White adipose tissue, WAT: specialized fat tissue primarily responsible for lipid storage
Brown adipose tissue, BAT: specialized fat tissue primarily responsible for adaptive thermogenesis; cells are dense with mitochondria
Adipokines: also called adipocytokines; are endocrine- and paracrine-signaling molecules secreted by adipose tissue
Leptin resistance: refers to the phenomenon of reduced anorexigenic actions of leptin in the obese state
Body mass index, BMI: a measure for relative body fat based on an individual's weight and height
Metabolic syndrome: a disorder that defines a combination of metabolic and cardiovascular risk determinants associated with obesity and insulin resistance
Nonalcoholic fatty liver disease, NAFLD: characterized by fatty infiltration of the liver in the absence of alcohol consumption
Introduction to Adipose Tissue
Adipose tissue is not merely an organ designed to passively store excess carbon in the form of fatty acids esterified to glycerol (triglycerides). Mature adipocytes synthesize and secrete numerous enzymes, growth factors, cytokines, and hormones that are involved in overall energy homeostasis. Many of the factors that influence adipogenesis are also involved in diverse processes in the body including lipid homeostasis and modulation of inflammatory responses. In addition, a number of proteins secreted by adipocytes play important roles in these same processes. In fact, recent evidence has demonstrated that many factors secreted from adipocytes are pro-inflammatory mediators and these proteins have been termed adipocytokines or adipokines. There are currently over 50 different adipokines recognized as being secreted from adipose tissue. These adipokines are implicated in the modulation of a range of physiological responses that globally includes appetite control and energy balance. Specific metabolic processes regulated by adipose tissue include lipid metabolism, glucose homeostasis, inflammation, angiogenesis, hemostasis, and blood pressure.
The major form of adipose tissue in mammals (commonly referred to as “fat”) is white adipose tissue (WAT). WAT is composed of adipocytes held together by a loose connective tissue that is highly vascularized and innervated (Figure 45-1). White adipocytes are rounded cells that contain a single large fat droplet that occupies over 90% of the cell volume. The mitochondria within white adipocytes are small and few in number. The mitochondria and nucleus of the white adipocyte is squeezed into the remaining cell volume. Molecular characteristics of white adipocytes include expression of leptin but no expression of uncoupling protein 1, UCP1 (designated UCP1−, leptin+ adipocytes).
Histological section of adipose tissue demonstrating distinctive morphology of WAT and BAT. White adipocytes occupy the left side of the image and brown adipocytes the right side. As described below, white adipocytes are generally rounded with over 90% of the cell volume taken up by a single fat droplet. The few small mitochondria and the nucleus are compressed to the very edge of the white adipocyte. The brown adipocytes are smaller in overall size, polygonal in shape, contain several small lipid droplets, and ...