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Primordial Germ Cell Migration in Invertebrates
In Drosophila, the whole migration process has been estimated to take 10 hours. It begins with the formation of PGCs; from dividing nuclei becoming encircled by cell membranes, occurring at the posterior pole of the embryo. Division of the nuclei stops once they have a cell membrane. PGCs’ transcription process is also thought to be actively subdued once formed.

In Drosophila, PGC migration begins with passive movement along the dorsal side of the embryo, during gastrulation. This is followed by more passive movement, due to the invagination of the posterior midgut primordium, which leads to the PGCs in the centre of the embryo, surrounded by epithelial cells that have been folded back on themselves. . There is then a split into two groups, left and right respectively, as they actively migrate laterally across the epithelium to exit the gut, facilitated by fibroblast growth factor (FGF) signalling and a repulsion-based mechanism using enzymes encoded by the Wunen gene. This is followed by active movement dorsally along the basal side of the embryo. Through directional migration - which requires multiple genes to work, one being the Columbus (clb) gene, which codes for Drosophila HMG CoA reductase - the germ cells move towards the somatic gonadal precursor cells and associate with them. These two associated cell types then migrate together anteriorly, until they coalesce into the embryonic gonad at the future site of the mature gonad.