| Summary: | Cannulation of the vitelline vessels of the 11.5 day rat conceptus has allowed the introduction of both macromolecular and particulate substances directly to the embryo, thus bypassing the digestive and metabolic activity of the visceral yolk sac. This has facilitated the study of the fate of such substances and their metabolism by embryonic tissues. Vitelline injection does not affect the growth and differentiation of the conceptus in vitro providing that inert solutions (e.g. azo-blue coloured Hank's balanced solution) in volumes of less than 1.0 ul are employed. The vasculature of the 11.5 day embryo has been studied extensively using both scanning and transmission electron microscopy. This has shown that all the embryonic blood vessels can be classified as capillaries, both fenestrated (bridged by a diaphragm) and continuous endothelia being observed. However, the endothelial lining of the heart is discontinuous; this is probably related to the metabolic demands of the surrounding tissues. This method of study has enabled the transport mechanisms operational in the embryonic vessels to be elucidated. By injection of both radiolabelled and colloidal gold labelled immunoglobulin G and bovine serum albumin, the fate of protein substrates has been studied. Whereas immunoglobulin G was internalised by embryonic cells, probably by a receptor-mediated mechanism, and was not subsequently degraded, bovine serum albumin was captured by a non-specific adsorptive mechanism and routed to the lysosomal system for digestion. In comparison, invertase and polyvinylpyrrolidone were taken up to a much lesser extent, presumably by fluid phase (non-adsorptive) pinocytosis. When particulate substances were used as substrates, they were found to be mainly confined to cells of the endothelial lining of the blood vessels, with relatively little material penetrating to the mesenchymal layer. In contrast, when iron dextran was used, it was found mainly in the mesenchymal cells, although the explanation of this phenomenon is unknown. The methodology employed in this study has provided some useful insights into the means by which passive immunity (IgG) and nutritional molecules (other proteins) are acquired and dealt with by the embryo. The technique of cannulation of the vitelline vessels will in future provide the means to study the fate of many other macromolecules, in the embryo, without the influence of the extraembryonic tissues.
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