Abstract
Phase-contrast computed tomography (PCCT) is an X-ray-based imaging method measuring differences in the refractive index during tissue passage. While conventional X-ray techniques rely on the absorption of radiation due to differing tissue-specific attenuation coefficients, PCCT enables the determination of the electron density (ED). By the analysis of respective phantoms and ex vivo specimens, we identified the components responsible for different electron densities in murine adipose tissue depots to be cellular fat and mitochondrial content, two parameters typically different between white adipose tissue (WAT) and brown adipose tissue (BAT). Brown adipocytes provide mammals with a means of non-shivering thermogenesis to defend normothermia in a cold environment. Brown adipocytes are found in dedicated BAT depots and interspersed within white fat depots, a cell type referred to as brite (brown in white) adipocyte. Localization and quantification of brown and brite adipocytes in situ allows an estimate of depot thermogenic capacity and potential contribution to maximal metabolic rate in the cold. We utilized PCCT to infer the composition of white, brite, and brown adipose tissue from ED of individual depots. As proof of principle, we imaged mice 10, 20, and 30 days of age. During this period, several WAT depots are known to undergo transient browning. Based on ED, classical WAT and BAT could be clearly distinguished. Retroperitoneal and inguinal WAT depots increased transiently in ED during the known remodeling from white to brite/brown and back to white. We systematically analyzed 18 anatomically defined adipose tissue locations and identified changes in fat content and mitochondrial density that imply an orchestrated pattern of simultaneous browning and whitening on the organismic level. Taken together, PCCT provides a three-dimensional imaging technique to visualize ED of tissues in situ. Within the adipose organ, ED provides a measure of mitochondrial density and fat content. Depending on experimental setting, these constitute surrogate markers of cellular distribution of white, brite, and brown adipocytes and thereby an estimate of thermogenic capacity.
Highlights
Brown adipose tissue (BAT) is a mammalian heater organ providing adaptive, non-shivering thermogenesis to defend normothermia in a cold environment
Thermogenic brown adipocytes reside both in characteristic, homogeneous BAT depots and interspersed within white adipose tissue (WAT) depots
While genetic mouse models with brown adipocyte specific luciferase expression can be repeatedly subjected to in vivo imaging, the methodology suffers from interfering parameters that change over time during experiments, i.e., light absorption by fur and overlaying WAT tissue of differing thickness, skin pigmentation or altered luminogenic substrate distribution owing to changes in body mass and composition (Galmozzi et al, 2014; Mao et al, 2017)
Summary
Brown adipose tissue (BAT) is a mammalian heater organ providing adaptive, non-shivering thermogenesis to defend normothermia in a cold environment (for a review, see Klingenspor et al, 2017). Thermogenic brown adipocytes reside both in characteristic, homogeneous BAT depots and interspersed within white adipose tissue (WAT) depots. While genetic mouse models with brown adipocyte specific luciferase expression can be repeatedly subjected to in vivo imaging, the methodology suffers from interfering parameters that change over time during experiments, i.e., light absorption by fur and overlaying WAT tissue of differing thickness, skin pigmentation or altered luminogenic substrate distribution owing to changes in body mass and composition (Galmozzi et al, 2014; Mao et al, 2017)
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