Ultrastructural and biochemical changes in the intestinal epithelium during the process of active
triglyceride absorption were studied in rats fed a
zinc-deficient diet as compared with those of pair-fed and ad libitum-fed
zinc-supplemented controls. The rate of
triglyceride absorption markedly decreased in
zinc-deficient rats. Despite a significant reduction in pancreatic
lipase activity, the digestion of
triglycerides proceeded normally in the
zinc deficient rats, as evidenced by no apparent signs of
diarrhea (or
steatorrhea) and by the appearance of the hydrolytic products such as
free-fatty acids and
monoglycerides in the intestinal mucosa. The mucosa uptake of digested
lipids and resynthesis of
triglycerides in the mucosa from deficient rats were normal. Ultrastructural and chromatographic analysis of the mucosal
lipids indicated a massive accumulation of lipid droplets, predominantly in the form of
triglycerides. The primary defect in
lipid absorptive processes in
zinc-deficient rats occurred in the formation of
chylomicrons. The lipid droplets in the mucosa of deficient rats were physically unstable. This instability was shown by coalescence of droplets which did not appear to be membrane-bound. Coalescing lipid droplets ranged from 2.0 to 4.0 micron in diameter. The absorptive cells were not able to discharge lipid droplets of this size into the intercellular spaces and hence into the lamina propria, resulting in the accumulation of the large droplets within the mucosa. This exit block to the movement of lipid droplets out of the mucosal cell appeared to be due to the failure, in
zinc-deficiency, of the mucosal synthesis of
proteins required for the formation of
chylomicrons. Ultrastructural observations demonstrated changes in the subcellular organelles related to
protein synthesis, including a marked reduction in granular endoplasmic reticulum and a quiescent appearance of the Golgi-complex.