الفهرس 
GENERAL ABSTRACTOnly 14 pages are availabe for public view
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Abstract
Milk fat is the most variable component of milk, it is easily changed by nutritional and environmental factors rendering it a central point of interest for the daily market and scientists. Dietary manipulation has profound influence on milk fat synthesis and its fatty acid profile. However, our basic understanding of how dietary manipulations regulates mammary lipogenesis is limited by the use of mammary biopsy to obtain representative tissue sample for gene expression analysis, as repeated biopsies over short time interval from the same animal is not possible.
Therefore, the general objective of this dissertation was to measure the sequential changes in milk fat concentration, milk fatty acid profile and mammary lipogenic gene expression that occur during the transition from normal to dietary induced either elevated or depressed milk fat concentration, using new non-invasive alternative technique by extracting RNA from cytosolic crest trapped in large fat globules. Two experiments were conducted: 1) feed restriction that induce negative energy balance, body fat mobilization, and elevated milk fat concentration, 2) dietary induced milk fat depression that resulted in depression in milk fat concentration.
In the first study ten multiparous Holstein cows were used in completely randomized repeated measure design, cows were blocked by parity and milk yield, ca. 36.4 kg/d milk and 84 (±17) days in milk at the start of the experiment. All cows were fed ad- lib the basal control diet for the first 14 days of the experiment, and data recorded during this period were used as covariate in the statistical analysis. This was followed by 4 days of feed restriction to only 60 percent of the estimated ad libitum intake for 5 cows and the other 5 cows continue receiving their ad libitum intake of their basal diet. Then at day 19 to day 20 all cows were fed ad libitum the same basal diet to follow up for any carry over effect of feed restriction.
During the whole experiment course, blood serum NEFA, milk composition, milk fatty acid, and mammary lipogenic gene expression were measured. Feed restriction resulted in increase in serum NEFA, milk fat percent and preformed fatty acids, and downregulation of several mammary lipogenic genes, associated with decrease in milk yield and de novo synthesized fatty acids. The downregulation of the selected mammary lipogenic genes is in agreement with previous fed restriction studies used mammary biopsy technique to study gene expression in the mammary gland.
In the second study milk fat depression was dietary induced using CLA supplement. Ten multiparous Holstein cows averaging 39.9 kg milk and 100 (±11) days in milk at the start of the experiment were used in completely randomized design. At the first 18 days of the experiment all cows were fed the basal control diet. Data recorded during this period were used as covariate in the statistical analysis. from day 19 to day 24, cows were split into two equal groups, one group continue feeding on the basal diet and the other group received the basal diet topdressed with CLA supplement, at dose of 200g/d. At day 25 the CLA supplementation was stopped and all cows received the basal control diet and data recorded at day 25 to 27 were used to investigate for carry over effect, milk yield was recorded during the whole experiment period plus extra 4 days, to follow up the complete recovery of milk yield. Top dressing the diet with CLA supplement for 6 consecutive days resulted in moderate reduction in milk fat percentage and yield, associated with moderate reduction in de novo synthesized fatty acids, and increase in milk yield. Due to limitation of the invasive mammary biopsy technique, the effect of short term dietary manipulation of mammary lipogenic gene expression was difficult to be investigated. from these two experimental models we conclude that extraction of RNA from cytosolic crest trapped in milk fat globule during its formation, can be potent non-invasive alternative to mammary biopsy, and can reflect the changes in mammary lipogenic gene expression in mammary epithelial cells. Thus, can be used successfully to study the very short and short term dietary changes and mammary gene expression interaction.