On the development of the stratification of the inner plexiform layer in the chick retina

This study investigated the development of the subdivision of the chick inner plexiform layer (IPL). The approach included an immunohistological analysis of the temporal and spatial expressions of choline acetyltransferase, of the neural-glial-related and neural-glial cell adhesion molecules (NrCAM and NgCAM, respectively) and axonin-1, and of inwardly rectifying potassium (Kir) channels in 5- to 19-day-old (E5-E19) embryos. Ultrastructural investigations evaluated whether synaptogenesis accompanies the onset of differentiation of the IPL. We found that the differentiation of the IPL started at E9. Distinct cholinergic strata appeared, NrCAM immunoreactivity showed a poorly defined stratification, and Kir3.2 was expressed in the IPL and in the inner nuclear layer. From E10 until late E14, NgCAM- and axonin-1-immunoreactive strata emerged in an alternating sequence from the outer to the inner IPL. During this period, the NrCAM pattern sharpened, and eventually five bands of weaker and stronger immunoreactivity were found. Conventional synapses formed at the beginning of E9, and stratification of the IPL also began on the same day at the same location. Synaptogenesis and stratification followed a gradient from the central to the peripheral retina. The topographic course of differentiation of the IPL generally corresponded to the course of maturation of ganglion and amacrine cells. Synaptogenesis and the expression of G-protein-gated Kir3.2 channels accompanied the onset of stratification. These events coincide with the occurrence of robust and rhythmic spontaneous neuronal activity. The subsequent differentiation of the IPL seemed to be orchestrated by several mechanisms.