Neoglycoprotein-binding sites (endogenous lectins) in the Fallopian tube, uterus and blastocyst of the rabbit during the preimplantation phase and implantation.

Abstract

Regulation of the initial phase of embryo implantation may involve the recognition interplay of glycoconjugates and respective receptors such as endogenous lectins on both cellular surfaces. Whereas changes in glycoconjugate composition have been detected in preparation for embryo implantation and described in detail, knowledge on endogenous lectins has remained scant. Affinity probes (carrier-immobilized carbohydrate structures as ligand part on a histochemically inert backbone) are used in the present investigation in order to gain further insights in this area. Cryostat sections of rabbit Fallopian tubes and uteri in nonpregnant and early pregnant [tubes: 3 days post coitum (d p.c.); uteri: 3, 5, 7 and 9 d p.c.] states were studied for binding patterns of a series of biotinylated (neo)glycoproteins. A high density of binding sites was detected with beta-galactosides (with decreasing intensity: beta-D-galactose-BSA, asialofetuin with its triantennary glycan chains, lactose-BSA). Considerably less binding (but with the same pattern) was obtained with beta-N-acetyl-D-glucosaminide-BSA and is interpreted to originate from a cross-reactivity of such sites which may bind physiologically to Gal-beta1,3/4-GlcNAc sequences. In contrast, no evidence for the presence of binding molecules with specificities for a-D-mannose-BSA, maltose-BSA, N-acetyl-galactosaminide-BSA and N-acetyl-D-neuraminic acid-BSA was obtained in these tissues under the same conditions. The epithelium of the Fallopian tube showed a high density of beta-galactoside-binding sites at the apical cell poles (including the cytoplasm and membrane region) already in the nonpregnant state. At 3 d p.c., a strong reaction in all epithelial cells of the isthmus and a marked decrease in the ampulla were noted. The putative lectin(s) appear(s) to be synthesized and secreted by the tubal epithelium. A physiological role in forming the mucoprotein layer of the blastocyst coverings by precipitating the appropriate mucin-type molecules can be considered. Within the endometrium, the beta-galactoside-binding molecules were almost exclusively localized at the apical cell pole of epithelial cells, whereas there was hardly any binding in the epithelial cytoplasm or in the endometrial stroma. The reaction was very weak in the non-pregnant state but increased considerably until 5 d p.c., starting in the luminal-most parts of the epithelium. While the reaction was rather homogeneous at the surface of the luminal epithelium at 5 d p.c., the degree of heterogeneity increased stepwise from 7 to 9 d p.c. In the implantation chamber, the density of these beta-galactoside-specific 'receptors' was further enhanced in particular at the epithelial surface of the placental folds. In contrast, the reaction was less intense at the antimesometrial uterine epithelium and in interblastocyst segments of the uterus, and it remained weak in the middle and deep crypts. The trophoblast showed a high density of galactoside-binding sites at its surface, and less in the cytoplasm. Neoglycoprotein binding to the blastocyst coverings observed at 7 d p.c. was strong in particular at the outer and inner surfaces. Physical factors (e.g. differential texture at surfaces) are discussed to influence the staining patterns of these extracellular coverings. Nevertheless, the observations made on the tubal and the uterine mucosa suggest that the putative lectin(s) detected here is (are) secreted by these epithelia and could be involved in the structural organization of the various layers of the blastocyst coverings with their remarkable content of oligosaccharide chains. This effect on topological aspects of the zona pellucida equivalents may be important for the interplay between trophoblast and uterine epithelium and the cascade leading to implantation initiation.