The Identification of Chylomicra and Lipoproteins in Tissue Sections and Their Passage Into Jejunal Lacteals

Abstract

The electron microscopic appearances of chylomicra and lipoproteins have been investigated. The particles were isolated from rat chyle by differential flotation in an ultracentrifuge. Various fixing and embedding media were used. The two kinds of particles were then identified in thin sections of the jejunum of rats. The chylomicra had diameters of from 1,000 A to 1 ~; the lipoproteins ranged from 100 to 1,000 A. They were identified by their sizes and their similarities to the isolated particles after the various fixing and embedding procedures, In addition, the relative amounts of the two kinds of particle varied greatly under different dietary conditions. The chylomicra had a thin rim, probably of phospholipid. Section B records the passage of the two kinds of particle into the lacteals in the villi of the jejunum. Both chylomicra and lipoproteins were seen passing through many open junctions. From permeability considerations it would seem that this is the most important route. These open junctions appear to act as "inlet valves" which prevent backflow as the contractions of the villi pump material out of the lacteals. Both chylomicra and lipoproreins were also seen entering the endothelial cells and lying inside them. The lipoproteins entered via "normal" caveolae and were seen in "normal" vesicles (~ 500 A); the chylomicra necessarily occupied much larger organelles. Both kinds of particles were also seen in caveolae on the luminal surface of the endothelium, but it was impossible to be certain that these were not just particles entering the cells from the lumen. The chylomicra often seemed to be washed out of these caveolae as many large, empty ones were seen on the luminal sides of the cells. Frequently, these caveolae had dark membranes.

An Electron Microscope Study of the Intestinal Absorption of Medium Chain and Long Chain Triglycerides in the Rat

“…In the core of the intestinal villus, it is generally considered that chylomicra enter into the central lacteal through the open gaps of endothelial junctions (CASLEY-SMITH, 1962;LADMAN et al, 1963;HORSTMANN and BREUKER, 1972), while DOBBINS and ROLLINS (1970) have suggested on the basis of their investigations using peroxidase, ferritin or chylomicron as a tracer that pinocytotic transport may also be very significant for chylomicron transport. The present study, however, is compatible with the opinion of CASLEY-SMITH and other workers.…”

Synthesis and transport of lipoprotein particles by intestinal absorptive cells in man

“…The evidence which supports this belief is the almost complete disappearance of lipoprotein particles from fasting absorptive cells of rats (7) and men after biliary exclusion 8 and their absence from human jejunum after bile has been completely excluded by a short circuit operation for obesity. 9 The morphological evidence suggests that lipoprotein particles follow the same pathway in the fasting state as do chylomicrons and lipoprotein particles after a fatty meal. Lipoprotein particles were evident within endoplasmic reticulum, some of which connected with Golgi cisternae.…”

“…These particles were presumed to be lipoprotein because they were identical in appearance to those which Casley-Smith (9) found in the vicinity of rat intestinal lacteals and which he showed to be morphologically similar to those isolated from rat chyle by ultracentrifugation.…”

The ultrastructure of lipid absorption and transport in bovine intestinal mucosa

“…Lipid was observed to enter the lacteals through fenestrations in the endothelium, or across the endothelial cytoplasm (Casley-Smith, 1962). …”

“…Particles similar to those isolated and identified as chylomicrons by Casley-Smith (1962) were prepared from intestinal lymph of a milk-fed calf and examined by electron microscopy ( Figure 33D). Size variation ranges from 90 to 360 mp, generally being larger than the apical or Golgi droplets observed in fixed tissue.…”