An ultrastructural study using electron microscopy was undertaken of yolk and yolk sac to elucidate digestive mechanisms and to gather evidence to further substantiate the belief that intracellular digestion of yolk takes place in yolk sac epithelial cells during incubation. Initial work focused on the ultrastructure yolk in unincubated eggs using transmission electron microscopy of thin sections of yolk which had been fixed using conventional and novel fixatives. The next phase of the study involved the ultrastructural comparison of granules from sub-blastodermic fluid from 9 day old embryos, and extracellular yolk attached to the yolk sac of 15 day old embryos with the granules from yolk of unincubated eggs. The third phase of the study was the ultrastructural description of yolk sacs at 3 ages (3 days, 8 days and 15 days of incubation). A transport system for yolk lipid and its digestion products to the embryo was demonstrated ultrastructurally using the Imidazole-buffered Osmium Tetroxide protocol of Angermuller and Fahimi (1982) which enhanced lipid staining. We observed no ultrastructural changes in extracellular yolk granules and matrix particles at any age studied. Granules and matrix particles are taken up by endocytosis, and ultrastructural changes of yolk granules occur in epithelial cells, in acid phosphatase-containing vacuoles (which we have identified as secondary lysosomes), which are part of the functioning intracellular digestive system. We have ultrastructurally demonstrated the transport of lipids, which becomes more evident in 13 day old embryos. On the basis of the above evidence we conclude that yolk is digested in epithelial cells of the yolk sac, and the digestion products enter the blood sinuses and travel in the bloodstream to deliver nutrients to the developing embryo. (Abstract shortened by UMI.)
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/6930 |
| Date | January 1994 |
| Creators | Allan-Wojtas, Paula. |
| Contributors | Hincke, M. T., |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 170 p. |
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