The egg white and egg yolk are the two main sources of nutrients for the developing avian embryo. The egg white should be transferred into the yolk in order to be consumed by the embryo. How the egg white ultimately reaches the egg yolk is largely unexplored in the turkey Meleagris gallopavo. Here, we explored the routes of egg white transfer in fertilized turkey eggs. Initially, we tested the electrophoretic pattern of the proteins in different egg compartments throughout development. Then, we used lysozyme as a reference protein to follow the egg white transfer, and we measured its activity using Micrococcus lysodeikticus as a substrate. We found that several presumptive egg white protein bands appeared in the different egg compartments. Also, the electrophoretic patterns in the intestinal fluid and thick yolk were marked by the disappearance of large bands and the appearance of small ones at late developmental stages. Moreover, we detected a chronological appearance of lysozyme activity in the different egg compartments. The activity appeared in the extraembryonic and amniotic fluids on day 15, in the intestinal fluid on day 16 and in the thick yolk on day 17, and it increased in general with the progress of development. Our results suggest that the main route of egg white transfer is albumen sac -extraembryonic cavity -amniotic cavity -intestinal lumen -egg yolk. Furthermore, the transferred egg white proteins seem to undergo digestion in the intestinal lumen and egg yolk at late developmental stages.
The egg white and egg yolk are the two main sources of nutrients for the developing avian embryo. Egg white ultimately reaches egg yolk before being consumed by the embryo. Previously, in the turkey Meleagris gallopavo, we showed that the major egg white transfer into the yolk through the amnion and intestinal lumen of the embryo started on day 17 and became obvious on day 19. We also suggested that the transferred egg white and endogenous yolk proteins undergo digestion at late developmental stages. However, the protease activity throughout development and the type of activated proteases are completely unexplored in the yolk of turkey eggs. Here, we measured the general proteolytic activity in the egg yolk during different developmental stages using casein as a substrate. Furthermore, we determined the type of activated proteases by employing different types of protease inhibitors. Protease activity in the egg yolk was basal throughout development until day 19 when activity significantly increased, remaining high thereafter. Moreover, acidic aspartic protease(s) were primarily activated in the yolk. Our results suggest that the transferred egg white and endogenous yolk proteins are mostly degraded by the aspartic proteases that become highly activated late in development.
Yolk proteins undergo digestion either inside the egg yolk or in the surrounding yolk sac membrane (YSM) before being consumed by the developing avian embryo. However, the mechanisms underlying the digestion of yolk proteins during embryogenesis are largely unexplored in the pigeon Columba livia domestica. To better understand these mechanisms, the present study examined the classes of activated proteases in the egg yolk and the gene expression patterns of cathepsin B (CTSB) and cathepsin D (CTSD), which encode for lysosomal cysteine and aspartic proteases, respectively, in the YSM. We investigated the activated proteases by applying different types of protease inhibitors to yolk samples taken from incubation day 16. Then, we detected the mRNA levels of CTSB and CTSD in the YSM at incubation days 6, 8, 10, and 12-17. Both cysteine and aspartic proteases appeared to be activated in the egg yolk. Moreover, CTSB expression increased progressively and reached the maximum value on day 13; however, it decreased significantly on days 14 and 15 and further reduced toward hatching (day 17). In contrast, CTSD expression was weak and fluctuated insignificantly during development. Our results suggest that the degradation of yolk proteins at late developmental stages largely occurs in the egg yolk itself, probably by the activated cysteine and aspartic proteases. Furthermore, cathepsin B in the YSM seems to have a primary role in protein digestion, but this role decreases toward hatching.
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