Bovine biology series
Part - 38 Mammary gland (4/5)
This is the fourth installment of five lessons on the mammary gland. In these two final lessons, we are going to get to the crux of the matter....how milk is actually made in the udder.
We already know that milk is made in the secretory tissue of four mammary glands that comprise the udder. But what is secretory tissue?
This tissue is remarkable in that hormonal control, active blood flow, smooth muscle influence, and the physical action of the nerve cells all work in concert to supply the required nutrients, stimuli and energy so that milk can be made. And we know that somewhere between 500 to 650 parts of blood flow through this tissue so that milk is made. Again, the evolution of the mammary gland in mammals is truly remarkable at the cellular level, as well as macroscopically when we judge an udder for support, size, teat placement so that it may hold in a lifetime of production 100,000, 200,000 pounds of milk or more!
Let us go inside the secretory tissue.
When an animal begins lactating, there is considerable increase in food and water intake, so that after clearing the liver, these nutrients circulate in the bloodstream. Even the digestive system of the animal is altered so that a higher level of nutrients can be absorbed. And as you might expect, other peripheral tissues, such as muscles and the reproductive portion of an animal, have reduced levels of blood flow so that there is more available for the mammary glands. Or said this way, if other requirements are put on the cow, like pregnancy, or walking great distances, or the stress of breathing poor quality air, then the animal will adjust accordingly.....and blood flow is adjusted away from the udder.
The point is simple: at the secretory tissue level, maximum blood flow is required so those nutrients are ready to flow from blood to milk. We call these blood nutrients precursors, because their ultimate fate is something else, in this case milk.
In a single alveolar epithelium cell (alveolus), blood flows nearby, as does a nerve cell and some part of a smooth cell.
The membrane that is separating the blood vessel and the alveolus is porous, of course. Just as the membrane of the cell is porous so that the newly made milk can leave the alveolus and be stored in the lumen or opening. Or said this way: on one side of the alveolus are blood vessels that supply precursors; on the opposite side is an opening - lumen for which newly synthesized milk is stored until it is removed by physical action.
Glucose and acetate, a volatile fatty acid both circulate in the bloodstream. They enter the alveolus and make their way to the mitochondria, a spot analogous to an energy-producing plant. Here, they drive the metabolic processes so that as other precursors from the blood are delivered to the alveolus, the various molecules of protein, sugar and lipid can be made.
One interesting note: the secretory tissue spares the consumption of glucose so that enough is left circulating in the body, maintaining a level of 40-60 mg/100 ml blood.
Why? Well, if the secretory tissue scavenged all of the glucose in the blood as it traveled through the gland, soon the animal might go into shock for a lack of blood glucose available for the brain, or heart, or other muscle movement. So in times of high milk production, the secretory tissue will utilize more acetate and spare glucose for peripheral use.
However, glucose is used nevertheless in the alveolus, especially in the formation of lactose.
Lactose, or milk sugar, is a disaccharide composed of one molecule of glucose and another of galactose. Glucose is the only precursor of lactose; thus it cannot be made from a fatty acid like acetate. Two molecules of glucose enter the alveolus; one is converted to galactose. The enzyme lactose synthase catalyzes the reaction of these two molecules, forming lactose. This combination takes place in the part of the alveolus called the Golgi apparatus.
The enzyme lactose synthase is composed of two subunits: galactosyl transferase, found in almost all body cells as well as secretory cells, and alpha-lactalbumin, a whey protein. This second subunit is found only in secretory alveolus.
What is interesting about this enzyme is that progesterone, the pregnancy hormone, suppresses the synthesis of alpha-lactalbumin. This partially explains why the animal, if pregnant, will produce less milk for a lack of this one enzyme. Too, that is why at the onset of lactation at calving (and the rapidly increasing levels of prolactin, or the milk hormone), progesterone levels are no longer required to support a fetus, so the level is dropped and thus more alpha-lactalbumin is available.
Again, a delicate balance exists between how much glucose is withdrawn in the secretory tissue of the mammary glands, and how much is available for doing other things. That is why a bottle of CMPK has dextrose in it, so that it may converted into glucose and thus, support the overall well being and demand of the cow.
What is truly remarkable is that the acetate molecule can serve as a different type of energy producing role so that glucose is spared for lactose synthesis. The animal avoids hypoglycemia.
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