Introduction

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White and brown adipose tissues have long been considered as two distinct and independent entities, underestimated by researchers.

White adipose tissue (WAT) has largely been ignored by researchers for many years. One major reason for this is its diffuse distribution and its apparently uninteresting localisation in the mammalian body, as if filling the interstitial spaces among the various organs like a connective tissue. WAT is distributed at the cutaneous (dermis), subcutaneous, perivisceral (mediastinal, retroperi-toneal, intraperitoneal: omental, mesenteric, etc.) and intravisceral (parotid, bone marrow, parathyroid, pancreas) levels. When the cellular nature of this connective tissue was first unveiled, the functional interpretation came quickly and intuitively: given its high fat content (triglycerides), WAT had to be an energy depot for short or prolonged fasting periods; however, not being used in the physio logical daily routine, it could not be very important.

Its histological aspect (Fig. 1) was similarly uninspiring, consisting of spherical cells 70-90 ^m in diameter with a thin rim of cytoplasm surrounding a single lipid droplet (triglycerides); scarce cytological tissue elements (macrophages, mast cells and fibroblasts); and unimpressive vas-cularity and innervation.

The description of mammalian brown adipose tissue (BAT) had a separate history. The term adipose is due to its high triglyceride content, whereas brown is due to its darker hue compared with white fat, a colour that is visible to the naked eye and stems from its different cellular and tissue anatomy: smaller (30-50 ^m in diameter) and not properly spherical (polyhedral) cells with a rounded nucleus occasionally located in a central position.

The distinctive feature of BAT is the multilocu-lar organisation of cytoplasmic triglycerides into

Fig. 1. Human subcutaneous white adipose tissue made up of unilocular adipocytes. Light microscopy 550x

multiple separate droplets that are easily seen at the light microscope (Fig. 2). The electron microscope shows large and cristae-rich mitochondria filling the cytoplasm (Fig. 3), another distinguishing characteristic from white fat. The histological features of BAT are also different: vessels are much more diffuse and the nerves appear to infiltrate the parenchyma, forming numerous neuro-adipocytic synaptic contacts. It is the high capillary density of this tissue and the large amount of mitochondria found in brown adipocytes that confer on BAT its brown colour. Despite a similar interstitial distribution, the mammalian BAT is easier to localise precisely compared with WAT, especially in smaller animals.

The function of BAT - heat production - and its large proportion in hibernating animals were described in the 1960s. This led to circumscription of the importance of this tissue, also because it was increasingly clear that its amount was negligible in large mammals, man included.

In the late 1970s, Stock's group in London revived the biomedical interest in BAT. In addressing the issues connected with human eating habits, especially in relation to the growing proportion of obese individuals in Western countries, these researchers - wondering why laboratory rats did not become obese - tried feeding them a diet similar to

Fig. 2. Mouse brown adipose tissue composed of multilocular adipocytes. Note the dark dots in the cytoplasm corresponding to large mitochondria. Resin-embedded tissue. Light microscopy 1200x
Fig. 3. Electron micrograph of the cytoplasm of a brown adipocyte. Note the large mitochondria rich in cristae. Transmission electron microscopy 10 000x

the one widely diffused in the West: the fat-rich, so-called cafeteria diet. Unexpectedly, although the rats did gain weight, they did not do so by the amount that had been forecast based on the calculation of the calories administered [1]. There therefore had to be a protective mechanism, whose enhancement could maybe help address human obesity and its complications (diabetes, hypertension, cardiovascular disease).

The significantly increased tissue function of BAT observed in rats fed the cafeteria diet prompted the hypothesis that the factor protecting against obesity could be the energy consumed by the tissue in response to food administration (diet-induced thermo-genesis). Indeed, a very large amount of energy is theoretically dissipated through thermogenesis. It has been calculated that a gram of BAT can develop 300 Watts, i.e. 300 times the amount developed by all other mammalian cell types [2]; thus, 1 kg of BAT is capable of dispersing about 6200 kcal/day, corresponding to the energy requirements of an individual doing a heavy manual job (while most of us expend about one third of this energy). Therefore, a few hundred grams of fat tissue in our body can use the amount of energy required for the functioning of the whole body, and 50 g of BAT can use 10-15% of the whole energy turnover [2,3].

In 1993, the hypothesis that BAT could be an anti-obesity factor received further support from experiments by the Boston group of Lowell and Flyer, who obtained genetically manipulated mice lacking BAT and reported that they became obese [4]. However, subsequent research on mice lacking the mitochondrial protein UCP1 - which is responsible for BAT's thermogenic activity and thus for its ability to disperse energy - showing that they were sensitive to cold (did not produce heat) but did not become obese [5], reopened the question of the role of BAT. In contrast, mice made to produce UCP1 ectopically in WAT were lean and resistant to obesity [6,7].

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