NEOGLYCOPROTEIN-BINDING SITES (ENDOGENOUS LECTINS) IN THE FALLOPIAN-TUBE, UTERUS AND BLASTOCYST OF THE RABBIT DURING THE PREIMPLANTATION PHASE AND IMPLANTATION

Regulation of the initial phase of embryo implantation may involve the recognition inter-play of glycoconjugates and respective receptors su ch as endogenous lectins on both cellular surfaces. Whereas changes in glycoconjugate composition have been detected in preparation for embr yo implantation and described in detail, knowledge on endogenous lecti ns has remained scant. Affinity probes (carrier-immobilized carbohydra te 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 n onpregnant and early pregnant [tubes: 3 days post coitum (d p.c.), ute ri: 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 chain s, lactose-BSA). Considerably less binding (but with the same pattern) was obtained with beta-N-acetyl-D-glucosaminide-BSA and is interprete d to originate from a cross-reactivity of such sites which may bind ph ysiologically to Gal-beta 1, 3/4-GlcNAc sequences. In contrast, no evi dence 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 cond itions. 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) ap pear(s) to be synthesized and secreted by the tubal epithelium. A phys iological role in forming the mucoprotein layer of the blastocyst cove rings by precipitating the appropriate mucin-type molecules can be con sidered. Within the endometrium the beta-galactoside-binding molecules were almost exclusively localized at the apical cell pole of epitheli al cells, whereas there was hardly any binding in the epithelial cytop lasm or in the endometrial stroma. The reaction was very weak in the n on-pregnant state but increased considerably until 5 d p.c., starting in the luminal-most parts of the epithelium. While the reaction was ra ther 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-specif ic 'receptors' was further enhanced in particular at the epithelial su rface of the placental folds. In contrast, the reaction was less inten se at the antimesometrial uterine epithelium and in interblastocyst se gments of the uterus, and it remained weak in the middle and deep cryp ts. 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 particula r 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 inv olved in the structural organization of the various layers of the blas tocyst coverings with their remarkable content of oligosaccharide chai ns. This effect on topological aspects of the zona pellucida equivalen ts may be important for the interplay between trophoblast and uterine epithelium and the cascade leading to implantation initiation.