The Fine Structure of Brown Adipose Tissue in the Newborn Mouse and Rat

Abstract

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.The interscapular brown fat of rodents was formerly thought to have a function related to winter dormancy and it was therefore called the hibernating glan~, but it is now generally believed to be a special form of adipose tissue. Although it has been a subject of renewed interest to histologists, biochemists, and virologists in the past few years, its physiological significance remains largely unknown. For a review of the early literature on this tissue, a description of its histology, and a tabulation of its occurrence in various" mammalian species, the reader is referred to the classical paper by Rasmussen (17) in 1923. Later work on the microscopic structure, histochemistry, and metabolism of brown adipose tissue, and its

Heat production in the new‐born rabbit and the fat content of the brown adipose tissue.

“…This is the probable explanation of the present findings and it is supported by the electron microscopic appearance of brown adipose tissue. In the cell which is replete with fat, mitochondria are seen closely applied to the surface of the vacuoles (Napolitano & Fawcett, 1958). In the cell which is depleted of fat, mitochondria may be seen lying free in the cytoplasm out of contact with the fat vacuoles (W. Aherne & D. Hull, unpublished) The initially constant concentration of fat in the brown adipose tissue despite the declining fat content was an unexpected finding.…”

A Case of Severe Hypermetabolism of Nonthyroid Origin With a Defect in the Maintenance of Mitochondrial Respiratory Control: A Correlated Clinical, Biochemical, and Morphological Study

“…Paracrystalline inclusions have been described from some types of mitochondria, such as those of the retina (63), the brown fat body (64), human liver cells in jaundice (65)(66)(67), and frog embryos (68). There are also paracrystalline inclusions in the cytoplasm of the thyroid cells (69) which show a very similar ultrastructure.…”

Septal Cells and Post-Natal Development of Rat Lungs, Electron Microscopic and Biochemical Studies

“…Occasionally, their cytoplasm extended in the interstitial space between epithelial cells and capillaries. (Napolitano and Fawcett 1958), rather than a single large drop of signet shape of the white adipose cells (Napolitano 1963). In the earlier stage of differentiation lipid was localized in the vicinity of cuboidal nucleus (Fig.…”