Cold-induced Conversion of Connective Tissue Skeleton in Brown Adipose Tissues.

Thermogenesis via fatty acid-induced uncoupled mitochondrial respiration is the primary function of brown adipose tissue (BAT). In response to changes in ambient temperatures, the weight and specific gravity of BAT change, depending on the quantity of lipid droplets stored in brown adipocytes (BA). Such conditions should result in the reconstruction of connective tissue skeletons, especially of collagen fiber networks, although the mechanisms have not been clarified. This study showed that, within 4 hr of exposing mice to a cold environment, collagen fibers in the extracellular matrix (ECM) of BAT became discontinuous, twisted, emancipated, and curtailed. Surprisingly, the structure of collagen fibers returned to normal after the mice were kept at room temperature for 19 hr, indicating that the alterations in collagen fiber structures are physiological processes association with adaptation to cold environments. These dynamic changes in connective tissue skeletons were not observed in white adipose tissues, suggesting that they are unique to BAT. Interestingly, the vascular permeability of BAT was also augmented by exposure to cold. Collectively, these findings indicate that dynamic changes in ECM collagen fibers provide high flexibility to BAT, enabling the adjustment of tissue structures and the regulation of vascular permeability, resulting in adaptation to changes in ambient temperatures.