An electron microscopic study of the intestinal villus. II. The pathway of fat absorption

The intestinal pathway for absorbed fat was traced in thin sections of intestinal villi from rats fed corn oil by stomach tube after a fast of 24 to 40 hours. For electron microscopy the tissues were fixed in chilled buffered osmium tetroxide and embedded in methacrylate. For light microscopy, other specimens from the same animals were fixed in formal-calcium, mordanted in K(2)Cr(2)O(7), and embedded in gelatin. Frozen sections were stained with Sudan black B or Sudan IV. About 20 minutes after feeding, small fat droplets (65 mmicro maximal diameter) appear in the striated border between microvilli. At the same time fat particles are seen within pinocytotic vesicles in the immediately subjacent terminal web. In later specimens the fat droplets are generally larger (50 to 240 mmicro) and lie deeper in the apical cytoplasm. All intracellular fat droplets are loosely enveloped in a thin membrane, the outer surface of which is sometimes studded with the fine particulate component of the cytoplasm. This envelope, apparently derived from the cell surface by pinocytosis, has at this stage evidently become a part of the endoplasmic reticulum. Just above the nucleus numerous fat droplets lie clustered within the dilated cisternae of the Golgi complex. As absorption progresses fat droplets appear in the intercellular spaces of the epithelium, in the interstitial connective tissue spaces of the lamina propria, and in the lumen of the lacteals. All of these extracellular fat droplets are devoid of a membranous envelope. The picture of fat absorption as reconstructed from these studies involves a stream of fat droplets filtering through the striated border, entering the epithelial cell by pinocytosis at the bases of the intermicrovillous spaces, and coursing through the endoplasmic reticulum to be discharged at the sides of the epithelial cell into extracellular spaces. From the epithelial spaces, the droplets move into the lamina propria and thence into the lymph. If the lumen of the endoplasmic reticulum is considered as continuous with the extracellular phase, then the entire pathway of fat absorption may be regarded as extracellular. However, it is impossible to evaluate from the electron microscopic evidence thus far available the quantitative importance of particulate fat absorption by the mechanism described.

Triton hyperlipemia in dogs. II. Atheroscieross, diffuse lipidosis, and deletion of fat stores produced by prolonged administration of the non-tonic surface-active agent

Fourteen dogs, fed a regular diet and given 250 mg/kg of triton (a non-ionic surface-active agent) intravenously every 4th day, exhibited a progressively severe hyperlipemia. Serum triglycerides were the first to increase. Cholesterol, mostly in the free form, and phospholipids showed elevation only at a later stage and increased at almost identical rates. The plasma-free fatty acid concentration was from 2 to 3 times above normal. With establishment of sustained hyperlipemia, there was reduction, followed by total disappearance, of the high density D 1.063 to 1.21 lipoprotein. Most of the cholesterol and phospholipids (70 to 75 per cent of the total) were found in the D 1.006 to 1.063 lipoprotein class, the remainder in the D < 1.006 class. Triglycerides were almost evenly distributed between these two classes. The concentration of the serum lipoprotein proteins was within normal limits. All of the animals died within from 4 to 5 months after receiving the first injection of triton. Autopsy findings consistently showed: (a) numerous lipidladen macrophages in the liver, spleen, and lymph nodes; (b) significant depletion of all fat stores; (c) presence of lipids, either free or engulfed in macrophages (foam cells), in the subintima of the coronary arteries, aorta, and pulmonary arteries, indicating an early stage of atherosclerosis. Concurrent daily administration of heparin (5 mg per kilogram of body weight) did not substantially change the course of the disease. Withdrawal of triton from animals that had been receiving the detergent for from 3 to 4 months, elicited a slow return to normal of the lipid pattern. In two dogs killed when normolipemia was reestablished, all tissues were normal with the minor exception of a few hepatic macrophages still laden with sudanophilic material. It is postulated that the primary action of the injected triton was on the lipid moieties of plasma lipoproteins with formation of complexes, which, as foreign bodies, were preferentially taken up by the cells of the reticuloendothelial system. Depletion of fat stores was probably secondary to increased lipid mobilization, as an attempt by these tissues to supply energy to the parenchymal cells unable to utilize triton-bound lipids.