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Milk Composition & Synthesis
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Milk Synthesis
Milk Synthesis Process

Note: This section is replicated from the same section that you read in the Mammary Micro-Structure Module. This material also is relevant to the Milk Synthesis sections.

Milk synthesis processes : The precursors of milk components leave the blood and enter the extracellular fluid between the capillaries and the epithelial cells. Precursors then are taken up from the extracellular fluid through the basolateral membrane of the epithelial cell. Once inside the cell the precursors enter the appropriate synthetic pathway. In addition, some pre-formed proteins, such as immunoglobulins, are transported intact through the cell.

There are 5 routes by which milk precursors or components enter milk in the alveolar lumen, including uptake of amino acids, uptake of sugars and salts, uptake of milk fat precursors, uptake of preformed proteins (immunoglobulins, and the paracellular pathway. The diagram below indicates the mechanisms of uptake and utilization of amino acids for protein synthesis, glucose for lactose synthesis, fatty acids and glycerol for milk fat synthesis, immunoglobulins for transport across the cells, and the paracellular pathway.

Diagram illustrating pathways of substrates and secreted products in mammary epithelial cell.

Amino acids to proteins: Amino acids are absorbed through the basal membrane of the cell by several specific amino acid transport systems. Once inside the cell, amino acids are covalently bound together to form proteins at the polysomes (poly-ribosomes) on the rough endoplasmic reticulum (RER). Proteins that are synthesized at the RER include the proteins to be secreted (such as the milk proteins casein, ß-lactoglobulin, and a-lactalbumin) and membrane bound proteins (such as proteins involved in cell-cell contacts and membrane bound enzymes). Newly synthesized proteins are transferred from the RER to the Golgi apparatus where they are processed for transport out of the cell. Remember that casein is secreted as a micelle; the micelle is formed in the Golgi from the casein molecules, calcium and phospohorous. Caseins and other proteins undergo post-translational processing in the Golgi. Proteins that remain in the cell are synthesized by the ribosomes in the cytoplasm; these would include all the cellular enzymes, structural proteins in the cells such as keratin, and all other cellular proteins.

Milk proteins and lactose are transported to the apical membrane of the cell via secretory vesicles that bud off of the Golgi; these secretory vesicles are bounded by a lipid bilayer membrane. These secretory vesicles make their way to the apical membrane by a mechanism involving microtubules (made of polymerized tubulin). Tubulin is one of several cytoskeletal proteins which form the cellular scaffolding, providing the cell with structure; keratin is another cytoskeletal protein. The secretory vesicles do not transfer to the basolateral membrane. At the apical membrane, the membrane of the secretory vesicle fuses with the inner surface of the apical membrane, resulting in an opening through which the vesicle contents are discharged into the alveolar lumen.

Glucose to lactose : Glucose enters the cell via the basolateral membrane via a specific transport mechanism. Some glucose is converted to galactose. Both glucose and galactose enter the Golgi and enter into a reaction resulting in formation of lactose (see Lactose Lesson). The formation of lactose in the Golgi results in drawing water into the cell, into the Golgi, and ultimately becoming part of milk. Note that the 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. Note that lactose (and therefore much of the water of milk) is secreted via the secretory vesicles along with the milk proteins.

Milk fat precursors to milk fat : Precursors of milk fat synthesis are also taken up by the epithelial cells at the basolateral membrane. Acetate and ß-hydroxybutyrate are important precursors of fatty acid synthesis in mammary cells in some species (ruminants, especially). These precursors are absorb through the basolateral membrane. In addition, preformed fatty acids, glycerol, and monoacylglycerides are absorbed at the basolateral membrane. All these components enter into the synthesis of triglycerides of milk (see milk Fat Lesson). 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. [Imagine standing inside a balloon and trying to punch your hand through the balloon's wall. The balloon's wall would wrap around your hand.] So, 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. Note that 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.

Transport of Milk Components Not Synthesized in the Epithelial Cells : A number of other components pass across the epithelial cell barrier essentially unchanged from their form in the blood. These include immunoglobulins which bind to specific receptors on the basolateral surface of the cells, are taken "into" the cell in endocytic vesicles, and are transported to the apical side of the cell via the endocytic vesicles (or transport vesicles), where the membrane of the transport vesicles fuses with the inner surface of the apical membrane of the cell and releases the immunoglobulin into the lumen of the alveolus. As the transport vesicles traverse the cell they do not seem to interact with the Golgi, secretory vesicles or the lipid droplets. Some serum albumin may be transported across the epithelial cells by this mechanism. There is not a serum albumin receptor, however, serum albumin molecules probably are internalized into the cell along with the immunoglobulins which are taken up by the transport vesicles.

Paracellular Pathway : Because of the tight junctions between epithelial cells, there is little or no "flow" of anything between the cells, except perhaps water and some ions. Anytime something passes between the cells through the tight junction, this is called the paracellular pathway. When the udder is inflamed, such as during mastitis or involution, or when oxytocin is causing milk ejection, the tight junctions open some or become 'leaky'. This allows lactose and potassium to move from the lumen into the extracellular space, and for sodium and chlorine to move into the lumen from the extracellular space. This results in a change in electrical conductivity of the milk (as used in detecting mastitis), as well as an increase in concentrations of lactose and other milk-specific components in the blood. Lactose can be measured in the urine of a cow during the peripartum period. Milk proteins can be detected in the cow's blood during lactation and early involution.

Other components that can enter the lumen without passing through the epithelial cells are leukocytes (discussed in Mastitis Module). The leukocytes comprise the vast majority of the somatic cells in the milk. These cells pass between the epithelial cells and in the process they "break open" the tight junctions between the epithelial cells and enter via the paracellular pathway. Of course, this also allows other extracellular components like salts to diffuse into the lumen and milk components to diffuse out of the lumen into the extracellular fluid. [This is one reason why there is a change in electrical conductivity in the milk during mastitis].

Synthetic Activity in Mammary Cells : Cells within an alveolus appear to be synchronized in synthetic activity. In some alveoli the cells are all full of lipid droplets and secretory vesicles, while in other alveoli the cells are all devoid of those structures. (synchronization is seen in other tissues such as the seminiferous tubules in the testis)

Secretory activity seems to occur in two phases:

  • Formation of intracellular secretory structures like lipid droplets and secretory vesicles. Progressive distention of alveolar cells. Cells are tall, columnar.
  • Release of products into the lumen. Cells become more cuboidal, lumen fills with milk. Intracellular synthesis may decrease during this time.

If you incubate mammary tissue in the presence of radiolabeled amino acids, the radioactive trace moves through the cell as a pulse.

  • Stays in the cytoplasm ~3-15 min. [Represents in newly synthesized proteins.]
  • Appears in the Golgi within 15-30 min. [Represents newly synthesized proteins being processed.]
  • Label is increased in the lumen 30-60 min. later. [Represents secreted proteins.]

If you inject radiolabeled precursors of milk components into an animal, they are found in milk at varying times.

  • Those that enter by equilibrating across the apical or Golgi membranes (Na+, K+, Cl-) take ~1 hr to reach max. specific activity in the milk.
  • Those that enter by synthesis in the Golgi (lactose, casein, Ca, citrate, phosphate) take 2-3 hr. to reach maximum specific activity in the milk.
  • Those that enter as part of milk fat synthesis take 5-7 hr. to reach maximum specific activity in the milk.

Milk Synthesis Process
Milk Composition & Synthesis