«White adipose tissue
Characteristics of brown adipose tissue, "physiological anti-obesity tissue"
The role played by brown adipocytes is different from that played by white adipocytes. First of all because they are smaller cells, whose dark color is due to the presence of cytochromes contained in the numerous mitochondria. Unlike white adipocytes, brown adipocytes do not contain a single large fat mass but many small drops of triglycerides, called lipid vacuoles. Consequently, the nucleus and the cytoplasm are not located in the periphery but clearly distinguishable inside the cell. In addition to a morphological difference there is also one of a functional nature.
If the organism suffers from hypothermia, the brown adipocytes respond by mobilizing their triglycerides, from whose catabolism dissipable energy is released in the form of heat.
This phenomenon is called thrill-free thermogenesis, to distinguish it from the classic thrill (involuntary muscle contraction aimed at producing heat).
The brown adipose cell, which is the cell of the organism richest in mitochondria, contains a mitochondrial protein, called, UCP-1 (decoupling protein), which is the real marker of this adipocyte and intervenes in thermogenesis. sympathetic stimulus, thanks above all to the B3-adrenergic receptors, the thermogenetic activity is activated. Mice genetically deprived of these receptors undergo a phenomenon of transdifferentiation of the brown adipose tissue, which transforms into white adipose tissue making them massively obese despite the increased physical activity and the normocaloric diet.
The brown adipose tissue has a rich sympathetic innervation which makes it particularly sensitive to the activity of catecholamines, hormones secreted rapidly in response to acute psychophysical stress.
The brown adipose tissue is activated, not only in response to a lowering of temperature, but also in case of excessive caloric intake with the diet. In theory this phenomenon, based on the dispersion of the caloric surplus in the form of heat, should guarantee the homeostasis of the body weight, independently from the alimentary excesses.
In overnourished rats, an increase in thermogenesis was shown, with a preventive effect on the development of obesity. The brown adipose tissue responded to this condition with the same metabolic and structural changes activated during cold thermogenesis. Not surprisingly, as soon as they ate the food. temperature increases by about 0.5 / 1 degree, precisely because of this form of postprandial thermogenesis mediated by brown adipose tissue, which tends to keep the body's energy balance stable despite the caloric surplus of the meal.
The experimental animal exposed to the cold for ten days transforms the phenotype of its adipose organ into a predominantly brown phenotype. The surprising thing is that not only do the percentages of white / brown adipocytes change, but that the total number of fat cells remains constant. ; this means that under certain conditions the mature white adipocytes can transform into brown adipocytes, and vice versa.
In genetically obese rats, brown adipose tissue has a reduced thermogenetic capacity.
The reduced presence of brown adipocytes in an adult individual therefore seems to be one of the many pathogenetic mechanisms underlying obesity.
According to the most recent studies, the adipose tissue of mammals (including humans) has the intrinsic ability to transform white adipocytes into brown adipocytes, and vice versa. The brown adipose tissue, in fact, is not numerically constant in its cell population, but expands and shrinks as needed. This event is due to phenomena of hyperplasia and conversion of white adipocytes into brown adipocytes; the coexistence of these cells is in fact antithetical (the white ones accumulate lipids while the brown ones burn them). The discovery of these biological mechanisms opens the door to future therapeutic developments in the treatment of obesity; theoretically, in fact, to defeat it it would be sufficient to increase the percentage of brown adipocytes, which is also very useful in the prevention of diabetes.
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