Chapter 6 Adipose Tissue. 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 . .
A specialized type of connective tissue in which adipocytesor fat cells predominate.Located in many areas throughout the body, Adipose tissue represents 15–20% of the body weight in men.in women of normal weight, 20–25% of body weight.
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.
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.
Adipocytesthemselves 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.
A unilocularadipocyte 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.
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.
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.
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.
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.
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.
Triglyceridesare transported by lymph and blood from the
Intestine and liver in lipoprotein complexes known as
chylomicrons(Chylo) and very low—density lipoproteins
In capillary endothelial cells of adipose tissue, these
complexes are partly broken down by lipoprotein lipase,
releasing free fatty acids and glycerol.
where they are re—esterified to glycerol phosphate, forming
These resulting triglycerides are stored in droplets until needed.
Norepinephrinefrom nerve endings stimulates the cyclic AMP
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
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.
Although all white adipose tissue appears histologically and physiologically similar, differences in gene expression have been noted between
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 unilocularadipocytes lose nearly all their fat and become polyhedral or spindle-shaped cells with very few lipid droplets.
The color of brown adipose tissue or brown fat is due to both the numerous mitochondria (containing coloredcytochromes) scattered through the adipocytes and the large number of blood capillaries in this tissue.
Adipocytesof 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.
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.
Brown adipose tissue resembles an endocrine gland in that its cells assume an almost epithelial arrangement closely associated with blood capillaries.
unlike white fat, liberated fatty acids of multilocularadipocytes 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 thermogeninor uncoupling protein (UCP-1), a marker unique to brown fat.
Thermogenin permits the backflow of protons previously transported to the intermembranous space without passing through the ATP-synthetase system in the mitochondrial globular units.
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.