Biosynthesis of Milk

Milk is synthesized in the mammary gland. Precursors of milk come from the bloodstream. It is estimated that the production of 1 liter of milk requires 500 liter of blood moving through the mammary gland to provide the milk precursors. Some materials in the milk come unchanged from blood. These include minerals, some hormones and some proteins (e.g. immunoglobulins). Only precursors of milk protein and carbohydrates are present in blood. The primary substrates extracted from blood by the lactating mammary gland include glucose, amino acids, fatty acids, beta hydroxybutyrate, and salt.

Within the mammary gland there is milk producing unit called  alveolus. It contains a single layer of epithelial secretory cells surrounding a central storage area called the lumen, which is connected to a duct system. The secretory cells are surrounded by a layer of myoepithelial cells and blood capillaries.

Proteins synthesis begin in Golgi vesicles within the secretory cells. Amino acids for synthesis are supplied from blood.  

Lipids: Acetate and ß-hydroxybutyrate are important precursors of fatty acid synthesis in mammary cells of ruminants. These precursors are taken up by the epithelial cells at the basolateral membrane. In addition, preformed fatty acids, glycerol, and monoacylglycerides are also absorbed at the basolateral membrane. All these components enter into the synthesis of triglycerides of milk. Milk fat triglycerides are synthesized on the smooth endoplasmic reticulum (SER) and form small droplets. Numerous small lipid droplets will fuse together as the growing lipid droplet moves toward the apical membrane. At the apical membrane the large lipid droplet forces out the apical membrane of the cell, the apical membrane surrounds the lipid droplet until it pinches off and enters the lumen. Therefore, in the lumen of the alveolus, the milk fat globule (or milk lipid globule as it is now called) is surrounded by a membrane. This membrane originally was part of the epithelial cell's apical membrane. Inside the cell the lipid is not membrane bound and is called a lipid droplet, while after secretion in the lumen, the milk lipid globules are surrounded by a membrane.

C4-C14 fatty acids are synthesized in the cells.

C16 and greater fatty acids are preformed as a result of rumen hydrogenation and are transported directly in the blood.

Lactose: Glucose enters the cell via a specific transport mechanism. Some glucose is converted to galactose. Both glucose and galactose enter the Golgi apparatus to synthesize lactose. The formation of lactose in the Golgi results in drawing water into the cell, into the Golgi, and ultimately becoming part of milk. Golgi apparatus is involved in processing of milk proteins, synthesis of lactose, and the osmotic draw for water. The Golgi apparatus is very important to the synthesis of skim milk components. Lactose along with water is secreted via the secretory vesicles along with the milk proteins. Milk is in osmotic equilibrium with the blood and is controlled by lactose, K, Na, Cl; lactose synthesis regulates the volume of milk secreted.

Milking stimuli, such as a sucking calf, a warm wash cloth, the regime of parlour etc., causes the release of a hormone called oxytocin. Oxytocin is relased from the pituitary gland, below the brain, to begin the process of milk let-down. As a result of this hormone stimulation, the muscles begin to compress the alveoli, causing a pressure in the udder known as letdown reflex, and the milk components stored in the lumen are released into the duct system. The milk is forced down into the teat cistern from which it is milked. The let-down reflex fades as the oxytocin is degraded, within 4-7 minutes. It is very difficult to milk after this time.