Seasonal changes in ultrastructure of brown adipose tissue in the common shrew (Sorex araneus l.).

Seasonal changes have been detected in the ultrastructure of brown adipose tissue (BAT) of the common shrew, Sorex araneus (a true nonhibernator) living under natural conditions and collected at the time when the most representative growth phase of the animal for the given season could be expected. In summer and autumn, BAT is characterized by the presence of large, regular, spherical lipid droplets and mitochondria closely adhering to one another. During winter, mitochondria possess densely packed cristae and are dispersed in the cytoplasm, sometimes invaginating into lipid droplets; the latter are diminished and often irregular in contour. The BAT in winter specimens is distinguished also by a large amount of blood capillaries penetrating the tissue. In spring, mitochondria of BAT are found more frequently adhering to each other, and are characterized by loosely arranged cristae. In addition to the spherical lipid droplets, agglomerations of lipid material may be found in the cytoplasm. The observed seasonal fluctuations in the ultrastructure of BAT in the shrew correspond to the metabolic rhythm of this animal. The latter point is discussed.