Following the administration of 2 ml of C14-labeled oleic acid to rats by stomach tube, osmiophilic droplets and particles, from 10 to 300 mμ in diameter, were demonstrated in the intestinal lumen. The smaller droplets of fatty acid were frequently found between the microvilli of intestinal epithelial cells and were of the same size range as has been proposed for lipid micelles (10 mμ). The radiochemical examination of the lumen contents revealed that the activity was still present in free fatty acids. Larger osmiophilic droplets, averaging 150 mμ in diameter, were found in cytoplasmic vesicles within the epithelial cells.These findings suggest that electron microscopic visualization of lipid droplets cannot distinguish between fatty acid and mono-, di-, and triglycerides. Therefore, conclusions on the nature of osmiophilic lipid droplets in intestinal absorption must be based upon a correlation with biochemical findings.Chemical and radiochemical findings in the absorption of fatty acid indicate that the fatty acid enters the chyle primarily as triglyceride, that the triglyceride content of the intestinal wall is increased, and that there is a significant transfer of the C14 label from fatty acid to triglyceride at some stage during absorption. These observations mean that at least some of the osmiophilic droplets seen with electron microscopy in the intestinal epithelial cells in fatty acid absorption must be triglycerides. The relationship of these findings to the synthesis of triglyceride from fatty acids during internalization and transcellular passage is discussed.
