ANSC 438 Home / Beginning / Milk Composition / Mammary Structure
Mammary Development / Mother & Neonate / Lactation / Mastitis

Independent Study
Modules

Mammary Macro-structure
Dairy Cow Udder Anatomy

9

Secretory tissue in the interior of the gland is made up of:

  • Connective tissue - This includes fibrous connective tissue of the parenchymal tissue and the fatty tissue of the fat pad. An example of fat pad is illustrated in the image below.
  • Secretory tissue - This is made up of secretory epithelial cells. These are the cells that produce milk during lactation. These also are part of the parenchymal tissue.

The relative amount of connective and secretory tissue varies from animal to animal.

The image to the right is of a beef heifer's udder cut in cross-section. The fat pad and parenchyma are indicated. In this case, all of the fat pad would be considered as part of the udder's connective tissue. The fat pad is required for correct development of the parenchymal components as the ducts branch and grow into the fat pad. The parenchyma is composed of epithelial structures, in this case ducts and immature lobules, and the connective tissue that is around the ducts and lobules, as well as the connective tissue within the lobules (see the later sections on mammary histology for more on lobules). There are no secretory cells as such in this non-lactating heifer, however, the epithelial cells of the lobules will eventually become the cells that secrete milk during lactation.

Udder of beef heifer cut in cross-section showing fat pad and parenchymal tissue.

Secretory tissue in the udder is organized into lobes, with each lobe made up of many lobules. Each lobule contains 150-220 microscopic alveoli. Lobules and alveoli are covered in more detail in the Micro-Structure section of this lab.

Lobe - Groups of lobules are surrounded by a connective tissue sheath and comprise a lobe. Each mammary gland is made of numerous lobes.

Lobules - Clusters of 150-220 alveoli are encapsulated by a connective tissue sheath and are organized as a lobule (~.7-.8 mm dia.).

Alveoli - (acini) Alveoli are sack-like structures where milk is synthesized and secreted. An alveolus is the discrete milk producing unit. It is estimated to hold ~1/5 of a drop of milk. The lumen of the alveolus is lined by a single layer of secretory epithelial cells. The epithelial lining is surrounded by contractile myoepithelial cells. Myoepithelial cells contract in response to the hormone oxytocin, resulting in milk being squeezed out of the alveolar lumen and into the small ducts. Outside of the myoepithelial cells the alveolus is surrounded by a connective tissue basement membrane. The capillary bed on the outside of the alveolus is part of the stromal tissue (connective tissue) between alveoli. A group of alveoli can be visualized as a clump of grapes, with the stems acting as the small ducts leading from the alveoli.

Mammary Structure