Extraction of Absorbed Lipid (linoleic Acid-1-14c) from Rat Intestinal Epithelium during Processing for Electron Microscopy

Intestinal epithelial cells absorb ingested neutral lipids, apparently as water-soluble micelles of bile salts, monoglycerides, and/or fatty acids (3, 7). Electron microscopy of intestinal cells fixed in osmium tetroxide reveals some of the absorbed lipid as electron-opaque droplets in the apical cytoplasm distal to the microvilli and subjacent terminal web (10). Experiments by Strauss and Ito (17) and Strauss (18) suggest that these lipid droplets in the apical cytoplasm are the higher glyceride products synthesized from absorbed fatty acids and monoglycerides. The absorbed precursors of the higher glycerides must pass through the microvilli and/or terminal web to the apical cytoplasm. A few investigators (5, 11, 13) have reported electron-opaque particles (lipid) in the microvilli and/or terminal web of intestinal cells from fat-fed animals. However, these observations have not been as consistent or as frequent as expected when there is ample ultrastructural evidence of lipid absorption in the cellular area distal to the microvilli and terminal web. We believe that extraction during tissue preparation could be the explanation for inconsistent and infrequent evidence of lipid in the microvilli and terminal web. Recent reports of lipid extraction from cells (amoeba (4), lung (9), liver (14), mammary glands (15), heart (16)) during preparation for electron microscopy have led us to the hypothesis that some absorbed lipid is likewise extracted from intestinal epithelial cells. Our experiments were designed to explore this possibility.