Interscapular fat from newborn rats and mice was fixed in buffered 1 per cent osmium tetroxide and thin sections of the methacrylate-embedded tissue were studied with the electron microscope. The findings have reaffirmed the epithelioid character of brown adipose tissue, and have provided additional information on the relation of its cells to each other and to the rich capillary bed. For the most part, the earlier description of the fine structure of brown adipose cells by Lever, has been confirmed, but our observations on the mitochondria and their relation to fat droplets have led us to different conclusions concerning the role of these organelles in lipogenesis. Mitochondria were often found to be very closely associated with lipide inclusions, but no actual communication between the two was observed and no evidence was found to support the hypothesis that mitochondria are transformed into lipide droplets. Large dense bodies which showed a highly ordered fine structure suggesting a crystalline protein were seen in the matrix of some mitochondria. The cytoplasm of the adipose cells contained fine granules that seemed to be of two kinds: particles of uniform size (∼150 A) and appreciable density that are believed to be ribonucleoprotein, and granules of lower density and more variable size that are tentatively interpreted as a form of glycogen. The Golgi complex of the adipose cells was small and the endoplasmic reticulum almost entirely absent. The significance of the poor development of these organelles is discussed in relation to current concepts of their function.

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