The Glycosylation Pattern of Secretory Granules in Binucleate Trophoblast Cells is Highly Conserved in Ruminants

The fetal (F) and maternal (M) stroma is labelled in all micrographs to facilitate the identification of the respective placental compartments. A Tragulus, DBA staining. Two chorionic villi (fetal stroma marked with F) are separated by a maternal septum (M). Three BNC (arrows) with strongly stained granules are located in the trophoblast epithelium. One BNC (arrowhead) is now part of a fetomaternal syncytial plaque and is in contact with the maternal stroma. (Magnification bar = 20 μm for all figures). B Red Deer, DBA staining. The fetomaternal contact line is marked by arrowheads. BNC with stained granules can be seen in the trophoblast epithelium (above the fetomaternal contact line) and as part of fetomaternal trinucleate cells in the uterine epithelium (below that line). C Goat, WFA staining. BNC with intensely stained granules are marked by arrows. Since there is more staining of other structures (mainly maternal stroma) the cells are less conspicuous. D Goat, SBA staining. Intensively stained granules can be seen in BNC (arrows), but also basally in the uterine epithelium (arrowhead). E Red Deer VVA staining. F Goat HPA staining. Staining of BNC granules is slightly less intense than with other GalNac binding lectins.

A Tragulus PHA-L (magnification bar = 20  μm for all figures). BNC show an intense staining of the cytoplasm, while the nuclei remain unstained. B Water Deer PHA-E staining, showing moderate binding to granules. C Fallow Deer ECA staining. The BNC are just recognisable as unstained shadows (arrowhead). D Fallow Deer ECA + Neuraminidase. Treatment with neuramindase unmasks the ECA binding glycotope and results in a moderate staining of BNC granules (arrowheads). E Tragulus SNA-1 staining. BNC are clearly visible. F Impala SNA-1 staining. The BNC remain unstained, while maternal and fetal capillary endothelia show a moderate labelling.

Schematic drawing of the main N-glycan carried by PAGs in bovine BNC [46]. The tetraantennary glycan is attached to an asparagine (Asn) residue of the protein. Dotted circles mark the structural patterns which are important for lectin binding to this carbohydrate. The terminal α2,3-linked sialic acid (NeuAc) in this structure is not recognised by MAA-lectin, probably due to steric hindrance by the GalNAc linked to the subterminal Gal [46].

Phylogenetic tree of the ruminants (modified from [5]). Species used in this study are highlighted. The binucleate trophoblast cells (BNC) are present in all ruminants, while the cotyledonary placenta type is found only in the higher ruminants (pecora).