Confocal and electron microscopy to characterize sialoglycoconjugates in mouse sublingual gland acinar cells

Double lectin labeling for confocal microscopy and lectin-protein A-gold binding for electron microscopy were applied to the mouse sublingual gland in order to study surface and cytoplasmic sialoglycoconjugates. For this purpose, serially cut sections were submitted to sialidase followed by incubation with lectins recognizing usually acceptor sugars for terminal sialic acids. At the electron microscope level, the residues subtended to sialic acid were individually identified on adjacent sections by an indirect technique of labeling, whereas with confocal microscopy the above sugars were simultaneously visualized on the same section by a double staining method using fluorescein isothiocyanate (FITC)- and tetramethylrhodamine isothiocyanate (TRITC)-conjugated lectins. Acinar cells were found to contain the terminal sequence sialic acid-beta-galactose in abundance while the sequence sialic acid-alpha-N-acetylgalactosamine appeared to be present in modest amounts. Both sialoglycoconjugates were homogeneously codistributed inside acinar cells. The combination with a saponification method also allowed the occurrence of C4 acetylated sialic acids linked to beta-galactose to be discovered, at the electron microscope level, on acinar cell secretory products.