Synapse formation and modification between distal retinal neurons in larval and juvenile Xenopus

A serial section analysis of photoreceptor synaptic bases was undertaken in the clawed frog Xenopus laevis. The developmental period from tadpole stage 48 through metamorphosis was studied. Horizontal cells contacted rod and cone photoreceptors at ribbon synapses; the number of such contacts per receptor base was constant for rods, but increased for cones as a function of developmental stage. In pre-metamorphic animals bipolar cells contacted receptors only through basal junctions; their number in cone bases increased dramatically during development but was unchanged in rod bases. A densitometric estimation of the cleft width of basal junctions showed that it ranged from 10 to 18 nm, but the junctions could not be divided reliably into the 'wide' and 'narrow' categories reported for other vertebrate species. Near metamorphic climax a new type of ribbon-related bipolar cell junction appeared. Gap junctions between horizontal cells and conventional synapses of horizontal cell onto bipolar cell processes were first seen in mid-larval developmental stages.