The formation of photoreceptor synapses in the retina of larval Xenopus.

The first appearance and early development of synapses between photoreceptors, bipolar cells and horizontal cells were studied in the retina of larval Xenopus, between stages 37/8 and 46. Synapse morphology was reconstructed from examination of serial sections. Additional data were obtained from E-PTA stained tissue, Photoreceptors initially contacted horizontal cell dendrites via ribbon synapses at the photoreceptor basal surface (stages 37/8--39). Later, horizontal cell dendrites penetrated the receptor as digitiform processes (stages 39--40) which subsequently expanded extensively within the invagination (stages 40--44). At stages 41--46, a photoreceptor ribbon generally was associated with two horizontal processes in a synaptic dyad complex; each ribbon participated in 1 or 2 such complexes. Bipolar cell dendrites made contact with receptors via superficial or invaginating basal-type junctions. Such junctions were observed first at stage 39, but increased greatly in number beginning at stage 42. Basal junctions were characterized by marked staining of the paramembranous receptor cell cytoplasm and a 9--13 nm wide cleft containing an electron-dense material. No association of bipolar cell dendrites with photoreceptor ribbons was noted. Junctional complexes presumed to be gap junctions first were observed at stage 39 between photoreceptor bases and basal processes emitted by neighbouring photoreceptors.