Female reproductive system

Mammary Gland

Gwen V. Childs, Ph.D.
Reading assignment: pp 483-486

Test your knowledge of Mammary gland structure and function

1. Define the overall classification of the mammary gland.
2. How does the architecture of the resting mammary gland differ from that of the lactating mammary gland?
3. What is the secretory products of mammary glands and how are they secreted?
4. What hormones stimulate development of the gland?
5. What stimulates synthesis of the products?
6. What stimulates secretion from the gland?
7. How do behavioral actions assist in lactation?
8. How do neural reflexes assist in lactation?
9. How is the nipple structured to lactate?

Classification and the resting state

Mammary glands are compound tubuloalveolar glands.  They have 15-20 lobes that radiate out from the nipple separated by collagenous connective tissue.  The lobes drain into a lactiferous duct which is expanded to form a lactiferous sinus for milk storage. This leads to the nipple.  This is drawn in the cartoon to the left.

The photograph below shows a lobe of a non-lactating mammary gland. 

In the resting state, the alveoli are not well developed.  The glandular portion is surrounded by myoepithelial cells (which will form a basket that helps lactation when active). The photos below show resting glands. The pink "ribbon" like outer layer is the myoepithelial cell. 

The ducts are lined by simple columnar epithelium in the beginning and stratified cuboidal epithelium as they become lactiferous ducts. The lactiferous ducts become stratified squamous (keratinized) as they approach the nipple. The following photograph shows a section through a duct and lactiferous sinus. Note that in some places the epithelium looks like stratified cuboidal and in others it looks stratified squamous. Underneath is the elastic connective tissue characteristic of the nipple.

Lactating Mammary Glands

As pregnancy develops, the ducts branch and form secretory alveoli.  These are composed of cuboidal cells that produce the milk.  They are surrounded by myoepithelial cells, which are a basket-like arrangement of cells that contract to squeeze the milk out of the glandular cells into the duct. The glands in the alveoli shown below have droplets of milk and the lumen is filled with milk.

Secretory Products

Initially, the glands produce colostrum, an antibody-rich fluid designed to nourish and protect the newborn.  It contains IgA, minerals, vitamin A, sodium and choride, lactalbumin and is also cellular including lymphocytes and monocytes.

Then, within a few days after birth, prolactin from the pituitary activates the secretion of milk.  Milk contains both proteins and lipids.  The lipids are released by apocrine methods.  Droplets of milk coalesce to form larger droplets at the apical surface.  They project as cytoplasmic blebs that are pinched off. The proteins in milk, however, are released by exocytosis. The secretion of lipid is seen in the following electron micrograph (L) of a lactating mammary gland. Vesicles containing the proteins are found in the glands as well.

What stimulates development of the mammary gland and lactation?

Prolactin is responsible for the development of mammary glands and the production of milk and colostrum. Also, estrogen and growth hormone are important for development.   The act of mothering (touch, handling the infant, anticipation, etc) stimulates prolactin release which stimulates milk production. This continues as long as the mother is handling her infant.

 However, without oxytocin, the milk products cannot be secreted into the ducts. Oxytocin is stimulated by sucking (reflex from nerves leading from the nipple).  This hormone then stimulates contraction of the myoepithelial cells which expels the milk into the duct system. The release of prolactin stimulates milk production by both breasts.  The release of oxytocin occurs as the infant suckles one breast.  However, some milk ejection occurs in the other breast.

 Nipple:

Skin and connective tissue that is heavily vascularized and innervated to aid in lactation. The wrinkling comes from smooth muscle in the wall; the organization and contraction of the muscle fibers is responsible for the erection of the nipple. This region is shown in the following photograph. The darker connective tissue is highly elastic.  Bundles of smooth muscle are abundant.

Lactiferous sinuses narrow and converge on the nipple at multiple openings.   Skin of nipple may have sebaceous and sweat glands.

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