Abstract
Purpose: :
The majority of ganglion cells (GCs), the output neurons of the retina, are found in the most proximal layer, the ganglion cell layer (GCL). However, a small number of GCs are known to also be present in the inner nuclear layer (INL) and are typically regarded as "displaced." Displaced ganglion cells (dGCs) have not been well characterized, in part because a specific marker for dGCs has not been available. The present study describes a population of dGCs in the retina of a transgenic mouse line that expresses the fluorescent protein Chlomeleon.
Methods: :
We have studied a population of cells in the INL of the CLM1 transgenic mouse line that expresses the chloride-sensitive fluorescent protein Chlomeleon (driven by a Thy-1 promoter). Using confocal microscopy, immunocytochemistry, intracellular tracer (Neurobiotin) injections, and retrograde rhodamine dextran tracing, we have characterized these cells by studying their morphology, distribution and determined if they project to the superior colliculus.
Results: :
A population of Chlomeleon-expressing neurons in the INL, with an average soma size of 15 µm x 13 µm (± 2 µm), were identified definitively as dGCs given 1) the presence of an axon following intracellular injection of tracer, 2) the absence of immunoreactivity for the amacrine cell markers GABA or glycine, and 3) that >75% were retrogradely labelled with rhodamine dextran applied to the superior colliculus. None of the CLM1-dGCs were immunoreactive for three common GC markers (calbindin, calretinin, melanopsin) and no tracer coupling was noted following tracer injection. The average number of CLM1-dGCs per retina was 149 ± 31 (mean ± SD). The majority (80%) of CLM1-dGCs were located in the periphery of the retina but were distributed equally within each quadrant (dorsal, ventral, temporal, nasal). All the CLM1-dGCs injected with tracer had dendrites that ramified in sublamina a of the IPL and specifically within the S2 substrata.
Conclusions: :
The morphology and consistent stratification of CLM1-dGCs suggests that they consist of a small number (class) of similar types. Therefore, the CLM1 mouse retina provides a useful model system in which to systematically study a population of dGCs.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • anatomy • ganglion cells