Early retinal development in the zebrafish, Danio rerio: light and electron microscopic analyses.

The morphological differentiation of the zebrafish retina was analyzed by using light (LM) and transmission electron (TEM) microscopy between the time of initial ganglion cell differentiation (approximately 32 hours postfertilization; hpf) and shortly after the point when the retina appears functional (approximately 74 hpf), i.e., when all major cell types and basic synaptic connections are in place. The results show that the inner retinal neurons, like the photoreceptor and ganglion cells, differentiate first within the ventronasal region, and differentiation subsequently spreads asymmetrically into the nasal and dorsal regions before reaching the ventrotemporal retina. In addition, we show that the attenuation of the optic stalk occurs in parallel with ganglion cell differentiation between 32 and 40 hpf. The first conventional synapses appear within the inner plexiform layer simultaneously with the first photoreceptor outer segment discs at 60 hpf; functional ribbon triads arise within photoreceptor synaptic terminals at 65 hpf; and synaptic ribbons occur within bipolar cell axon terminals at the time larvae exhibit their first visual responses (approximately 70 hpf). Although development is initially more advanced within the ventronasal region between 50 and 60 hpf, development across the retina rapidly equilibrates such that it is relatively comparable within all quadrants of the central retina by 70 hpf. An area within the temporal retina characterized by tightly packed and highly tiered cones emerges with subsequent development. Retinal differentiation in the zebrafish corresponds with that generally described in other vertebrates and can be correlated with the development of visual and electroretinographic responses in the animal.