The Organization of the Inner Nuclear Layer of the Rabbit Retina

Abstract

The initial goal of this study was to establish an accounting the major classes of cells present in the inner nuclear layer (INL) of the rabbit's retina. Series of 80-100 radial sections 1 μm thick were cut from retinal blocks dissected at intervals along the vertical meridian. They were photographed at high magnification in the light microscope. By visualizing the initial segments of processes leaving the somata, we could identify each cell as a bipolar, amacrine, horizontal, or Müller cell. The identifications made by light microscopy were confirmed by electron microscopy of alternating ultrathin sections. On average, bipolar cells made up 41% of the total INL cells, amacrine cells 32%, horizontal cells 1.5%, and Müller cells 24%. These fractions varied relatively little across the retina or among different animals. We next immunolabeled the rod bipolar cells of wholemounted retinas with antibodies against protein kinase C, using FITC as the visualizing agent. The same retinas were counterstained with a DNA-binding probe that fluoresces at longer wavelengths. Serial optical horizontal sections of the double-labeled wholemounts were made by confocal microscopy. On average, rod bipolars accounted for 10% of the total INL cells. By subtraction, the cone bipolars made up 31% of the total cells. We conclude that cone bipolars substantially outnumber rod bipolars, even in a retina in which rods outnumber cones by more than 20:1. Using the base of reference created here, a similar analysis can be carried out for other subclasses of retinal neuron. Because the analysis does not depend on absolute cell densities or corrections for shrinkage, data acquired by different histochemical techniques may be combined.