Single-cell insights into development of the bovine placenta

A central determinant of pregnancy success is proper development of the conceptus (embryo/fetus and associated extraembryonic membranes including the placenta). Although the gross morphology and histology of the bovine placenta has been well studied, the cellular and molecular mechanisms regulating placenta development and trophoblast differentiation and function remain essentially undefined. Here single cell transcriptome (scRNA-seq) analysis was performed on the day 17 bovine conceptus and chorion of day 24, 30 and 50 conceptuses (n = 3-4 samples per day) using the 10X Genomics platform. Bioinformatic analyses identified cell types and their ontogeny including trophoblast, mesenchyme, and immune cells. Loss of IFNT expressing trophoblast uninucleate cells (UNC) occurred between days 17 and 30, whereas binucleate cells (BNC), identified based on expression of placental lactogen (CSH2) and specific pregnancy-associated glycoproteins (PAGs) genes, first appeared on day 24. Several different types of UNC were present in day 24, 30 and 50 samples, but only one (day 24) or two types of BNC (days 30 and 50). Cell trajectory analyses provided a conceptual framework for UNC development and BNC differentiation, and bioinformatic analyses identified candidate transcription factors governing differentiation and function of the trophoblasts. The digital atlas of cell types in the developing bovine conceptus reported here serves as a resource to discover key genes and biological pathways regulating its development during the critical periods of implantation and placentation.