RNASeq reveals conservation of function among the yolk sacs of human, mouse and chicken. Yolk Sac

The human yolk sac is often considered vestigial. Here, we report RNA-sequencing analysis of the human and murine yolk sacs and compare with that of the chicken. We relate the hu- man RNA-sequencing data to coelomic fluid proteomic data. Conservation of transcripts across the species indicates the human secondary yolk sac likely performs key functions early in development, particularly uptake and processing of macro- and micronutrients, many of which are found in coelomic fluid. More generally, our findings shed light on evolutionary mech- anisms giving rise to complex structures such as the placenta. We propose that although a choriovitelline placenta is never established physically in the human, the placental villi, exo- coelomic cavity, and secondary yolk sac function together as a physiological equivalent.