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The Structure of the Udder
The udder is divided into thin layers of supporting tissue made from ligament. These divide the udder vertically but also make a horizontal division. Beneath this division, the milk collects in chambers known as cisterns, while the milk is produced above the divide in clusters of tissue known as lobes. The producing and collecting centers are joined by a series of narrow ducts that pass through the layer of supportive tissue.
The Lobe, Lobule and Alveoli
The lobe is subdivided into a series of smaller balloon-like structures known as lobules. These store arrangements of cells and cavities known as alveoli. The tiny cavities, known as lumen, are walled with milk-producing cells. When the teat of the animal is stimulated, this sends a hormone called oxytocin to the brain, which causes cells around the lumen to contract and force the milk downwards into the ducts and from there into the cistern. This is known as the "let down reflex".
The Milk-Producing Cell
Milk production is carried out by the cells that line the lumen. They have a distinctive, cuboid shape and are responsible for the production of the main constituents of the milk as well as the transference of other constituents from the blood into the lumen. Fat, lactose (a type of sugar), and casein (a group of proteins), are all synthesized within a section of the cell called the endoplasmic reticulum and transported to the edge of the cell through the Golgi complex. Here, they must pass through the cell membrane, which they do by various mechanisms. Globules of fat stretch the cell membrane until they are fully surrounded and then split the membrane in order to enter the lumen. Globules of casein and lactose aggregate within their own membranes that then fuse with the cell membrane that repairs the damage caused to the cell membrane by the exiting fats, and deposits the nutrients outside the cell.
Blood Supply
Though the main constituents of milk are produced in the udder, others are drawn out of the blood through narrow-walled blood vessels that surround the alveoli like a net. These blood vessels supply vitamins, minerals and certain fats and proteins to milk. In cows, around 350 liters of blood must be circulated through the udder in order to deliver the volume of these nutrients found in one liter of milk.