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F. Hallbook, P.–H.D. Edqvist; Newborn horizontal cells migrate bi–directionally across the neuroepithelium during retinal development . Invest. Ophthalmol. Vis. Sci. 2004;45(13):5394.
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Purpose: This work was initiated by an observation in our previous work (Karlsson et al 2001 Development. 128:471–479), in which we found that avian horizontal cells (HC) that express nerve growth factor and its receptor TrkA, were translocated from the vitreal to the ventricular side of the retina. Here we have investigated how avian prospective HCs, defined as cells positive for the transcription factors Lim1 (Lhx1) and Prox1, migrate during retinal development. The current view holds that HCs migrate only a short distance from their place of birth to their definite position. However, based on the distinct spatio–temporal changes of the Lim1/Prox1 labelling patterns we propose that newborn Lim1+ HC precursors undergo a previously undocumented pattern of migration before attaining their final position in the external part of the inner nuclear layer. Methods: Standard histochemical methods in combination with ocular injections of BrdU, cytochalasin D or colcemid, biolistic gene transfer of GFP to retinal slices followed by time–lapse microscopy. Results: The pattern of the early expressed transcription factors Lim1 and Prox1 indicated 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, equivalent to embryonic day 4.5 and 8. Intra ocular injections of a BrdU showed that the migrating cells were post–mitotic and injection of cytochalasinD at stage 29 interfered with the horizontal cell migration to their final destination. Furthermore, ventricle–directed migration was recorded by time–lapse microscopy in retinal slices after biolistic gene transfer of GFP expression vector. Immunohistochemistry showed that transfected cells in slices, which also had been translocated in a ventricular direction were positive for the horizontal cell markers Lim1 and Prox1. Conclusions: 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.
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