The Developmental Changes in the Extra-Embryonic Vascular System in the Circulating Phase of Primordial Germ Cells in Aves

There are many reports regarding the molecular control of the asymmetric developments that occur during the early embryonic period in many experimental animals, including avian species. Generally, only the left ovary develops and becomes a mature reproductive organ in matured avian females. In mature avian males, the left testis tends to be dominant in respect to size. However, these reports do not to provide an answer to why avian primordial germ cells (PGCs) mainly colonize in the left gonadal rudiment just after their circulation period. in the present study, dynamic changes in the vascular networks were observed throughout the circulating phase of PGCs in the early embryonic stages, mainly in chickens. India ink was injected into the bloodstream of chicken and quail embryos at stages 13 to 17 in order to visualize the changes in the vascular networks in the extra and intra embryonic regions. The blood flow pathway was also observed via the movement of the injected India ink particles. Throughout stages 13 to 17, the branching level of the vitelline veins from the dorsal aortas was at the 19th and 20th somites in the quail and chicken embryos respectively. Furthermore, the posterior vitelline vein did not develop by stage 14; it began to develop at stage 15 or later. The blood flow in the gonadal region became asymmetrical in the left and right sides in parallel with the development of the posterior vitelline vein. The asymmetry we observed in the blood flow might be the cause of the asymmetric proportions of the PGCs just after their circulation in the left and right gonadal regions.