Morphologic Techniques for the Study of Brown Adipose Tissue and White Adipose Tissue

Light microscopy (LM) allows study of the anatomy of the adipose organ. For instance, it permits investigation of its microscopic organization (possible lobular subdivision) and the arrangement in the tissue of vessels, nerves, and main cell types (1 ). The unilocular or multilocular organization of adipocytes is already clearly visible at low magnification (�2.5), but the identification of other cell types, such as mast cells (particularly frequent in brown tissue), histiocytes (particularly frequent in white tissue, especially in fasting animals and in animals under diet restrictions), and inflammatory cells, requires magnification of at least �40–100. Other cell types such as fibroblasts, preadipocytes and pericytes, are not easily recognized at LM. This technique also provides data on the functional state of the adipose tissue (AT): the size of white adipose tissue (WAT) adipocytes is indicative of the animal’s nutritional state and areas of capillary bed expansion appear in fasting animals. In brown adipose tissue (BAT), multilocularity is observed in various degrees, depending on the functional state of the organ. When this is stimulated by cold exposure or diet, lipid vacuoles are small and numerous, at rest (i.e., at warm environmental temperature or in fasting animals), they increase in size and gradually merge into fewer vacuoles, and eventually into a single vacuole. The functional activation of mitochondriogenesis in BAT also determines increased eosinophily of adipocyte cytoplasm. LM observation requires specimen processing by the following steps: fixation, dehydration, embedding, sectioning, and staining (2 ).