Insulin-like growth factor ontogeny in the avian embryo and hatchling
Insulin-like growth factors I and II (IGF-I and IGF-II) once bound to the receptor are responsible for enhancing overall tissue growth. The three experiments explored the presence of IGF protein in amniotic and allantoic fluids, mRNA abundance in extra-embryonic membranes and small intestine, and formation of tight junctions in chickens, ducks, and turkeys during incubation and post-hatch growth. The IGF-II concentrations in the amniotic and allantoic fluids were higher than the IGF-I concentrations from stage 26 through stage 44 of embryonic development. A peak in IGF-I concentration occurred in chicken and turkey amniotic fluid midway through the incubation period. The chicken IGF-II concentration in the amniotic fluid was 2 times greater than the turkey and 2.8 times greater than the concentration found in the duck.
The IGF-I, -II, and -R transcript abundance in the amniotic and allantoic membranes was greater in the turkey compared to duck and chicken. Within the chicken's small intestine, duodenum IGF-I and IGF-II mRNA was lowest at day -4 of incubation. Chicken and duck IGF abundance decreased at day of hatch in the jejunum while turkey has the highest levels of IGF-II and -R expression during the time period. Whole segment expression of IGF was lower compared to mucosa in the duck while no propensity for higher IGF expression in the segment or mucosa was present in chickens and turkeys.
Tight junction formation in the small intestinal segments of the chicken, duck, and turkey found jejunal enterocyte cell perimeter decreases to hatch followed by an increase to 3 days post-hatch. The percent of enterocyte membrane involved in tight junction formation decreases to hatch and increases from day 1 to 3 post-hatch. The uniform size in the epithelial cell perimeter at day of hatch and uniform percentage of cell membrane involved in tight junctions from day of hatch to 1 day post-hatch in all species insinuates tight junctions are complete at day of hatch. The observed changes in enterocyte structure, location of organelles, and accumulation of lipid droplets in the villus verify observations made by other researchers.
Overall, the changing abundance of IGF protein and mRNA in the extra-embryonic membranes and fluids as well as the small intestine during the final days of incubation and post-hatch reveal the importance of IGF in the developing embryo. Determining that tight junctions are intact by day of hatch implies barrier function is complete. However, further studies are needed to evaluate the interaction between tight junction formation and IGF in the developing small intestine.