American Journal of Clinical Nutrition, Vol 22, 257-265, Copyright © 1969 by The American Society for Clinical Nutrition, Inc.

Morphologic Aspects of Lipid Absorption

¹ From the Gastrointestinal Research Laboratory, Veterans Administration Hospital, and Department of Medicine, Duke University Medical Center, Durham, North Carolina 27705

Even though there is extraction of 13-26% of unsaturated neutral lipids and phospholipid during electron microscopic processing of intestinal tissue, morphologic studies of fat absorption can be considered a faithful reproduction of the events taking place. Pinocytosis of fat is not a mechanism of fat absorption. Micelles have not been convincingly demonstrated by morphologic methods. Overwhelming morphologic and biochemical data show that fat entry into absorptive cells is by passive diffusion of free fatty acids and monoglycerides. During fat absorption, morphologically demonstrable lipid droplets first appear within profiles of the smooth endoplasmic reticulum as chylomicrons. The protein component of chylomicrons is probably manufactured in the rough endoplasmic reticulum. Chylomicrons next appear within Golgi vacuoles and are probably extruded from the absorptive cell by way of the vacuoles by a process of reverse pinocytosis, though this has never been demonstrated. The rate-limiting process of fat absorption is that of exit of fat from the cell and events taking place within the Golgi apparatus may be the specific site of this rate-limiting step. Chylomicron passage to lacteals within the extracellular space is probably controlled by intestinal pressure changes and by muscular contractions of villi. Entry into the lymphatic lumen is largely via gaps in endothelial cells.