Adipose Tissue and Adipokines in Health and Disease

Abstract

Mammals are provided with an organ that has been neglected by scientists in the past: the adipose organ. This organ is formed by a series of well-defined depots mainly located at two corporal levels: superficial (subcutaneous depots) and deep (visceral depots). In adult rodents, two main depots are the anterior and posterior subcutaneous depots. The first consists of a central body located in the area between the scapulae and several elongated projections abutting toward the cervical region and the axillae. The second is extended from the dorsolumbar area to the gluteal, with an intermediate region located in the inguinal area. The main visceral depots are tightly connected with viscera. In adult rodents, the main visceral depots are mediastinic, perirenal, perigonadal, mesenteric, and retroperitoneal. The weight of the adipose organ is about 20% of the body weight and therefore it is one of the biggest organs in the body. Its color is mainly white but some areas are brown. In young-adult rodents, maintained in standard conditions, the interscapular region and parts of the cervical and axillary projections of the anterior subcutaneous depot, as well as parts of the mediastinic and perirenal depots, are brown. These two colors correspond to the two tissues: white and brown adipose tissues. The relative amount of the two tissues varies with age, strain, environmental and metabolic conditions, and subsequently, the distribution of the two colors is also variable and implies the ability of reversible transdifferentiation of the two types of adipocytes. During pregnancy and lactation, the subcutaneous depots are transformed into mammary glands. Each depot of the organ receives its own neurovascular peduncle that is specific for the subcuta- neous depots and is usually dependent on the peduncle related to the connected organ in the case of visceral depots. The vascular and nerve supply is much more dense in the brown areas than in the white areas. Their density changes in conjunction with the number of brown adipocytes. It has been shown that the white adipose tissue of obese mice and humans is infiltrated by macrophages and that the level of infiltration correlates with body mass index and mean size of adipocytes. This infiltration seems to be an important cause for the insulin resistance associated with obesity. We recently observed that macrophages are mainly located at the level of dead adipocytes in white adipose tissue of obese mice, obese humans, and in transgenic mice, which are lean but have hyper- trophic adipocytes (HSL knockout mice). The suggested function of these macrophages is mainly to reabsorb the lipid droplet from dead adipocytes.