Figure 2.

Genesis of the neuronal types of the vertebrate retina. (A) Chronological order of neuron birth in the vertebrate retina is depicted based on classical birth dating studies done across many vertebrate species. The first neurons born are the retinal ganglion cells, followed by cone photoreceptors, horizontal cells, amacrine cells, rod photoreceptors, and bipolar cells. Note that birth orders are overlapping and that we did not depict Muller cells in the schematic but they are the latest born cell-type. (B) Model of retinogenesis in the zebrafish embryo. Neuroepithelial progenitors (gray) divide asymmetrically at the apical side and give birth to one neuron either a retinal ganglion cell (magenta) or amacrine cell (light yellow) and one neuronal precursor committed to cone photoreceptor (cyan), horizontal (yellow), or bipolar (blue) cell fate. The committed precursors feature distinct morphology, expression of fate determinants and/or mitotic position. Cone photoreceptor precursors (cyan) show columnar epithelial morphology and divide within the developing photoreceptor layer at the apical surface of the retina. Horizontal cell precursors (yellow) are nonpolar and divide along the apico-basal axis of the INL, whereas bipolar cell precursors (blue) show bipolar morphology and can divide at apical or subapical positions. PR, Photoreceptors; HC, horizontal cells; BC, bipolar cells; AC, amacrine cells; dAC, displaced amacrine cells; RGC, retinal ganglion cells.