The study of the numerous varieties of benign and malig-nant adipose tissue tumors provides further insight into, and confirmation of, the sequence of adipose tissue differ-entiation described above. As with epithelial tumors and tu-mors of fibroblast origin, the variety of adipose tissue tumors reflects the normal pattern of adipose tissue differ-entiation; that is, discrete tumor types can be described that consist primarily of cells resembling a given stage in normal adipose tissue differentiation. The most common benign tumor of adipose tissue of adulthood is the lipoma. It is more common than all other soft tissue tumors combined. Lipomas are subclassified by the morphology of the predominant cell in the tumor. For in-stance, the conventional lipoma consists of mature white adipocytes, whereas a fbrolipoma has adipocytes sur-rounded by an excess of fibrous tissue and an angio lipoma contains adipocytes separated by an unusually large num-ber of vascular channels. The majority of lipomas show structural chromosome aberrations that include balanced re-arrangements, often involving chromosome 12. Lipomas are usually found in subcutaneous tissues in middle-aged and elderly individuals. They are characterized as well-defined, soft, and painless masses of mature adipocytes usually found in the subcutaneous fascia of the back, thorax, and proximal parts of the upper and lower limbs. Treatment of lipomas usually involves a simple surgical excision. Malignant tumors of adipose tissue, called liposarco-mas, are rare. They are typically detected in older individu-als and are mainly found in the deep adipose tissues of the lower limbs, abdomen, and the shoulder area. Liposarco-mas may contain both well-differentiated, mature adipocytes and early, undifferentiated cells (Fig. F9.2.1). Tumors containing more cells in earlier stages of differenti-ation are more aggressive and more frequently metasta-size. Typically, liposarcomas are surgically removed, but if a tumor has already metastasized, then both chemotherapy and radiation therapy can be utilized as a preor postsurgi-cal treatment. Although the term lipoma relates primarily to white adi-pose tissue tumors, tumors of brown adipose tissue are also found. Not surprisingly, these are called hibernomas. They are rare, benign, and slow-growing soft tissue tumors of brown fat most commonly arising in the periscapular region, axillary fossa, neck, or mediastinum. Most  hibernomas contain a mixture of white and brown adipose tissue; pure hibernomas are very rare.
 FOLDER 9.2 Clinical Correlation: Adipose Tissue Tumors
FIGURE F9.2.1 • Well-differentiated liposarcoma. This photomicrograph was obtained from a tumor surgically removed from the retroperitoneal space of the abdomen. Well-differentiated liposarcoma is characterized by a predominance of mature adipocytes that vary in size and shape. They are interspersed between broad fibrous septa of connective tissue containing cells (majority of them are fibroblasts) with atypical hyperchromatic nuclei. A relatively few scattered spindle cells with hyperchromatic and pleomorphic nuclei are found within connective tissue. 340. (Courtesy of Dr. Fabiola Medeiros.)
Thermogenic activity of brown adipose tissue is facilitated by UCP-1 that is found at the inner mitochondrial membrane.
The mitochondria in eukaryotic cells produce and store energy as an electrochemical proton gradient across the inner mitochondrial membrane. As described earlier (see page 55), this energy is used to synthesize ATP when the protons return to the mitochondrial matrix through the ATP synthase enzyme located at the inner mitochondrial membrane.