Electron microscopic study of the role of lipid micelles in intestinal fat absorption.

In vitro micellar solutions of oleic acid, monoolein, and sodium taurocholate were studied electron microscopically. They contained osmiophilic particles 30-200A in diameter. Osmium staining alone was sufficient to demonstrate the particles; lead staining had little effect on their appearance. The intestinal intraluminal contents from rats during the absorption of unsaturated fat also contained osmiophilic particles, 40-200A, and numerous similar particles were found between the microvilli and engaged in the fine filamentous coating of microvilli. In the lumen only, larger emulsion-type droplets were also seen. The small particles were demonstrable in osmium-fixed material, with or without lead stains, and staining with lead only increased contrast of the particles. Spherules measuring about 1000A in diameter with walls about 100A thick were observed in the terminal web during fat absorption, at which time they were slightly larger and more numerous than in fasting rats. It is proposed that during fat absorption micellar particles are engaged in the filamentous material covering the microvilli and then enter the absorptive cell either by molecular diffusion across the plasma membrane or by being incorporated into the walls of thick-coated spherules which then pass into the subapical cytoplasm.