Synaptic arrays of the inner plexiform layer in the developing retina of Xenopus

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During embryonic and larval development in Xenopus laevis, arrays of synapses made by amacrine cells form in two phases: an initial phase of rapid synaptic addition and a second phase of slower addition. In the region near the optic nerve, at which all measurements were made, these synapses first appear at stage 40 (approx 66 hr postfertilization). Connectivity increases at a rate of 8.6 synapses per hr per inner nuclear layer (INL) nucleus until stage 47 (132 hr postfertilization). After this phase the rate of formation decreases to 1.19 synapses per hr per INL nucleus. Synaptic arrays made by bipolar cells have only one phase of addition. A synapse made by a bipolar cell may be identified by its presynaptic ribbon, the first of which are seen at stage 40. Ribbons are added to the IPL neuropil at a rate of 4.6 ribbons per hr per INL nucleus until stage 47. After this the number of ribbons per INL nucleus in the area near the optic nerve remains constant. Although they may be found, amacrine to amacrine synapses (serial conventional) remain at low numbers throughout larval and early postmetamorphic life. This is unlike the condition found in Rana pipiens where a dramatic increase in amacrine to amacrine connectivity occurs at metamorphosis.