106 THE RECEPTIVE BEEF COW ENDOMETRIUM: POTENTIAL KEY FEATURES FAVOURING THE COMMUNICATION BETWEEN EMBRYONIC AND MATERNAL TISSUES

A receptive state of the endometrium is crucial for proper communication with the pre-implantation embryo and, thereby, a perquisite for a successful pregnancy. Both reduction of transmembrane mucin and initiation of apoptotic events have been proposed as key features prompting embryo/endometrial cell interaction and facilitating maternal receptivity of the murine and human embryo. In beef cows, however, the signature for receptivity needs to be elucidated. Therefore, we aimed to characterise “receptive” versus “refractory” endometrial tissues with the focus on (1) transcription profiles related to mucin, apoptosis and proliferative pathways, and 2) phenotypic features; that is, epithelial transmembrane mucin and apoptotic cell rates (ACR). Using a bovine model, preovulatory follicle growth was manipulated to produce 2 groups: cows with larger preovulatory follicle, longer proestrus, and higher receptivity (LF-LCL) versus cows with smaller preovulatory follicle, shorter proestrus, and lower receptivity (SF-SCL). Seven days post-induced ovulation, endometrium was collected. Transcriptome profiles of endometrial tissue (n = 3/group) were determined using Illumina RNA sequencing analyses. In addition, paraffin embedded endometrial samples (n = 6/group) were stained with Alcian Blue for semiquantitative analyses of mucin staining intensity and treated with antibodies against activated Casp3 to determine ACR. RNA-seq data showed that cell surface MUC1 gene expression was drastically reduced (fold-change –3.33) in LF-LCL tissue (P < 0.05). In contrast, the LF-LCL endometrial tissue displayed an up-regulated expression of genes involved in apoptosis pathways, such as Casp9, CRADD, DAPL1 (fold-change 1.80, 2.10, and 3.60, respectively; P < 0.05). Moreover, the expression of genes related to proliferation, such as MYC and NOV, was significantly down-regulated in LF-LCL endometrial tissue (fold-change –2.2 and –6.3, respectively; P < 0.01). Histology of endometrial samples revealed that the signal for transmembrane, anti-adhesive, mucin at the LF-LCL epithelium was consistently low, whereas the SF-SCL tissue group displayed variable amounts of transmembrane mucin at the apical epithelial boarder. Immunohistochemistry showed that gland cells from LF-LCL endometrial tissue displayed a significantly higher ACR compared with gland cells of the SF-SCL tissue (29.6 ± 1.52% v. 11.9 ± 1.26%; P < 0.05). In the surface epithelium and the stromal tissue, Casp3 positive cells were rare and ACR was similar between groups. In conclusion, a down-regulated expression of the transmembrane mucin might indicate a receptive condition of the endometrium. Furthermore, the prominent apoptotic characteristics in the LF-LCL tissue suggest that the receptive endometrium must surpass the proliferative status in order to differentiate to more specialised functions, such as embryotrophy.