Overview of Adipose Tissue and Its Role in Obesity and Metabolic Disorders

As the result of its apparent structural and histological simplicity, adipose tissue (AT) functions initially were limited to energy storage, insulation, and thermoregulation. Only decades later was the extraordinarily dynamic role of AT recognized, revealing its participation in a broad range of physiological processes, including reproduction, apoptosis, inflammation, angiogenesis, blood pressure, atherogenesis, coagulation, fibrinolysis, immunity and vascular homeostasis with either direct or indirect implications in the regulation of proliferation. The functional pleiotropism of AT relies on its ability to synthesize and, in some cases, secrete a large number of enzymes, hormones, growth factors, cytokines, complement factors, and matrix and membrane proteins, collectively termed adipokines. At the same time, white AT expresses receptors for most of these factors, warranting a wide cross-talk at both local and systemic levels in response to metabolic changes or other external stimuli. In this chapter, mounting evidence on the specific characteristics of AT from different depots is outlined in relation to fat distribution and comorbidity development. The current knowledge in this field is reviewed with a broad perspective ranging from classification, structure, and distribution to the key functional roles of AT with a particular focus on the role of adipokines and their involvement in the metabolic disorders accompanying obesity.