Fig. 4

Development of circuitry in the mammalian retina. Three stages of embryonic development observed in the ferret retina, as reported by Johnson et al. (1999). The ferret is particularly convenient because the young are born at a very immature stage, and the eyes do not open until 2 weeks after birth (middle panel). Left panel At birth, the cone and rod photoreceptors (C and R) are not identifiable on the basis of morphology and display a simple bipolar shape; however, they are identifiable by certain molecular markers, such as rhodopsin. Very interestingly, the photoreceptors send processes (downwards in this diagram) directly to the retina’s output layer, the so-called inner plexiform layer (IPL); however, it has not yet been established whether the neurons they contact are amacrine cells (A) or ganglion cells (G). At this stage, the horizontal cells (H) and bipolar cells are barely differentiated. Middle panel After a further 2 weeks, by the time of eye opening, the horizontal cells and bipolar cells (B) have matured, and the photoreceptors now retract the processes that they had previously sent to the IPL and instead make new contacts with the horizontal and bipolar cells; this layer of synaptic connections forms the outer plexiform layer (OPL) of the adult. Right panel At an even later stage of development, the photoreceptors extend an inner segment (IS) and a light-sensitive outer segment (OS); it is not until this stage that conventional visual transduction becomes possible [from Lamb et al. (2007) Fig. 5; first published in Nature Reviews Neuroscience 8, 2007 © Nature Publishing Group, a division of Macmillan Publishers Limited]