BIOSYNTHESIS OF MILK COMPONENTS - Scribd
Milk biosynthesis – aspidwefulsitanecrepuhojec
Biosynthesis and Secretion of Milk / Diseases eBook: Bruce L
The udder is known as an exocrine gland because milk is synthesized in specialized cells grouped in alveoli, and then is excreted outside the body through a duct system that functions like the tributaries of a river.
Milk production demands a lot of nutrients that are brought to the udder by the blood. To produce 1 kg of milk, 400 to 500 kg of blood must pass through the udder. In addition, the blood carries hormones that control udder development, milk synthesis, and the regeneration of the secretory cells between lactations (during the dry period).
The alveolus is a functional unit of production in which a single layer of milk secretory cells are grouped in a sphere with a hollow center. Capillary blood vessels and myoepithelial cells (muscle-like cells) surround the alveolus, and the secreted milk is found in the internal cavity (lumen).
The functions of the alveolus are:
Pregnancy has an inhibitory effect on milk yield. Most of the reduction in milk yield occurs after the fifth month of pregnancy. By the 8th month of pregnancy, milk yield may be 20% less for that month compared with a non-pregnant cow. The inhibitory effect of pregnancy is not likely due to fetal requirement, which does not increase considerably until the last two months of pregnancy. It is believed that the increase in estrogen and progesterone levels as pregnancy progresses, inhibits milk secretion.
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Age and body weight at calving
The amount of milk produced by the cow increases with advancing lactations (age). This is due in part to an increase in body weight, which results in a larger digestive system and a larger mammary gland for the secretion of milk. Another reason for increased milk production with age is due to the effects of recurring pregnancies and lactations. Data on milk production with cows suggest that 20% of the increase in milk production is due to increased body weight and 80% to the effects of recurring pregnancy and lactations. Recurring pregnancies and lactation can result in increases of 30% in milk production from the first to the fifth lactation.
Cows are usually milked twice daily. Milking twice a day yields at least 40% more milk than once a day. Increasing milking frequency to 3 x day increases milk yield by up to 20% (range 5-20%). The increase is usually highest for first lactation cows and declines as the cow gets older. The most likely reasons for increased milk production as frequency of milking increases are:
1) less intramammary pressure generated with frequent milking
2) increased stimulation of hormone activity favorable to milk production
3) less negative feedback on the secretory cells due to the accumulation of milk components.
The practice has been implemented to increase milk yield and utilize facilities more efficiently. However, the practice is still being debated. There are several problems associated with 3 x per day milking. It may increase the incidence of mastitis as the cows get exposed to the milking machine more often. The practice is also not recommended for poorly managed herds as existing problems will be aggravated.
Physiology of Milk Synthesis - PowerPoint PPT Presentation
After calving, lactation continues for an extended period, in the dairy cow typically 300 days. Hormonal influences are critical to the maintenance of lactation. A few weeks after parturition the cow will begin her oestrus cycle again and show heat. She will be usually be artificially inseminated at an appropriate heat around 70-90 days after calving. The objective is to get the cow calving about once per year. Milk yield declines as pregnancy progresses. In fact the hormonal changes during pregnancy and the increasing flow of nutrients diverted towards the foetus serve to down regulate milk secretion.
At around 300 days milking is usually stopped and the cow “dried off”.
During pregnancy, the final development of the mammary gland occurs to prepare for milk production (lactogenesis = process by which mammary alveolar cells acquire the ability to secrete milk), which commences as the calf is born. The first milk produced is called colostrum and is particularly rich in antibodies.
Biosynthesis and secretion of milk proteins: a review.
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The oxytocin effect usually takes place 20 to 40 seconds after the initial stimulation (e.g. udder wiping) and lasts for only about 6 minutes. It is most important that the milk is removed while this reflex is operating. The udder should be handled no earlier than 60-90 seconds before teat cup application, otherwise the beneficial effects of good milk ejection will be lost. Once the effect of oxytocin wears off, milk that has not been removed will flow back from the udder sinuses into the ducts.
The extra pressure within the udder will reduce the synthesis of milk after the incomplete milking, and if this situation occurs repeatedly the cow will reduce her yield accordingly. In dairy cows the milk ejection reflex is probably essential for 50 to 60% of the total yield at a milking.
The composition and biosynthesis of milk lipids
Synthesis of protein
The caseins and the serum proteins (such as ß-lactoglobulin, and α-lactalbumin) found in the milk are synthesized from the amino acids taken up from the blood. These proteins, synthesized in the rough endoplasmic reticulum (RER), are packed in micelles before they are released in the lumen of the alveolus.
The immunoglobulins are synthesized by the immune system, and these usually large proteins are drawn from the blood into the milk. The permeability of the secretory cells to immunoglobulins is high during the synthesis of colostrum, but decreases sharply with the onset of lactation.
Synthesis of fat
Acetate and butyrate produced in the rumen are used, in part, as the building blocks of the short-chain fatty acids found in milk. About 17-45 % of the fat in the milk is built from acetate and 8-25 % from butyrate. Diet composition has a strong influence on milk fat concentration. Lack of fiber depresses the formation of acetate in the rumen, which in turn results in the production of milk with a depressed concentration of fat (2-2.5 %).
In general, only half the amount of fatty acids in milk fat is synthesized in the udder, the other half comes from the predominantly long-chain fatty acids found in the diet.
Biosynthesis of Milk Proteins - Edinburgh Research Explorer
Regulation of milk volume
The amount of milk produced is controlled primarily by the amount of lactose (glucose + galactose) synthesized by the lactose synthase enzyme in the Golgi apparatus. Lactose secretion into the cavity of an alveolus increases the concentration of dissolved substances (osmotic pressure) relative to the other side of the secretory cells where the blood flows. As a result, the concentration of dissolved substances on each side of the secretory cells is balanced by drawing water from the blood and mixing with the other milk components found in the cavity of the alveolus.
For normal milk, a balance is reached when there is 4.5 to 5% lactose in the milk. Thus lactose production acts as a “valve” that regulates the amount of water drawn into the alveoli and therefore the volume of milk produced.
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