Dissecting the development of the early lineages and primordial germ cells in the bovine embryo

As cattle have not been traditionally considered a model species and the molecular details of germ cell development don't directly inform production practices, the specification of primordial germ cells in the bovine embryo has remained understudied and poorly understood. Recent work by our laboratory builds on previous investigations to establish the molecular profile of primordial germ cells (PGC) at the critical moment when they are being specified in the embryo during the gastrulation stage. Combining advanced immunolocalization, confocal imaging and single-cell RNA sequencing, we identified PGC in the bovine embryo approximately on day 16 of development by co-expression of the core transcription factors OCT4, SOX17, PRDM1, and TFAP2C as demonstrated for several other species in which the embryo develops a bilaminar disc at the onset of gastrulation. Soon after specification, between days 20 and 22 of embryo development, early migratory PGC repress transcripts responsible for the establishment of somatic lineages. Notably, these cells do not seem highly proliferative during the early migratory stage, another aspect of early germ cells that is conserved in cows and other species such as pigs. Advancing the study of germ cell specification and development during bovine embryonic development, particularly at stages when human embryos are unavailable for investigation, places cows as an additional domestic species capable of providing crucial information about events that are paramount for fertility. As the field of in vitro gametogenesis continues to rapidly evolve, the study of bovine PGC and fetal germ cell development will provide invaluable information to facilitate the development and advancement of future assisted reproduction technologies for the improvement of agricultural animals and human reproduction.