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High concentrations of dietary zinc have been reported to improvegrowth performance and health in piglets. It was the goal of the present study to characterize underlying mechanisms ofdifferent levels ofdietary zinc oxide on the gutmorphology and immune responses during four weeks after weaning.
In this thesis, Chapter 1introduceszinc oxide as feed additivewhich isknown to have effectsagainst diarrheaproblemsin pigs,and stressesthe significance to investigate the underlying mechanisms of dietary zinc on intestinal mucosalimmunity in weaned piglets.
Afterwards, Chapter 2reviewsprotective mechanisms in the intestinal mucosa andspecially focuseson the innate defense factors includingmucins, trefoil factors, antimicrobial peptides, and toll-like receptors. The characteristicsof weaning-associated changes in gut morphologyand intestinal immune responsein pigletsaredescribed. Also the roleof trace element zincin the gastrointestinal tractis illustrated.
The aim and hypotheses of this thesis areemphasized in Chapter 3.
The effects of dietary zinc oxide on intestinal morphological and immunological characteristics in the jejunum (Chapter 4) and the colon (Chapter 5) of weaned pigletswereevaluated, respectively.
Chapter 4 focuseson theinvestigation of changes of the proximal and distal jejunumto different dietary zinc levels. The resultsshowed that villus height and crypt depth differed age-dependently, whereas the dietary concentration of zinc oxidehad no effect. Similarly, the mucin chemotypes were modified by age.Dietary zinc oxidehad no effect in the proximal jejunum. In the distal jejunum, age and zinc impactedthe mucin chemotypes in goblet cells. The abundance of sulfomucins decreased and sialomucins increased with age. High level of dietary zinc oxidereduced the sulfomucins and increased sialomucins in the jejunal crypts. The phenotypes of lymphocytes in the epithelium of jejunum showed a relatively constant percentage of T cells as well as NK cells. The highestconcentration of dietary zinc led to a reduced abundance of CD8+γδT cells. The relative proportion of different cytotoxic T cells was age-dependent. The percentage of CD4−CD8β+ increased, whereas the relative amount of CD4+CD8β+ decreased with age. The expression of MUC2 and MUC20 was not influenced by age or dietary zinc concentration. High zinc intakes resulted in a reduced gene expression of β-defensin 3 and did not affect the expression of TFF3. Thus, it was concluded that zinc in the form of zinc oxide appears to have specific effects on the gut associated immune system of piglets that may explain improved health and barrier function whenpiglets were fed withhigh levels of zinc.
In Chapter 5, the changes of the proximal colon to dietary zinc werestudied.The colonic crypt area increased in an age-depending manner, and the greatest area was found after feeding medium concentration of dietary zinc. After feedingthe high concentration of dietary zinc, the number of goblet cells containing mixed neutral-acidic mucins and total mucins increased. Sialomucin containing goblet cells increased age-dependently. The expression of MUC2 increased with age and reached the highest level at 47 days of age. The expression levels of TLR2and 4decreased with age. The mRNA expression of TLR4 and the pro-inflammatory cytokine IL-8wasdown-regulated with high dietary zinc treatment, while piglets fed medium dietary zinc had the highest expression.
The effects of dietary zinc oxideon morphological and gut immunological characteristics in jejunum and colon of weaned piglets arediscussedin Chapter 6.
Taken together, the present study has revealed some interesting impacts of dietary zinc levels on morphology, mucins and epithelial immune parameters, which may help to understand the protective mode of high dietary levels of zinc against post-weaning diarrheain piglets.Dietary zinc levels had a clear impact on colonic morphology, mucin profiles and immunological traits in piglets after weaning. Those changes might support local defense mechanisms and affect colonic physiology and contribute to the reported reduction of post-weaning diarrhea.