milk is produced by the gland cells. Surrounding the gland cells are the myoepithelial cells which contract to cause milk ejection into the milk duct. The milk then travels down into the lactiferous ducts and into the lactiferous sinuses.

Milk is stored in these ducts and sinuses in the period between breastfeedings. Mothers continue to make milk between feedings and they make more milk during feedings.

When an infant breastfeeds, the infant draws the nipple and the areola into their mouth. The mother's nipple elongates to about twice its normal length. The nipple height is compressed between the tongue and the palate. Milk is ejected about 0.03 seconds after maximum nipple elongation ( Smith, 1988 ).

Suckling infants compress the areola with their gums which stimulates oxytocin release. The oyxtocin release causes an increase in the diameter of the milk ducts and movement of milk in the ducts toward the nipple. Mothers may feel a sensation of pins and needles, pressure, or pain in the breast with milk ejection. This sensation may disappear as lactation continues over a several month period of time. Using ultrasound examination of the breast in women who had been breastfeeding for at least 1 month, Ramsay and others found that initial milk ejection as manifested by an increase in diameter of the milk duct and movement of the milk fat globules, occurred at an average of 50 seconds after suckling began. The number of milk ejections that a woman had during a breastfeeding varied from 1 milk ejection in 26% of the women to 2-9 milk ejections in the other women. The mean number of milk ejections was 2.5 ejections/breastfeed. Between milk ejections, the diameter of the milk duct returned to the pre-ejection diameter. This suggests that milk storage does not occur in the larger milk ducts, but in the smaller ductules. Mothers who noticed breast sensations with the first milk ejection reported no breast sensations with subsequent milk ejections ( Ramsay, 2004 ).