In pigs, conceptus attachment to the uterine surface epithelium starts around Day 14 of pregnancy preceded by a pronounced vascularization at the implantation zones, initiating the epitheliochorial placentation. To characterize the complex transcriptome changes in the endometrium in the course of initial conceptus attachment, deep sequencing of endometrial RNA samples of pregnant animals (n = 4) and corresponding cyclic controls (n = 4) was performed using Illumina RNA-Seq. The obtained sequence reads were mapped to the porcine genome, and relative expression values were calculated for the analysis of differential gene expression. Statistical analysis revealed 1933 differentially expressed genes (false discovery rate 1%), 1229 with higher and 704 with lower mRNA concentration, in the samples from pregnant animals. Expression of selected genes was validated by the use of quantitative realtime RT-PCR. The RNA-Seq data were compared to results of a microarray study of bovine endometrium on Day 18 of pregnancy and additional related data sets. Bioinformatics analysis revealed for the genes with higher mRNA concentration in pregnant samples strong overrepresentation, particularly for immune-related functional terms but also for apoptosis and cell adhesion. Overrepresented terms for the genes with lower mRNA concentration in pregnant samples were related to extracellular region, ion transport, cell adhesion, and lipid and steroid metabolic process. In conclusion, RNA-Seq analysis revealed comprehensive transcriptome differences in porcine endometrium between Day 14 of pregnancy and corresponding cyclic endometrium and highlighted new processes and pathways probably involved in regulation of noninvasive implantation in the pig.
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