Fate of DNA from retinal cells dying during development: Uptake by microglia and macroglia (Muller cells)

The TUNEL technique of labelling fragmenting DNA was used to examine cell death in the developing retina of the rabbit, rat and cat. TUNEL-labelled structures included the still-intact nuclei of retinal cells and smaller, strongly labelled bodies interpreted as fragments of disintegrating nuclei (apoptotic or pyknotic bodies). With confocal microscopy, the cytoplasm around labelled nuclei was observed to be labelled, suggesting that DNA fragments spread into the cytoplasm of the dying cell. Also observed were cells whose nuclei were TUNEL(-) but whose cytoplasm was TUNEL(+), so that their morphology could be discerned. Evidence is presented that these are phagocytes, their cytoplasmic labelling resulting from the ingestion of the fragmenting DNA of a dying neighbour. Results suggest that in developing retina fragmenting DNA is phagocytosed principally by microglia and Muller cells, with a few neurones and no astrocytes active as phagocytes. in the postnatal material studied, microglia are the predominant phagocytes for cells dying in the ganglion cell and inner nuclear layers. Muller cells appear able to phagocytose cells dying in any retinal layer and, since microglia do not normally enter the outer nuclear layer, may be important for the phagocytosis of dying photoreceptors.