Glycans of the early human yolk sac.

The pattern of glycan distribution in the early human yolk sac has been investigated using a panel of lectins. Two 6-week and one 8-week human yolk sacs, and one 8-week fetal liver from live, ectopic pregnancies were fixed and embedded in epoxy resin. Lectin histochemistry was carried out on sections of these tissues using 23 biotinylated lectins and an avidin-biotin peroxidase revealing system. Mesothelial surfaces expressed most subsets of N-glycans (other than high mannose types), N-acetyl-lactosamine, sialic acid, and alpha 1,6-N-acetylgalactosamine. Endodermal surface and lateral membranes resembled those of mesothelium, but showed a preponderance of alpha 2,6-sialyl residues. Most intracellular granules contained N-glycan. There was a marked heterogeneity of granules in the endodermal cells, with different subsets varying in both staining and positional characteristics. The mesenchymal matrix bound most of the lectins used in the study, and expressed fucosyl residues which were also detected in the endothelium. Fetal liver parenchyma showed very similar staining patterns to those seen in the endoderm except for the distribution of N-acetylglucosamine, which was sparse. Despite some common features, each germ cell layer had a distinct 'glycotype', with some saccharides showing extreme topographical restriction.