AII amacrine cells in the distal inner nuclear layer of the mouse retina

We serendipitously found a distal Disabled‐1 (Dab1)‐immunoreactive cell in retina of the C57BL/6J black mouse. The somata of these cells are located in the outermost part of the inner nuclear layer (INL). Their processes extend toward the outer plexiform layer (OPL), receiving synaptic inputs from horizontal and interplexiform cells. In the current study, we name this cell the “distal Dab1‐immunoreactive cell.” Double‐labeling experiments demonstrate that the distal Dab1‐immunoreactive cell is not a horizontal cell. Rather, the distal Dab1 cell appears to be a misplaced AII cell, by being glycine transporter‐1‐immunoreactive and by resembling the latter cell in an electron microscopic analysis. A distal Dab1 cell had been reported in the FVB/N mouse retina, a model of retinitis pigmentosa (Park et al. [2004] Cell Tissue Res 315:407–412). However, here, we found this distal Dab1‐immunoreactive cell in the adult and normal developing mouse retinas. Hence, we show that such cells do not require the loss of photoreceptors as suggested previously (Park et al. [2004] Cell Tissue Res 315:407–412). Instead, two other pieces of data suggest an alternative explanation sources for distal Dab1 cells. First, we find a correlation between the number of these cells in the left and right eyes Second, developmental analysis shows that the distal Dab1‐immunoreactive cell is first observed shortly after birth. At the same time, AII cells emerge, extending their neurites into the inner retina. These data suggest that distal Dab1‐immunoreactive cells are misplaced AII amacrine cells, resulting from genetically modulated anomalies owing to migration errors. J. Comp. Neurol. 494:651–662, 2006. © 2005 Wiley‐Liss, Inc.