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Milk Composition & Synthesis
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Milk also contains a range of other components. Milk contains leukocyte cells, also known as somatic cells in cow milk. The image to the right includes several types of milk leukocytes (marcophages, lymphocytes, neutrophils, and eosinophils). Milk also contains some sloughed off epithelial cells from the teat inner lining, the ducts, and the alveoli.

The concentration of leukocytes in milk varies with the species (human milk has relatively high somatic cell counts; cow milk from healthy glands has low somatic cell counts), infection status of the gland, and stage of lactation.

 

Milk has numerous other components, many of which are grouped within the major biochemical components listed on other pages. These may include bioactive factors such as hormones and growth factors, enzymes, cellular proteins, and others.

For example, small amounts of cellular metabolites from glycolysis, citric acid cycle, pentose phosphate shunt, RNA and DNA synthesis and fatty acid synthesis are found in milk. Also, urea, ammonia and CO2. Milk glucose levels are low when compared with lactose levels and they may reflect intracellular glucose levels. Milk has a very low oxygen tension.

Milk may contain infectious agents, as well. For example the AIDS virus has been shown to be passed from mother to infant via the milk. In this particular case the virus was probably transmitted inside leukocytes normally found in the mother's milk. (See Vandeperre, et al 1991 New England J. Medicine 325:593; Nagamine et al 1991 J. Infect. Dis. 164:1024). Other viruses and bacteria may be exreted through the mammary gland into milk.

Remember that the mammary gland is an excretory gland. As such, most anything that is found in the blood of the lactating animal will be present in milk to one extent or another. This also becomes relevant when considering transfer fo toxins from mother to offspring via milk. Any lipophilic substance is likely to be sequestered in the milk fat and secreted in milk. Most aqueous-soluble compounds also are secreted, including a range of toxins. Interestingly, the mammary gland seems to act as a partial buffer to secretion of excess arsenic and lead. Although milk concentrations of As and Pb are increased in animals exposed to excess of these metals, the relative increase in milk As or Pb is lower than for other minerals.


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