Electron microscopic study of intestinal fat absorption in vitro from mixed micelles containing linolenic acid, monoolein, and bile salt

Sacs and segments of everted hamster gut were incubated in vitro in solutions of mixed micelles (containing taurodeoxycholate, monoolein, and linolenic acid) for various periods and at different temperatures. The appearance of electron micrographs of the tissues was consistent with the hypothesis that the lipids were taken up in small particles (of micellar or molecular dimensions) by means of diffusion. There was no stimulation of pinocytosis or alteration in pinocytotic vesicles in the terminal web during uptake. In the deep cytoplasm the lipids accumulated to form large osmiophilic droplets of two kinds: (a) large, solitary droplets contained only by the matrix of the cytoplasm, without confining membranes; (b) small droplets occurring in clusters, without evidence of coalescence, within the granular and agranular endoplasmic reticulum. There was a progressive accumulation of the small droplets with time. Similar droplets appeared in extracellular spaces, including the lacteals, after prolonged incubations at 37 degrees C. The droplets in the endoplasmic reticulum appear to be the end product of lipid absorption and to be closely related to chylomicrons. These processes resemble those occurring during absorption of triglycerides in vivo and support the idea that this occurs predominantly through the formation of mixed micelles of the type employed in this study.