Transcript Adipose Tissue Growth and Development

Adipose Tissue Growth and
Development
Fall 2006
Intro
 Adipose tissue is an anatomical term 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.
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 postprandial blood sugar)
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
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
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
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
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
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
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
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
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
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
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
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
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.
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
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
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
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
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
Factors Affecting Adipose Tissue
Composition and Lipid Metabolism
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Cellularity and Age
Anatomical location
Species
Sex
Genetics
Hormones
Nutrition
Environmental Temperature
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
Anatomical Location
 Characteristics of Adipose tissue from
various anatomical locations in pigs
 Tables 7.2 and 7.3
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
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
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
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
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
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
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
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