Cell death in the inner and outer nuclear layers of the developing cat retina

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Abstract
The occurrence and distribution of pyknotic profiles (dying cells) in the inner (INL) and outer nuclear layers (ONL) were mapped from radial sections through 13 kitten retinae aged from the 30th postconceptional day (30PCD) to the 81PCD (17th postnatal day). Cell death in the INL occurs in two consecutive waves, each beginning at the area centralis and proceeding toward the retinal edge. The first wave involves cells located in the inner part of the INL, adjacent to the inner plexiform layer. It begins on the 50PCD and lasts until the 66PCD, with a peak of 72,000 profiles on the 58PCD. This wave of cell death seems to be associated with the formation of the inner plexiform layer and may be due to the elimination of amacrine cells which have failed to establish sustaining connections with their appropriate target cells. The second wave involves cells located in the outer part of the INL, adjacent to the outer plexiform layer. It begins on the 60PCD and lasts until after the 81PCD, with a peak of 65,000 on the 71PCD. This wave appears to be due to the elimination of ectopic photoreceptor cells displaced by the formation of the OPL and presumably prevented from establishing sustaining connections with their appropriate target cells. A phase of cell death in the ONL begins on about the 42PCD and lasts until after the 81PCD, with a peak of 10,300 pyknotic profiles on the 60PCD. Pyknotic profiles in the ONL were always distributed fairly uniformly across the retina, and did not reflect the centropheripheral maturation of the outer plexiform layer, suggesting that cell death in this layer is not target-related. The findings of the present study indicate that naturally occurring cell death can be as extensive in populations of intrinsic neurons as it is in populations of extrinsic neurons.