Brown Fat Anatomy in Humans and Rodents

The Brown Adipose Tissue (BAT) is composed by mitochondrial rich, multilocular adipocytes, in strict topographical and functional relation with vasculature and noradrenergic nerves. Brown adipocytes are able to dissipate energy to produce heat, in a process known as non-shivering thermogenesis. Due to its contribution to energy expenditure, BAT is intensely studied for its potential to counteract metabolic diseases such as obesity, type 2 diabetes, dyslipidemia and cardiovascular diseases. BAT displays specific morphological characteristics that allow to assess its functional state. In this chapter we describe methodologies to properly dissect BAT depots, evaluate their gross anatomy, and assess its activation by light microscopy using peroxidase immunostaining and by laser scanning confocal microscopy using immunofluorescence. We also describe methodologies to study BAT ultrastructure by transmission and scanning electron microscopy, to visualize peroxidase immunostaining reactions at an ultrastructural level and to perform immunofluorescence reactions on paraffin-embedded samples, more often available in the clinical setting (due to the possibility to store them long-term) as opposed to fresh samples. The described techniques can be employed to study BAT morphology and activation in response to various stimuli (e.g., cold exposure; specific dietary composition) and in different pathological conditions (e.g., obesity; type 2 diabetes).