Bovine biology series
Part 39 Mammary gland (5/5)
In this, our fifth and final installment on the mammary gland, we complete a discussion of how milk is actually made in the udder.
Last month we learned how the pairing of a glucose molecule and a galactose molecule makes milk sugar lactose, by the enzyme lactose synthase. This combination takes place in the cellular body known as the Golgi apparatus. Lactose comprises 4.6 percent of the weight of milk.
Protein in milk can be divided into two fractions: casein, at 2.6 percent and whey, at 0.6 percent of the weight of milk.
Milk protein is comprised of amino acids linked together. When food is digested, proteins in the food (feed) are broken down into various amino acids. Once in the bloodstream, these free amino acids travel to every cell in the body, including the epithelial cell in the secretory tissue.
The genetic machinery in the cell, specifically the DNA, governs the formation of milk protein. The cellular DNA transcribes, or copies a molecule of itself that is call RNA, or ribonucleic acid. Once RNA is made, the molecule moves out of the nucleus, the home of DNA, into the cytoplasm, the part of a cell containing other metabolic machinery. The RNA then serves as the blueprint for the translation of protein. Once the protein molecule is assembled from amino acid and peptide precursors supplied by blood flow, the protein is diffused into the forming milk volume in the lumen of the secretory tissue.
Of note here is that the blood serum albumin’s and the very large molecular weight immune globulin’s are not synthesized in the udder, rather they are absorbed into the milk volume directly from the bloodstream.
From a nutritional standpoint, the amino acids getting to the udder can be divided into essential and non-essential. That is, the essential ones, arginine, histidine, isoleucine, leucine, lysine, threonine, methionine and valine must be fed in the ration as they cannot be made from the RNA machinery in the cytoplasm of an epithelial cell. The non-essential ones can of course, so the challenge in protein feeding centers on having these eight essential ones available in the bloodstream so milk protein synthesis can occur normally.
Milk fats in bovine milk are comprised of triglycerides with a large proportion of short chain fatty acids. The carbon chain is as short as 4 but can be as long as 16. These fatty acids are synthesized in the cytoplasm of the epithelial cell.
There are three precursors of milk fat: the first and most major source is acetate (acetic acid) transported from the rumen via the bloodstream. Acetate is part of all carbon chains from 4 through 14, and contributes to those chains having 16 carbon lengths. The second source is longer chained triglycerides, specific fatty acids called palmitic, stearic, oleic and linoleic. The final source is a minor one, that of acetyl CoA that is made from glucose in the epithelial cell.
Acetate is a basic building block of longer chained fatty acids. It has two carbons. And the carbon length of fatty acids in milk fat is generally in-groups of two. That is, butyric acid, 4 carbons; capric acid, 10 carbons; palmitic acid, 16; and stearic acid have 18 carbons. [Of note here is that lactic acid, very much a part of silages and thus a component of dairy rations, has a carbon length of three, thus it is not suited for the building blocks of milk fat as the multiple is wrong, three instead of acetic acid at two.]
In a way the formation of milk fat is nothing more than within the cytoplasm of an epithelial cell, acetate and to some extent other more minor precursors are put together in the formation of various lengths of fatty acids. Then these are made into triglycerides.
What then is a triglyceride? Well, this is a molecule consisting of three fatty acids joined together with a glycerol molecule. Glycerol in the epithelial cell is a derivative of blood glucose primarily and to a lessor extent can be obtained when triglycerides are broken down.
Milk fat is largely triglycerides in their final secretory form. They are stored in fat cells as triglycerides. Upon hydrolysis or breakdown, the three fatty acids are decoupled from the glycerol.
The other components of milk are minerals comprising the ash portion of milk. Minerals are derived in the blood of course, but the relationship between high levels of minerals in the blood and what that resultant level in the milk is not clear. It is known that sodium and potassium are constant in milk, as long as the membrane between the epithelial cell and the lumen is healthy. We know that cows that have an overtly damaged membrane in response to tissue infection (mastitis) have milk much higher in sodium due to the breakdown of membrane integrity. Thus these components plus chlorine help maintain the osmotic equilibrium in milk.
Water that is delivered to epithelial cells from blood moves into the milk volume in equilibrium with blood. So the volume of milk secreted is directly related to the concentration of these minerals in the milk volume.
Vitamins cannot be synthesized in the secretory tissue. They are made by bacteria in the rumen, converted from precursors in the liver, or can enter the bloodstream directly from the ration at the time of digestion.
Finally, this concept: lactation is largely controlled by a wide variety of hormones. Two important ones need some mention.
Prolactin is secreted by the anterior pituitary gland. This protein hormone (also called lactogenic hormone) stimulates and sustains the genetic machinery in epithelial cells to utilize their efforts in making milk.
Growth hormone (also secreted by the anterior pituitary gland) is a significant contributor to milk synthesis, in that this protein hormone partitions energy towards the secretory tissue of the cow, away from its other needs, such as adding body weight or supporting a pregnancy. Or said this way, growth hormone stimulates milk production at the expense of other energy demands. This obligates the manager of the cow, of course, to maintain the ration intake in such a manner that there is adequate energy for body weight, pregnancy, and general maintenance.
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