Reproduction Abstracts

Primordial germ cells undergo an orchestrated developmental program prior to establishing the mature gametes. Most of our knowledge on PGC development stems from studies in mice, however recent studies in humans revealed important differences in the mechanisms of PGC development. Since pigs share many embryological features with non-rodent species, like humans, and human PGC can only be studied from early gonadal stages, we have used pig embryos to study the developmental program of PGC from their inception until they reach the gonad. Embryos collected between days 16 and 70 were sectioned and stained for a panel of markers used to identify PGCs as well as to establish their epigenetic features. Early migratory PGC were identified by expression of Blimp1, AP2g, Sox17, Nanog, and Oct-4, and absence of Sox2. These cells show reduced levels of DNA methylation (5-mC), which was consistent with low levels of Dnmt3a and UHRF, and increased levels of 5-hmC and tet-1. In gonadal PGCs, 5-mC and 5-hmC are both very low/absent in male and female PGC. H3K9me2 is absent/low in migratory PGCs and increases in gonadal PGC, although it remains lower than in somatic cells. H3K27me3 is detected in migratory PGCs and decreases in gonadal PGCs compared to somatic cells. Histone H2A.Z is detected in migratory PGCs, and the signal increases in late gonadal stages. This systematic characterization of porcine PGCs identified important differences in the program of germ cell development compared to mice, and similar patterns of gene expression and epigenetic marks to those reported for humans.