The Expression of Proteases and the Oligopeptide Transporter in the Yolk Sac Membrane, Proventriculus, and Small Intestine During the Development of Embryo.

The role of the yolk sac membrane (YSM) and digestive tract in the processing of egg yolk proteins during embryogenesis is unexplored in the duck . Here, we investigated in the duck embryo the function of the YSM, proventriculus, and small intestine in protein digestion and uptake. We tested the expression of aminopeptidase N () and the oligopeptide transporter as well as the expression of cathepsin B () and cathepsin D () lysosomal genes in the YSM during incubation days 12, 14, 16-18, 20, 22, 24, 26, and 28 (the day of hatch). Also, we examined embryonic duck pepsinogen () expression in the proventriculus and and expression in the small intestine. In the YSM, expression was weak compared to that of , and the expression of , , and reached its maximum on day 24 and decreased afterwards. In the proventriculus, expression peaked on days 17 to 20 and decreased thereafter. The and expression levels were highest in the jejunum and ileum and reached their maximum on day 28. Our results suggest that the YSM plays a role in the degradation and uptake of the peptides that are digested by the activated yolk proteases, and it also functions in the lysosomal digestion of yolk lipoproteins. Furthermore, the proventriculus is possibly involved in the digestion of yolk proteins. Finally, the jejunum and ileum appear to be the primary sites for peptide digestion and absorption at the end of the incubation.