Migration behavior of PGCs and asymmetrical gonad formation in pond smelt Hypomesus nipponensis

In teleost fish, the gonad originates from primordial germ cells (PGCs) and somatic cells. However, it is not clear whether the final gonadal position is determined by anteroposterior and dextrosinistral differentiation of endodermal organs or by the distribution of PGCs. The pond smelt has a transparent body even after hatching, enabling clear observation of PGC distribution and endodermal differentiation. Here, we first examined normal embryonic development to define the spatio-temporal characteristics of our developmental model. Second, the origin of PGCs was investigated by in situ hybridization. Third, the migration route of PGCs was tracked by microinjection of GFP-nos3 3' UTR mRNA and visualization of PGCs by green fluorescent protein. Lastly, differentiation of gonadal and endodermal organs was examined histologically. Maternal vasa transcripts were detected at the ends of cleavage furrows, indicating that PGCs differentiated by inheritance of germplasm as in other teleosts. During gastrulation, PGCs migrated following somatic cell movement and lined both sides of the embryonic body. During the segmentation period, PGCs moved posteriorly and were distributed in a line among dorsal mesentery cells around the posterior part of the intestinal bulb in the 16^th to 24^th somite region at 3 days post hatching. At 1 month post hatching, the gonad was formed at the 20^th somite region. PGC distribution was biased to the left side of the body cavity, while the pancreas was formed on the right side. These results indicate that PGCs accumulate at the gonadal region by dorsal mesentery cells, and gonadal position is determined by the digestive system.