Adipose Tissue Growth and Development

1. Adipose Tissue Growth and Development Fall 2006

2. Intro • Adipose tissue is an anatomicalterm for loose connective tissue composed of adipocytes. Its main role is to store energy in the form of fat, although it also cushions and insulates the body • Free fatty acid is "liberated" from lipoproteins by lipoprotein lipase (LPL) and enters the adipocyte, where it is reassembled into triglycerides by esterising it onto glycerol.

3. Intro • Fat cells have an important physiological role in maintaining triglyceride and free fatty acid levels, as well as determining insulin resistance. • Factors that have been suggested to contribute to the development of obesity include: • Genetic factors and some genetic disorders (e.g., Prader-Willi syndrome) • Underlying illness (e.g. hypothyroidism) • Certain medications (e.g., atypical antipsychotics) • Sedentary lifestyle • A high glycemic diet (i.e., a diet that consists of meals that give high postprandialblood sugar)

4. Intro • Adipose tissue is a modified connective tissue that serves as an energy reserve • Energy dense refers to the amount of heat that can be expended and is greater for fats and lipids than proteins and CHO’s • Provides insulation, support, protection of vital organs and imparts varying flavors in meat products • Is a source of hormones

5. Anatomy of Fat • Occurs in specific sites called adipose tissue depots or fat depots • Relative size of the depots differ across species (Fig. 7.1) • Is seen in most all areas of the body • Visceral (around stomach and vital organs) • Subcutaneous • Between and within muscles

6. Visceral fat • Associated with the viscera • Mesenteric – located around the intestines • Lace fat • Caul – thin sheet of fat located over the stomach • Perirenal fat – kidney fat • Leaf fat – located between the lining of the thoracic cavity and the ribs

7. Subcutaneous Fat • Fat beneath the skin – backfat, external fat • Either second or third to be deposited • Consist of three layers • Outer layer – first to develop and acts as insulation • Middle layer – second layer to develop and is usually the thickest. Represents the bulk of the subcutaneous fat • Inner layer – last to develop and is very thin

8. Intermuscular Fat – seam fat • Represents the second or third fat deposited • Located between the muscles • Closely associated with the with the epimysium of the muscles

9. Intramuscular fat • Latest to be deposited and is within each muscle • Contributes the least to total carcass fat • Associated with the perimysium which surrounds the bundles of the muscles • Fatty degeneration – invasion of the bundles where fat is deposited in the endomysium surrounding individual muscle fibers

10. Adipogenesis • Origination of adipose tissue • Mesenchymal cells associated with connective tissue sites give rise to adipoblasts which are the early form of fat cells • Increased vascularization of connective tissue is seen at this time • Following early vascularization, a collection of adipoblasts (lobules) form and give rise to a larger lobe with a sheath of collagen fibers

11. Adipocyte Hyperplasia and Hypertrophy • Cattle and sheep – most adipocytes have unilocular (containing a single cavity) lipid at birth • Swine – multilocular lipid is seen • Hyperplasia occurs prenatally and proliferation (increased growth of cells) of the cell ceases once it differentiates into an aidpocyte

12. Hyperplasia and Hypertrophy of Fat Cells • The source of new cells is associated with connective tissue • The precursor differentiates itself with adequate nutrition and time • Increased age yields hypertophic growth in the presence of sufficient energy for fat deposition. • Hypertrophy is accomplished by the increase in diameter and volume of adipocytes • Repeated phases of hyperplasia may occur and are evident in obese individuals

13. Brown Fat Differentiation • Brown versus white fat cells • Brown cells contain more and larger mitochondria • Important to neonatal survival from the generation of heat • May serve as precursors for white fat cells

14. Adipose Tissue Metabolism • Triglycerides constitute the major form of lipid stored whereas phospholipids and sterols are important but not as abundant as Triglycerides • Rate of fat deposition • Absorption from the blood • Fatty acid synthesis and Triglyceride formation • Lipolysis

15. Adipose Tissue Metabolism • Lipogenesis • All events involved in absorption of nutrients from the blood and synthesis and esterification to form Triglycerides in an adipocyte • Lipolysis • Process of mobilization or release of fatty acids from Triglycerides • Net lipid acretion is equal to the rate of lipogenesis minus the rate of lipolysis

16. Lipogenesis • Major site is dependent on species • Mammary gland • Adipose tissues • Blood triglycerides come from either food ingested or hepatic synthesis • Fatty acids and glycerol are split from circulating triglycerides by an enzyme lipoprotein lipase • The liberated F.A.’s diffuse across the endothelium. • Glycerol is not metabolized by adipocytes and is targeted to the liver.

