65 Improvement of bovine early embryo development in vitro by coculture with endometrial epithelial cells

Abstract

We optimized a bovine endometrial epithelial cell (BEEC) line as a valuable research model for the study of very early embryo-maternal interactions in vitro. In this study, we aimed to (1) characterise the BEEC monolayers along the primary culture and first passages with respect to the expression of epithelial and mesenchymal cell markers and abundance of functional key transcripts; (2) to test whether direct or indirect contact with endometrial cells alter the quality of the embryos in vitro; and (3) to test the specificity of the effect. In Exp. 1, after isolation from slaughterhouse uteri at the early luteal phase, BEEC were cultured in DMEM/F12 phenol red-free medium supplemented with 10% fetal bovine serum (FBS) from primary culture until subculture 3. Fixed samples were immunostained for cytokeratin and vimentin. Transcript abundances for cellular lineage markers (KRT18 and VIM), oestrogen receptor (ESR1), interferon α/beta receptor 1 (IFNAR1), and prostaglandin G/H synthase 1 (PTGS1) and 2 (PTGS2) were evaluated by real-time quantitative PCR. Statistical analyses were carried out by ANOVA and Tukey test. Immunofluorescence data revealed that the BEEC line co-expresses cytokeratin together with a mesenchymal marker (Vimentin). This indicates that these epithelial cells underwent an epithelial-mesenchymal transition in vitro. Gene expression data showed a 6-fold increased (P < 0.001) abundance of VIM mRNA from the primary culture to the subculture 1, which remained constant until subculture 3; however, KRT18, ESR1, IFNAR1, PTGS1, and PTGS2 were similar between the passages, suggesting that the cells conserved their functional characteristics. In Exp. 2, groups of 15 morulas (Day 5.5) were cultured in SOF medium supplemented with 5% FBS in the absence (control) or in the presence (co-culture) of BEEC at passage 2, for 48 h. Embryos were placed on direct or indirect contact with a BEEC monolayer using a 96-well insert containing 8 μm pores. Developmental rates were compared by chi-square test and P-values were adjusted by Tukey’s test. The percentage of embryos that had developed from morula into blastocyst stage on Day 7.5 was significantly higher in the direct and indirect contact co-culture (65%; P < 0.05) groups compared with the control (53%) group. Moreover, 63% of the blastocysts were expanded, hatching, or hatched in the co-culture groups, whereas a rate of 46% was found in the control counterparts (P < 0.05). In Exp. 3, the same experimental conditions from Exp. 2 were used, but groups of 15 Day 5.5 morulas were cultured in control, or conditioned medium from BEEC (CondBEEC) or bovine fibroblasts (CondFib). Blastocyst development rate on Day 7.5 was higher in the CondBEEC group (71%; P < 0.001) compared with the control (54%) and CondFib (50%) groups. In conclusion, based on the markers studied, BEEC monolayers undergo epithelial-mesenchymal transition in vitro but preserve functional characteristics after few passages. The co-culture system improves bovine embryonic development from morula into blastocyst stage. This support is BEEC specific and does not rely on a direct cell-to-embryo contact.