MM Godlewski, J Bierła, A Kotunia, R. Zabielski (2008) Department of Physiological Sciences, Faculty of Veterinary Medicine, Warsaw University of Life Sciences – Livestock Science (under revision)
Rebuilding of the gut epithelium during the early postnatal period and after weaning are extremely fast and involve proliferation, differentiation and programmed cell death (apoptosis and autophagy) processes. However, the rebuilding of the gut just after the birth is far different from that after the weaning. After birth there is a significant increase in mitotic activity in the crypt area, and the subsequent generations of differentiated enterocytes greatly vary in their morphology and function. Just after birth the enterocytes are equipped, like during the late foetal-period, with unique canaliculo-tubular system located in the apical part of the enterocyte. Its presence enables transfer of colostral macromolecules, such as immunoglobulins, hormones, growth factors and many other biologically active proteins and peptides from the gut lumen into the gut mucosa and circulation. During the first postnatal days apoptosis is reduced resulting in enhanced mitosis/apoptosis ratio. According to our studies, the ratio is doubled as compared to unsuckling neonates or 1 d old piglets (Table 1). In neonates the speed of gut mucosa rebuilding depends apparently on luminal nutrition. Switch from colostrum and milk feeding to milk formula feeding markedly slows down the rebuilding processes. The reason is lack of or markedly reduced supply with hormones (e.g., insulin, glucagon-like peptide-2 – GLP-2, leptin) and growth factors (e.g., epidermal and insulin-like growth factors – EGF and IGF) present in colostrum and milk in large quantities. Slower mucosa rebuilding delays gut barrier closure, and enhances the probability of microbial invasion.
The rebuilding after weaning is influenced by a number of factors such as energy and protein intake and the composition of bioactive substances in solid food, modification of gut microbial ecosystem, stress, andmanyothers.Large reduction in villi length is associated with increased enterocyte apoptosis which may in turn weaken the epithelial integrity in the gut mucosa. We have observed that well performing pigs at weaning showed little modification of the mitosis/apoptosis index in the mid-jejunum mucosa. Namely, the enhanced apoptosis, if present, was associated with increased mitosis (Table 1). In the intestinal mucosa of pig, calf and rat neonates, we observed that cells die in the groups or packets, suggesting involvement of some paracrine factors. The death signal was transferred via tissue continuum as well as across the gut lumen, and the involvement of transforming growth factor b1 (TGFb1) and tumor necrosis factor b (TNFa) was demonstrated. In pig neonates, the dying cells and the neighboring cells expressed an enhanced TGF-RII, receptor for TGF. In the dying cells the death signal promoted via TGF-RII, was associated with enhanced expression active caspase 8, TGFb1, TNFa and Q Bid (Figure 1 and 2). Quantitative study showed that high expression of TGFb1 was positively correlated with expression of BID and negatively with BCL-2, illustrating the transmission of signal from TGF-RII through SMAD cascade and RunX protein. We hypothesize that TGF-b1 sensitizes the enterocytes for TNFa signaling and both cytokines control the apoptosis process in the gut epithelium. In our studies we use a quantitative automated method of evaluation of intestine mucosa remodeling, based on molecular markers of mitosis, programmed cell death (apoptosis andautophagy)andDNAdamage indexes. Microphotographs of immuno-labeled tissuesections were acquired by the means of FV-500 laser scanning confocal microscope (Olympus). Object recognition is performed using the Microimage - image analysissoftware(Olympus) on the basis of fluorescent intensity, and the acquired data are subjected to quantitative evaluation.
