Chapter 9: Adipose Tissue: White, Brown & Beige

Energy is efficiently stored within adipocytes in the highly concentrated form of triglycerides. Historically, only white and brown adipose tissue were recognized, but modern classification includes a third type: beige adipose tissue. White adipose tissue (WAT) is the most common type in adults, performing energy storage, insulation (forming the panniculus adiposus), and cushioning of vital organs. White adipocytes are large (sometimes 100 micrometers or more in diameter) and unilocular, containing a single large lipid droplet that displaces the nucleus to the periphery, creating a characteristic signet-ring appearance. White adipocyte differentiation from perivascular mesenchymal stem cells (Myf5 negative) is governed by the PPARgamma/RXR transcription factors, considered the "master switch". WAT secretes various adipokines (hormones, growth factors, cytokines) including leptin, a 16-kDa hormone that signals satiety to the hypothalamus for long-term weight regulation, along with insulin. The chapter also discusses the brain-gut-adipose axis, which includes short-term regulators like ghrelin (appetite stimulant) and peptide YY (PYY, appetite suppressant). Lipid mobilization (lipolysis) is activated neurally by norepinephrine from sympathetic nerves and hormonally by factors like glucagon and growth hormone. In contrast, brown adipose tissue (BAT) is abundant in newborns for nonshivering thermogenesis to generate heat, although remnants remain in specific adult locations such as the neck and retroperitoneal space. Brown adipocytes are smaller, multilocular (many small lipid droplets), highly vascularized, and filled with numerous large mitochondria containing Uncoupling Protein 1 (UCP-1), or thermogenin. UCP-1 uncouples fatty acid oxidation from ATP synthesis, dissipating energy as heat. BAT originates from Myf5-positive skeletal myogenic progenitor cells, with PRDM16/PGC-1 serving as the master regulator of differentiation. Beige adipose tissue is the third type, consisting of brown-like adipocytes (paucilocular, intermediate morphology) that emerge within subcutaneous WAT, primarily via white-to-brown transdifferentiation induced by cold exposure or physical activity. Both brown and beige adipose tissues secrete batokines to regulate body functions. Clinically, obesity (a body mass index of 30 kg/m(squared) or greater) is linked to hypertrophic adipocytes and chronic low-grade inflammation. Furthermore, the high glucose uptake activity of UCP-1 expressing brown and beige fat can cause interference or false-positive results when utilizing Positron Emission Tomography (PET) scans for cancer detection. Adipose tissue tumors include benign lipomas and rare malignant liposarcomas of white fat, as well as benign hibernomas of brown fat.