Electron micrograph showing portions of two adjacent adipose cells. The cytoplasm of the adipose cells reveals mitochondria (M) and glycogen (the latter appears as the very dark particles), x 15,000. Upper inset. Attenuated cytoplasm (Cy) of two adjoining adipose cells. Each cell is separated by a narrow space containing external (basal) lamina and an extremely attenuated process of a fibroblast, x 65,000. Lower inset. The external (basal) lamina (BL) of the adipose cells appears as a discrete layer where the cells are adequately separated from one another. F, fibroblast processes. x30,000.

to obesity. Lesioned rats deposit more body fat than control animals fed the same amounts of food.

Hormonal mobilization involves insulin, a hormone that inhibits the action of hormone-sensitive lipase and thus blocks the release of fatty acids. It also enhances the conversion of glucose into the triglycerides of the lipid droplet by the adipocyte. In addition, other hormones modify various steps in the metabolism of adipose tissue, including thyroid hormone, glucocorticoids, prostaglandins, and hormones of the pituitary gland.

S? brown adipose tissue

Adipocytes of brown, multiiocular adipose tissue contain numerous fat droplets

The cells of brown adipose tissue are smaller than those of white adipose tissue. The nucleus of a mature multiiocular adipocyte is typically in an eccentric position within the cell, but it is not flattened, as is the nucleus of a unilocular adipocyte.

In routine H&E-stained sections, the cytoplasm of the multiiocular adipocyte consists largely of empty vacuoles because the lipid that ordinarily occupies the vacuolated spaces is lost during preparation (Fig. 6.4). Multiiocular adipocytes depleted of their lipid bear a closer resemblance to epithelial cells than to connective tissue cells. The multiiocular adipocyte contains numerous mitochondria, a small Golgi apparatus, and only small amounts of rER and sER. The mitochondria contain large amounts of cytochrome oxidase, which imparts the brown color to the cells.

Brown adipose tissue is subdivided into lobules by partitions of connective tissue, but the connective tissue stroma between individual cells within the lobules is sparse. The tissue has a rich supply of capillaries that enhance its color. Numerous unmyelinated nerve fibers are present among the fat cells.

Metabolism of lipid in brown adipose tissue generates heat

Hibernating animals have large amounts of brown adipose tissue. The tissue serves as a ready source of lipid.

When oxidized, it produces heat to warm the blood flowing through the brown fat on arousal from hibernation. This type of heat production is known as nonshivering thermo,genesis.

Brown adipose tissue is also present in nonhibernating animals and again serves as a source of heat. In humans, multilocular adipose tissue is present in large amounts in the newborn, which helps offset the extensive heat loss that results from the newborn's high surface-to-mass ratio. The amount of brown adipose tissue gradually decreases as the body grows, but it remains widely distributed throughout the first decade of life. It then disappears from most sites except for regions around the kidney, adrenal glands, aorta, and regions in the neck and mediastinum. As in the mobilization of lipid in white adipose tissue, lipid is mobilized and heat is generated by multi-

locular adipocytes when they are stimulated by the sympathetic nervous system.

Thermogenic activity of brown adipose tissue is regulated by the unique uncoupling protein found in mitochondria

The mitochondria found in the cytoplasm of brown adipose tissue cells contain a unique uncoupling protein (UCP-1), which uncouples the oxidation of fatty acids from the production of ATP. At the molecular level, UCP-1 facilitates proton transport across the inner mitochondrial membrane. The movement of protons from the inner mitochondrial compartment dissipates the mitochondrial proton gradient, thus uncoupling respiration from ATP synthesis. The energy produced by the mitochondria is then used as heat. In experimental animals, UCP-1 activity has been shown to increase during cold stress.

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