Organization of the inner plexiform layer of the turtle retina: an electron microscopic study.

We have performed a serial-section electron microscopic study of the inner plexiform layer (IPL) of the retina of the turtle Pseudemys scripta elegans. A qualitative and quantitative assessment of the neuropil of the IPL has been done from photomontages taken from the linear visual streak area and peripheral retina. Counts of conventional, ribbon, serial, and reciprocal synapses, of ganglion cell dendrites, and of profiles containing large, dense-cored vesicles were made in five equal-thickness strata in each montage. Averages of these different features were plotted for each stratum in the linear visual streak and compared with peripheral retina. The trend was for stratum 2 to have the highest overall absolute number of amacrine and bipolar interactions, and also of serial synapses, both in the linear visual streak and in peripheral regions. Stratum 4 tended to have the second-highest number of synapses. The total number of synapses for the entire thickness of the IPL, regardless of stratification, is higher in the streak than in the periphery. The total amacrine-to-bipolar-synapse ratio in the IPL is the highest of any vertebrate studied to date (11.0 in the streak and 14.5 in the periphery) but the number of synapses/micron 2 was found to be similar to that reported for other vertebrates. Amacrine-to-amacrine synaptic contacts greatly outnumber other types of synapses; amacrines constitute the principal input to ganglion cells, whereas bipolar output is mainly onto amacrines. The trend for higher numbers of synaptic interactions in strata 2 and 4 of the streak region of the turtle IPL can be correlated with the branching of small-field amacrine and ganglion cells described in Golgi studies (Kolb: Philos. Trans. R. Soc. Lond. B 298:355-393, '82). In peripheral retina, branching of large-field amacrines and a lower number of bipolar pathways may account for the trend for larger numbers of amacrine synapses in strata 2 and 4. Profiles having large, dense-cored vesicles tend to occur most frequently in strata 1 and 5, which correlates well with the stratification in the IPL of the processes of immunoreactive amacrine cells described in other studies.