17. Lipogenesis • Once the fatty acids are in the adipocyte, they may be esterfide into triglycerides using glucose as the precursor of the glycerol backbone • Dietary fat serves as a second source of circulating fatty acids. • They are transported from the small intestine to the adipose tissue as triglyceride into lipoproteins called chylomicrons

18. Lipogenesis • Many times the dietary type of fatty acid is represented by the type of fatty acid in the adipose tissue • This occurs more in monogastrics • This does not occur in ruminants because the rumen changes unsaturated fats to saturated fatty acids • Fatty acids that are higher in unsaturation will have lower melting points and will affect the texture of the fat and thus be softer in chilled temperature. Ie. Pork fed peanut meal

19. Lipogenesis • Non-ruminants use glucose as a lipogenesis substrate whereas ruminants use acetate (a VFA produced in rumen fermentation) • However, young ruminants that are digesting like monogastrics have sufficient glucose to support lipogenesis

20. Lipolysis • Again, the release of fatty acids from adipocytes by breaking down triglycerides into fatty acids and glycerol where the fatty acids diffuse across cell membranes. • Enzymes such as lipases are responsible for this action

21. Composition of Adipose Tissue • 76-94 % lipid • 1-4 % protein • 5-20 % water • One of the most abundant types of fatty acids is oleic acid • Palmitodiolein is one palmitic and two oleic fatty acids combined • Palmitic and stearic are two saturated fatty acids that are abundant while oleic is an unsaturated fat that is most abundant

22. Factors Affecting Adipose Tissue Composition and Lipid Metabolism • Cellularity and Age • Anatomical location • Species • Sex • Genetics • Hormones • Nutrition • Environmental Temperature

23. Factors Affecting Adipose Tissue Composition and Lipid Metabolism • Cellularity and Age • Lipids, by % of tissue weight, are greater compared to water and protein for older animals than younger animals. • This is because of fat cell size, which increases with age • Triglycerides also increase in proportion to phospholipids with increased age • Both lipogenesis and lipolysis are occuring simultaneously yet when fats are increasing in deposition it is a result of greater lipogenesis

24. Anatomical Location • Characteristics of Adipose tissue from various anatomical locations in pigs • Tables 7.2 and 7.3

25. Species • Species differ based on differences in lipid metabolism • Hydrogenation of fatty acids by rumen microbes has a significant effect on the type of fatty acids available for triglyceride synthesis • Various species grow at different rates- rate of maturity vs number of adipocytes

26. Genetics • Fatter genetic lines possess a higher percentage of lipid in backfat than leaner genotypes • Leaner genotypes are associated with changes in adipose cellularity • Less adipocyte hypertrophy is a result of less recruitment of new adipocytes

27. Sex • First implies a hormonal status • Usually testosterone and estrogen are involved • Testosterone tends to inhibit while estrogen increases the size of adipocytes • Females contains more lipids

28. Hormones • Leptin- a hormone discovered while studying obesity and diabetic animals • Produced by adipocytes and circulating concentrations and are related to adipose tissue mass • Levels are elevated in obese animals and increase with overfeeding • Levels are low in leaner animals

29. Nutrition • The composition of the diet has a drastic impact on adipose tissue composition and lipid metabolism • High fat diets depress fatty acid synthesis, esp. in non-ruminants • Fatty acid composition impacts the composition of triglycerides that are stored in the adipocytes

30. Nutrition • Lambs fed a maintenance diet versus ad libitum can demonstrate how fat depots responds to nutritional regime • Figure 7.12 • Increased marbling scores are a result of high planes of nutrition • Diets with a.a. deficiencies will result in increased lipogenesis because lean tissue accretion cannot be maximized

31. Nutrition • Conjugated linoleic acid are isomers of linoleic acid and when fed to animals such as pigs and chickens, we see a reduction in fat deposition and it alters the f.a. profile

32. Environmental Temperature • Outside of the thermoneutral zone will have altered metabolism as the animals tries to adapt to the adverse environment • If the temp is lower, then animals will mobilize adipose tissue to support more heat production • Thus, feed intake will increase and increased energy requirements will be merited • If temp increases above the comfort zone, then feed intake decreases and inhibits generation of heat