ABSTRACT
In humans, the development of white adipose tissue (WAT) occurs to a large extent post-natally and continues throughout life, in contrast to the development of brown adipose tissue (BAT) which takes place mainly before birth and disappears thereafter. The acquisition of fat cells appears to be an irreversible process, as apoptosis has not been shown to be significant under physiological conditions. At the cellular level, this phenomenon raises the question of the characteristics of the cells constituting the adipose tissue organ. This leads in turn to the question of the nature of the factors that regulate the formation of new fat cells from dormant adipose precursor cells. Once adipose tissue is formed, adipocytes represent between one-third and two-thirds of the total number of cells. The remaining cells are blood cells, endothelial cells, pericytes, adipose precursor cells of varying degree of differentiation, and, most likely, fibroblasts. However, when studying WAT, the most provocative question which comes to mind when one considers the physiopathological consequences of overweight and obesity epidemic in developed and developing countries is “do we need WAT for normal life?” Part of the answer comes from evolution as, among invertebrates, adipose tissue repre sents an important organ in insects whereas its quantitative importance decreases in arachnids, crustaceans, and mollusks in which liver appears as a new organ. Among vertebrates, adipose tissue develops extensively in homeotherms, although its proportion of body weight can vary greatly between species (up to 40% of body weight in cetaceans) or within a species as it is the case in migrating birds and hibernating animals. But the most straightforward answer comes from the recent generation of transgenic mice largely devoid of WAT (vide infra: see below). The KO mice exhibit anatomical and physiological properties very similar to patients suffering from congenital generalized lipodystrophy which is characterized by severely decreased fat mass, hypertriglyceridemia and non-ketotic diabetes. It is now realized that WAT is an endocrine organ which secretes leptin in proportion to its mass. As leptin is known among many effects to stimulate the gonadal axis and to promote reproductive functions WAT, which represents the main energy storage organ in the body, has become a major player in connecting reproduction and energy requirements.
