A specialized type of connective tissue in which adipocytes or fat cells predominate Located in many areas throughout the body A dipose tissue represents 1520 of the body weight in men ID: 524602
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Slide1
Chapter 6 Adipose TissueSlide2
A
specialized type of connective tissue in
which
adipocytes
or fat cells predominate.
Located
in many areas throughout the body,
A
dipose
tissue represents 15–20% of the body weight in men.
in women of normal weight, 20–25% of body weight
. Slide3
key regulators of the body’s energy
metabolism.
Because
of a growing worldwide epidemic
of
obesity
and its associated problems,
including diabetes and heart disease
,
adipocytes
are now the most widely studied
cell
of
connective tissue.Slide4
Adipose tissue is the largest repository of energy (in the form of
triglycerides
, the neutral fats) in
thebody. The other organs that store energy, notably the liver and skeletal muscle, do so in the form of glycogen.However the supply of glycogen is limited and a large store of calories must be mobilized between meals. Because triglycerides are of lower density than glycogen and have a higher caloric value (9.3 kcal/g for triglycerides versus 4.1 kcal/g for carbohydrates), adipose tissue has evolved as a very efficient storage tissue.Slide5
A
dipocytes
themselves release hormones and a number of important factors.With its unique physical properties, adipose tissue or fat is a poor heat conductor.and it contributes to the thermal insulation of the body.It fills up spaces between other tissues and helps to keep some organs in place. Subcutaneous layers of adipose tissue help to shape the surface of the body, whereas deposits in the form of pads act as shock absorbers, chiefly in the soles and palms.Slide6
with
different
locations
, structures, colors, and pathologic characteristics. White adipose tissue, the more common type, is composed of cells that, when completely developed, contain one large central droplet of whitish-yellow fat in their cytoplasm. Brown adipose tissue contains cells with multiple lipid droplets interspersed among abundant mitochondria, which give these cells the darker appearance. Both types of adipose tissue have a rich blood supply.
Two
types of adipose tissue Slide7
White Adipose Tissue
Specialized for long-term energy storage, white adipose cells are spherical when isolated but are polyhedral when closely packed in adipose tissue. Each cell is very large, between 50 and 150 m in diameter and contains one huge droplet of lipid that makes up 85% of the cell's weight.
called
unilocular because triglycerides are stored in a single locus. Since lipid is removed from cells by the alcohol and xylene used in routine histological techniques, Slide8
A
unilocular
adipocyte appears in standard microscope preparations as a thin ring of cytoplasm surrounding the empty vacuole left by the dissolved lipid droplet, sometimes referred to as the signet ring cell. The large droplet causes these cells to have eccentric and flattened nuclei.The rim of cytoplasm that remains after removal of the stored triglycerides may rupture and collapse, distorting the tissue structure.Slide9
Figure 6–1Slide10Slide11Slide12Slide13Slide14
The thickest portion of the cytoplasm surrounds the nucleus of these cells and contains a Golgi apparatus, mitochondria, poorly developed
cisternae
of the rough ER, and free
polyribosomes. The rim of cytoplasm surrounding the lipid droplet contains cisternae of smooth ER and numerous pinocytotic vesicles. TEM studies reveal that each adipose cell usually possesses minute lipid droplets in addition to the single large droplet seen with the light microscope; the droplets are not enveloped by a membrane but show many vimentin intermediate filaments in their periphery. Each adipose cell is surrounded by a thin external or basal lamina.Slide15
White adipose tissue is subdivided into incomplete lobules by a partition of connective tissue containing a rich vascular bed and nerve network.
Fibroblasts, macrophages, and other cells make up about half the total number of cells.
Reticular
fibers form a fine interwoven network that supports individual fat cells and binds them together. Although blood vessels are not always apparent in tissue sections, adipose tissue is richly vascularized.Slide16
The
color
of freshly dissected white adipose tissue depends on the diet and varies from white to bright yellow, due mainly to the presence of
carotenoids dissolved in the fat droplets. Almost all adipose tissue in adults is of this type and it is found in many organs throughout the body. Age and gender determine the distribution and density of adipose deposits.Slide17
In the newborn, white adipose tissue has a more uniform thickness throughout the body.
As the child matures, the tissue tends to disappear from some parts of the body and increase in others.
Its distribution is partly regulated by sex hormones, which control adipose deposition in the breasts and thighs.
Slide18
Storage & Mobilization of Lipids
The white adipose tissue is a large depot of energy for the organism.
The lipids stored in adipose cells are chiefly triglycerides,
ie, esters of fatty acids and glycerol. Triglycerides stored by these cells originate in dietary fats brought to adipocytes as circulating chylomicrons, in triglycerides synthesized in the liver and transported to adipose tissue in the form of very low-density lipoproteins (VLDLs), and by the local synthesis of free fatty acids and glycerol from glucose to form triglycerides.Slide19
Chylomicrons
(=small) are particles up to 3 m in diameter, formed in intestinal epithelial cells and transported in blood plasma and mesenteric lymph.
They consist of a central core, composed mainly of triglycerides and a small quantity of cholesterol esters, surrounded by a stabilizing monolayer consisting of
apolipoproteins, cholesterol, and phospholipids. VLDL are smaller than chylomicrons (providing a greater surface-to-volume ratio) and have proportionately more lipid in their surface layer. VLDL also have different
apolipoproteins
at the surface and contain a higher proportion of cholesterol esters to triglycerides than do
chylomicrons
.Slide20
Lipid
storage and mobilization from
adipocytes
. Triglycerides are transported by lymph and blood from theIntestine and liver in lipoprotein complexes known aschylomicrons
(
Chylo
)
and very
low—density
lipoproteins
(VLDLs).
