Hormonal mobilization involves a complex system of hormones and enzymes that controls fatty-acid release from adipocytes. These include insulin, thyroid hormones, and adrenal steroids. Insulin is an important hormone that promotes lipid synthesis by stimulating lipid synthesis enzymes (fatty-acid synthase, acetyl-CoA carboxylase) and suppresses lipid degradation by inhibiting the action of hormone-sensitive lipase and thus blocking the release of fatty acids. Glucagon, another pancreatic hormone, and growth hormone from the pituitary gland both increase lipid utilization (lipolysis). In addition, elevated levels of tumor necrosis factor  (TNF-) have been implicated as a causative factor in the development of insulin resistance associated with obesity and diabetes.
Adipocytes of brown, multilocular adipose tissue contain numerous fat droplets.
The cells of brown (multilocular) adipose tissue are smaller than those of white adipose tissue. The nucleus of a muscle, other tissues
FIGURE 9.4 • Regulation of energy homeostasis. This schematic diagram shows the relationship of adipose tissue to the central nervous system and gastrointestinal system within the brain– gut–adipose axis that is responsible for regulating energy homeostasis.
mature multilocular 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 multilocular adipocyte consists largely of empty vacuoles because the lipid that ordinarily occupies the vacuolated spaces is lost during preparation (Fig. 9.5). Multilocular adipocytes depleted of their lipid bear a closer resemblance to epithelial cells than to connective tissue cells. The multilocular 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 features are listed in Table 9.2.
Brown adipose tissue, abundant in newborns, is markedly reduced in adults.
Brown 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 and avoid lethal hypothermia (a major risk of death for premature babies). In newborns, brown adipose tissue makes up about 5% of the total body mass and is located on the back, along the upper half of the spine, and toward the shoulders. The amount of brown adipose tissue gradually decreases as the body grows, but it remains widely distributed throughout the first decade of life in the cervical, axillary, paravertebral, mediastinal, sternal, and abdominal regions of the body. It then disappears from most sites except for regions around the kidney, adrenal glands, large vessels (i.e., aorta), and regions of the neck (deep cervical and supraclavicular), regions of the back (interscapular and paravertebral), and thorax (mediastinum).
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.
Brown adipocytes differentiate from mesenchymal stem cells under the control of PRDM16/PGC-1 transcription factors in the presence of catecholamines.