Quantitative electron microscopy shows uniform incorporation of triglycerides into existing lipid droplets |
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Authors: | Jinglei Cheng Akikazu Fujita Yuki Ohsaki Michitaka Suzuki Yuki Shinohara Toyoshi Fujimoto |
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Institution: | (1) Department of Anatomy and Molecular Cell Biology, Graduate School of Medicine, Nagoya University, 65 Tsurumai, Showa, Nagoya 466-8550, Japan |
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Abstract: | The lipid droplet (LD) is an organelle with a lipid ester core and a surface phospholipid monolayer. The mechanism of LD biogenesis
is not well understood. The present study aimed to elucidate the LD growth process, for which we developed a new electron
microscopic method that quantifies the proportion of existing and newly synthesized triglycerides in individual LDs. Our method
takes advantage of the reactivity of unsaturated fatty acids and osmium tetroxide, which imparts LDs an electron density that
reflects fatty acid composition. With this method, existing triglyceride-rich LDs in 3Y1 fibroblasts were observed to incorporate
newly synthesized triglycerides at a highly uniform rate. This uniformity and its persistence even after microtubules were
depolymerized suggest that triglycerides in fibroblasts are synthesized in the local vicinity of individual LDs and then incorporated.
In contrast, LDs in 3T3-L1 adipocytes showed heterogeneity in the rate at which lipid esters were incorporated, indicating
different mechanisms of LD growth in fibroblasts and adipocytes. |
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Keywords: | Lipid droplet Electron microscopy Triglyceride Cholesterol ester Adipocyte |
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