A fine structural and E-PTA study of photoreceptor synaptogenesis in the chick retina

Photoreceptor synaptogenesis in the embryonic and hatchling chick retina was studied with conventional EM techniques and ethanolic phosphotungstic acid (E-PTA). The photoreceptors line up between 11 and 13 embryonic days with their undifferentiated synaptic bases facing the outer plexiform layer (OPL). E-PTA staining at 11 embryonic days does not reveal any para-membranous specializations of the receptors but numerous stained punctae adhaerentes are observed in the OPL. At 13 embryonic days neurites of presumed bipolar and horizontal neurons are aligned parallel to the bases of the receptors and cytoplasmic protrusions of the receptors project between some of these neurites to form dyad appositions. An osmiophilic undercoating, which is not E-PTA positive at this time, is present on the cytoplasmic face of the receptor membrane in these apposition regions. Between 13 and 15 embryonic days the filopodial protrusions of the receptors continue to elongate further and become aligned with neurites in dyad and triad appositions. The osmiophilic undercoating now extends along the entire inner surface of the receptor pedicle protrusions and becomes E-PTA positive. Between 15 and 17 embryonic days focal aggregations of osmiophilic and E-PTA stained material appear along the membranes of the protrusions and there is some E-PTA staining of the postsynaptic densities and intervening cleft material. Between 17 and 21 embryonic days mature ribbon synapses are observed on the surfaces of the conical-shaped, receptor pedicles where the ribbons and their synaptic vesicles are associated with the dense aggregations (arciform densities), seen earlier as isolated focal aggregations, and the receptor undercoating is restricted to non-synaptic regions. E-PTA staining shows that ribbons are positively stained around their borders only and that they are contiguous with the intensely stained arciform densities. The cleft material and postsynaptic densities of some synapses first stain as V-shaped junctions and later as Y-shaped junctions. These observations suggest that ribbon synaptic junction formation begins with an alignment of pre- and postsynaptic membranes and the presence of the receptor presynaptic membrane undercoating, followed by the appearance of the presynaptic arciform densities and some staining of the cleft material and postsynaptic densities. These events are followed by the appearance of synaptic ribbons which are associated with the presynaptic arciform densities and by a further differentiation of the cleft material and postsynaptic densities.