Comparison of the metabolic activity of yolk sac tissue in the whole embryo and isolated yolk sac culture

In rat visceral yolk sac tissue cultured from 10.5 to 12.5 days of prenatal age, metabolism of the lipoxygenase inhibitor N-hydroxy-N-methyl-7-propoxy-2-naphtalinethanamine (QA 208-199, QAB) and the accumulation of its metabolites have been shown previously (1,2). In this study, the metabolic activities of visceral yolk sac tissues cultured either alone or together with the embryo were compared. The metabolite patterns in medium and visceral yolk sac tissue generated by intact conceptuses or by isolated visceral yolk sac tissues were similar. After 24 as well as 48 h of culture, the major in vivo metabolite, 7-propoxy-napthalene-2-ylacetic acid (QAA) and other, as yet unidentified metabolites, accumulated in embryo proper and visceral yolk sac tissues. QAB was not found in the embryo proper, and was only found in yolk sac tissues using the higher concentration. In particular the metabolites M5 and M6 exhibited a massive accumulation in the visceral yolk sac, whereas QAA, M3, and M4 accumulated to a much lesser degree. In isolated yolk sacs cultured for 48 h, tissue levels of QAA and M4 were similar to those in yolk sacs of cultured whole conceptuses, whereas M5 and M6 exhibited twofold higher levels in isolated yolk sacs. These findings were in agreement with the distinct increase of myeloid figures containing partly fragmented inclusion bodies in yolk sac tissues. These results suggest that the visceral yolk sac may be the major site of QAB metabolism in cultured rat conceptuses in vitro.