Lectin receptors on the peri- and early post-implantation mouse embryo

Ultrastructural localization of lectin receptors in the preimplantation ovine embryo

BACKGROUND

Preimplantation development of mammalia is characterized by cell surface changes functioning in intercellular communication and adhesion. The glycoconjugate role in cellular interactions has been analysed for several groups but not in sheep embryos. The binding patterns of eleven lectins during sheep preimplantation development were investigated and the role of glycoconjugates in early development was discussed.

METHODS

Ultrathin sections from preimplantation ovine embryos (3-7 days) were incubated with different colloidal gold conjugated lectins and the frequency of gold particles on the cell membrane, some organelles, and the zona pellucida was evaluated.

RESULTS AND CONCLUSIONS

We observed a higher staining of WGA, DBA, and SBA lectins in the intercellular contact zone with respect to the free cell surface of blastomeres during cleavage. This indicates that the N-acetyl galactosamine and N-acetyl glucosamine residues may be involved in sheep morula compaction. In contrast, the trophoblast cell displays an increase of staining of some lectins previously identified during cleavage (LcH, WGA, SBA, MPA, and PNA) on the free membrane, and a lack of sugar residues in the intercellular surface. This polarization of the trophoblast cell surface is not observed in the inner cell mass and could provide a mechanism for differentiation within the blastocyst. Intracytoplasmic vesicles show a cytochemical identity with lysosomes in the blastocyst (abundant GlcNAc and Man/Glc residues) that may reflect a functional relationship between both organelles in an intracellular cycle. The zona pellucida presents abundant GalNAc, GlcNAc, and Gal residues during preimplantation ovine development.

Gastrulation in the mouse embryo: ultrastructural and molecular aspects of germ layer morphogenesis

Ultrastructural studies and lineage analyses of gastrulating mouse embryos have revealed that different morphogenetic tissue movements are involved in the formation of the three definitive germ layers. Definitive ectoderm is formed by epibolic expansion of the pre-existing progenitor population in the embryonic ectoderm. Formation of the mesoderm and the endoderm is initiated by cellular ingression at the primitive streak. The mesodermal layer is established by cell migration and cell sheet spreading, but the endoderm is formed by replacing the original primitive endodermal population. To this date, genes that are expressed during mouse gastrulation mostly encode cell surface adhesion or signalling molecules, growth factors and their receptors, and putative transcriptional factors. Their precise role during gastrulation remains to be investigated.

Structural and functional properties of trophoblast cells of mouse egg-cylinders in vitro

Embryos at the egg-cylinder stage were obtained by culturing blastocysts in vitro on an agarose surface for 4 days. The adhesiveness and outgrowth activity of the trophoblast of the egg-cylinder were compared with those properties of the flushed adhesive blastocyst. Trophoblast cells of egg-cylinders were found to be more adhesive and their outgrowth activity greater than in the case of trophoblast cells of adhesive blastocysts. The cultured egg-cylinders presented two subpopulations of giant trophoblast cells. They represent probably two stages of differentiation of the same trophoblast population. The most distinctive morphological differences of these subpopulations were that the surface of one was covered with small blebs and the cytoplasm had relatively few ribosomes, while the surface of the other subpopulation was covered with large blebs and microvilli and the cytoplasm was rich in ribosomes and large, dense granules. The two types of giant trophoblast cells of the day 7 implantation site consist of one subpopulation whose cytoplasm has few organelles, and the other subpopulation, whose cytoplasm is rich in ribosomes, lysosome-like bodies, and heterophagosomes. Hence, we conclude that the in vitro grown trophoblast cells have counterparts in the in vivo implanting embryos. The lectin binding pattern of the agarose cultured egg-cylinder trophoblast cells was similar to that of the adhesive and/or invasive trophoblast cells grown in vivo. Thus, the in vitro grown egg-cylinders are appropriate material for the analysis of trophoblast cells at the invasive stage of implantation.

Lectin-induced abnormalities of mouse blastocyst hatching and outgrowth in vitro

We have examined the role of cell surface glycoconjugates during mouse blastocyst maturation, hatching, attachment, and outgrowth by monitoring the influence of six lectins on blastocyst development in vitro. Two lectins, concanavalin A and wheat germ agglutinin were toxic to blastocysts at the concentrations used. Bandierea simplicifolia lectin 1 (BSL-1) induced abnormal growth, developmental arrest at the hatching stage, and some disruption of cell contacts. Culture with Lotus tetragonolobus lectin-1 (LTA-1) also disrupted cell contacts and caused developmental arrest. The remaining lectins, Dolichos biflorus agglutinin (DBA) and Ulex europaeus agglutinin (UEA), retarded blastocyst hatching and outgrowth but did not induce any major defects, although differentiation of the inner cell mass was limited by both. This study demonstrates that very low concentrations of lectins can disrupt blastocyst development, suggesting that exposed surface saccharide moieties may be involved in interactions between blastomeres and their environment.

Fucosylated glycoconjugates appear on mouse embryos during blastocyst formation

Mouse embryos were analysed for expression of surface glycoconjugates using a panel of fluorescein-labelled lectins. Morulae and blastocysts but not early cleavage stages stained brightly with fucose binding lectins. By contrast, cleavage stages stained brightly with all the other lectins tested. These findings provide evidence for substantial reorganisation of the cell surface during blastocyst formation and are consistent with a role for fucosylated glycoconjugates in compaction.