Milk Biosynthesis Milk is synthesized in the mammary gland. Within the mammary gland is the milk producing unit, the 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, in turn, surrounded by a layer of myoepithelial cells and blood capillaries.
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. Route I: Amino acids-poly----------- ribosomes/RER----Golgi bodies-----secretory vesicles—casein micelle Route II: Preformed fatty acids, glycerol, monoacylglycerides /acetate/ ß-hydroxybutyrate---SER--------MFG Route III: Glucose--galactose—Golgi apparatus--- secretory vesicles--lactose Route IV: Immunoglobulins ---- endocytic vesicles-------------- immunoglobulins Route V: Paracellular Pathway for leukocytes/ somatic cells, salts
The raw materials for milk production are transported via the bloodstream to the secretory cells. It takes 400-800 L of blood to deliver components for 1 L of milk. • Proteins: building blocks are amino acids in the blood. Casein micelles, or small aggregates thereof, may begin aggregation in Golgi vesicles within the secretory cell. • Lipids: • 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: milk is in osmotic equilibrium with the blood and is controlled by lactose, K, Na, Cl; lactose synthesis regulates the volume of milk secreted • The milk components are synthesized within the cells, mainly by the endoplasmic reticulum (ER) and its attached ribosomes. The energy for the ER is supplied by the mitochondria. The components are then passed along to the Golgi apparatus, which is responsible for their eventual movement out of the cell in the form of vesicles. Both vesicles containing aqueous non-fat components, as well as liquid droplets (synthesized by the ER) must pass through the cytoplasm and the apical plasma membrane to be deposited in the lumen. It is thought that the milk fat globule membrane is comprised of the apical plasma membrane of the secretory cell.
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.
Milk is the source of nutrients and immunological protection for the young cow. The gestation period for the female cow is 9 months. Shortly before calving, milk is secreted into the udder in preparation for the new born. At parturition, fluid from the mammary gland known as colostrum is secreted. This yellowish coloured, salty liquid has a very high serum protein content and provides antibodies to help protect the newborn until its own immune system is established. Within 72 hours, the composition of colostrum returns to that of fresh milk, allowing to be used in the food supply. The period of lactation, or milk production, then continues for an average of 305 days, producing as much as 9000 or more kg of milk. This is quite a large amount considering the calf only needs about 1000 kg for growth. Within the lactation, the highest yield is 2-3 months post- parturition, yielding 40-50 L/day. Within the milking lifetime, a cow reaches a peak in production about her third lactation, but can be kept in production for 5-6 lactations if the yield is still good.
About 1-2 months after calving, the cow begins to come into heat again. She is usually inseminated about 3 months after calving so as to come into a yearly calving cycle. Heifers are normally first inseminated at 15 months so she's 2 when the first calf is born. About 60 days before the next calving, the cow is dried off. There is no milking during this stage for two reasons: • milk has tapered off because of maternal needs of the fetus • udder needs time to prepare for the next milking cycle • The life of a female cow can be summerized as follows: • Age • 0 Calf born15 mos Heifer inseminated for first calf24 mos First calf born - starts milking27 mos Inseminated for second calf34 mos Dried off36 mos Second calf born - starts milkingCycle repeats for 5-6 lactations.
References: http://www.mkgandhi.org/health/diet_reform/32cowmilk.html http://classes.ansci.illinois.edu/ansc438/milkcompsynth/histology_6.html https://www.uoguelph.ca/foodscience/dairy-science-and-technology/milk-production-and-biosynthesis http://www.diffen.com/difference/Buffalo_Milk_vs_Cow_Milk http://en.wikipedia.org/wiki/Dairy_cattle http://www.slowfood.com/slowcheese/eng/12/many-breeds http://en.wikipedia.org/wiki/Milk http://www.agritech.tnau.ac.in/animal_husbandry/animhus_cattle%20_breed.html http://en.wikipedia.org/wiki/Dairy_cattle http://animals.pawnation.com/cattle-breeds-4731.html http://www.ansi.okstate.edu/breeds/