In capillary endothelial cells of adipose tissue,
these
complexes are
partly
broken down by lipoprotein
lipase,
releasing
free fatty
acids and
glycerol. Slide21
The free fatty acids diffuse from the capillary into the
adipocyte
,
where they are re—esterified to glycerol phosphate, formingtriglycerides. These resulting triglycerides are stored in droplets until needed. Norepinephrine from nerve endings stimulates the cyclic AMP(cAMP
)
system,
which
activates hormone—sensitive lipase
to hydrolyze the
stored
triglycerides to free fatty acids and glycerol.
These substances diffuse into the capillary, where
free fatty
acids
bind to
albumin for transport to distant sites for use as an
energy
source
.Slide22Slide23
Adipose tissue also functions as an important endocrine organ.
Adipocytes
are the sole source of the 16
kDa polypeptide hormone leptin (Gr. leptos, thin), a "satiety factor" with target cells in the hypothalamus and other organs, which regulates the appetite under normal conditions and participates in regulating the amount of adipose tissue.Slide24
Although all white adipose tissue appears
histologically
and physiologically similar, differences in
gene expression have been noted between visceral deposits (in the abdomen) and subcutaneous deposits of white fat. Such differences may be important in the medical risks of obesity; it is well-established that increased visceral adipose tissue raises the risk of diabetes and cardiovascular disease whereas increased subcutaneous fat does not.
The
release of visceral fat products directly to the portal circulation and liver may also influence the medical importance of this form of obesity.Slide25
In response to body needs, lipids are mobilized uniformly in all parts of the body, although adipose tissue in the palms, soles, and
retroorbital
fat pads resists long periods of starvation. After periods of starvation, most
unilocular adipocytes lose nearly all their fat and become polyhedral or spindle-shaped cells with very few lipid droplets.Slide26
Histogenesis
of White Adipose Tissue
Like the
fiber-producing cells of connective tissue, adipocytes undergo differentiation from embryonic mesenchymal cells. Such differentiation is first seen with the appearance of lipoblasts. Early lipoblasts have the appearance of fibroblasts but are able to accumulate fat in their cytoplasm. Lipid accumulations are isolated from one another at first but soon fuse to form the single larger droplet that is characteristic of
unilocular
adipose tissue
cells.
Slide27Slide28
Brown Adipose Tissue
The
color
of brown adipose tissue or brown fat is due to both the numerous mitochondria (containing colored cytochromes) scattered through the adipocytes and the large number of blood capillaries in this tissue. Adipocytes of brown fat contain many small lipid inclusions and are therefore called
multilocular
.
The
many small lipid droplets, abundant mitochondria, and rich vasculature all help mediate this tissue's
principal
function of
heat production
. In comparison with white adipose tissue, which is present throughout the body, brown adipose tissue has a much more limited distribution.Slide29
Cells of brown adipose tissue cells are polygonal and generally smaller than cells of white adipose tissue but their cytoplasm contains a great number of lipid droplets of various
sizes.
These
adipocytes
have spherical and central nuclei and the numerous mitochondria have abundant long
cristae
.
Slide30Slide31
Figure 6–4.Slide32
Brown adipose tissue resembles an endocrine gland in that its cells assume an almost epithelial arrangement closely associated with blood capillaries.
The
tissue is subdivided by partitions of connective tissue into lobules that are better delineated than the lobules of white adipose tissue.
Cells of this tissue receive direct sympathetic innervation.Slide33
Function of Brown
Adipocytes
The
main function of the multilocular adipose cells is to produce heat by nonshivering thermogenesis. The physiology of multilocular adipose tissue is best understood in the study of hibernating species.
In
animals ending their hibernation period, or in newborn mammals (including humans) that are exposed to an environment colder than the mother's uterus, nerve impulses liberate
norepinephrine
into brown adipose tissue.Slide34
unlike
white fat, liberated fatty acids of
multilocular
adipocytes are quickly metabolized, with a consequent increase in oxygen consumption and heat production, elevating the temperature of the tissue and warming the blood passing through it. Heat production is increased in these cells because the mitochondria have in their inner membrane a transmembrane protein called thermogenin or uncoupling protein (UCP-1), a marker unique to brown fat.Slide35
Thermogenin
permits the backflow of protons previously transported to the
intermembranous
space without passing through the ATP-synthetase system in the mitochondrial globular units.Consequently, the energy generated by proton flow is not used to synthesize ATP and is dissipated as heat. Warmed blood circulates throughout the body, distributing the heat and carrying fatty acids not metabolized in the adipose tissue for use elsewhere. Slide36
Histogenesis
of Brown Adipose
Tissue
Brown adipose tissue also develops from embryonic mesenchyme, that emerges earlier than white fat during fetal development. In humans the amount of brown fat is maximal relative to body weight at birth, when nonshivering thermogenesis is most needed. The tissue largely disappears (by involution) or is replaced by white fat during childhood. In
adults it is found only in scattered areas, especially around the kidneys and adrenal glands, the aorta, and
mediastinum
.Slide37
The number of brown
adipocytes
increases again during cold adaptation,
Besides stimulating thermogenic activity, autonomic nerves also promote brown adipocyte differentiation and prevent apoptosis in mature brown cells.Slide38
End of Chapter 6