47.4 Sunday, Jan. 5 11:00 Mechanisms of primordial germ cell migration in the sea urchin, Lytechinus variegatus MARTIK, ML*; MCCLAY, DR; Duke University; Duke University email@example.com
The sea urchin small micromeres arise at the vegetal pole from an unequal 5th cleavage, and their progeny are specified to become the primordial germ cells of the embryo. We show, by high-resolution time-lapse microscopy, that the small micromeres reach the coelomic pouches via a directed homing mechanism. Throughout gastrulation, small micromeres adhere to one another by LvG-cadherin-mediated adherens junctions. Once gastrulation nears completion, the tip of the gut undergoes basement membrane remodeling that allows the small micromeres to undergo an epithelial-mesenchymal transition (EMT) and migrate over the archenteron to the posterior half of the forming coelomic pouch. Small micromere progeny that will become the primordial germ cells preferentially migrate to the left coelomic pouch while a smaller number reach the right coelomic pouch and are apoptosed with the larval support system during metamorphosis. Ectopically placed small micromeres also home to the coelomic pouches. When placed at the equator of the 16-cell embryo, the small micromeres undergo a precocious EMT at the mesenchyme blastula stage and actively migrate to the tip of the early archenteron during its invagination. Ectopic insertion of 32-cell-stage small micromeres into the blastocoel of an early gastrula host embryo is followed by attachment of the small micromeres to the archenteron tip as soon as they become motile, independent of LvG-cadherin adherens. Current aims are to understand the signaling and chemoattractant mechanisms by which the small micromeres undergo such a dramatic feat of finding their way home.