Everted sacs of intestine from golden hamsters were incubated at 37°C for at least 1 hour in vitro with emulsified lipid after removal of both pancreatic lipase and bile salts. The fine structure of intestinal epithelium is well preserved under these conditions. Absorption of fat by the intestinal mucosa in vitro closely resembles lipid absorption in vivo, as observed by both light and electron microscopy. The physiological significance of these observations is discussed. Tubular elements of the agranular endoplasmic reticulum are often strikingly abundant in the apical cytoplasm of intestinal absorptive cells. These have a role in the intracellular transport of fat since they frequently contain droplets of lipid derived from the incubation medium. The rate of fat accumulation in the epithelium appears to be proportional to the concentration in the medium.

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