Newborn horizontal cells migrate bi-directionally across the neuroepithelium during retinal development

Cell migration plays an important role during the development of the retina. In this work we have studied the migration of newborn horizontal cells in avian embryonic retina. Using the pattern of the early expressed transcription factors Lim1 and Prox1 we have shown that horizontal cells migrate bi-directionally from their site of birth, close to the ventricular side, to the adjacent (vitreal) side of the neuroepithelium, where they align just next to the prospective ganglion cell layer before migrating back again to their final laminar position in the external part of the inner nuclear layer. The migration occurs between Hamburger and Hamilton stages 24 and 33, which is equivalent to embryonic day 4.5 and 8. Between stages 26 and 30 the horizontal cells reside close to the ganglion cell layer and intra ocular injections of a cytochalasin D, an actin polymerisation blocker that inhibit migration, at stage 29 interfered with the migration of the horizontal cells to their final destination. Furthermore, using biolistic gene transfer with a green fluorescence protein expression vector of retinal slices we were able to record ventricle-directed migration by time-lapse microscopy. Combining biolistics with immunohistochemistry we showed that transfected cells, which have also been translocated in a ventricular direction were positive for the horizontal cell markers Lim1 and Prox1. The alternative path of migration that is described in this work differs from the generally accepted one for horizontal cells and this knowledge will influence the view of how the molecular determination of horizontal cells is specified.