Subtype-specific expression of Fgf19 during horizontal cell development of the chicken retina

The mechanisms underlying retinal cell diversification are crucial to proper neural development. Fibroblast growth factor 19 (Fgf19) is expressed by developing horizontal cells (HCs) in the chicken retina. Although there are two major HC subtypes, axon-bearing and axon-less, the precise subtype expressing Fgf19 remains uncertain. Here we characterize Fgf19-expressing cells by co-labeling with antibodies against Lim1 (LIM homeodomain 1, or Lhx1), Islet1, and Prox1 (prospero-related homeobox 1) which are axon-bearing HC, axon-less HC, and pan-HC markers, respectively. We found that a subset of Fgf19-expressing cells was positive for Prox1 and Lim1 in the vitread neuroepithelium at embryonic day 4 (E4). By E9, the majority of Fgf19-expressing cells became positive for Prox1 and Lim1 prior to arrival at the prospective HC layer. In contrast, Fgf19-expressing cells did not overlap with the Islet1-positive population at any stage examined. These results suggest that Fgf19 is expressed by the early migratory horizontal precursors, and later by the presumptive axon-bearing HCs.