Figure 5. Dynamic interactions between photoreceptors and infiltrating microglia underlying phagocytosis during photoreceptor degeneration.

Time-lapse confocal imaging (at a rate of one image stack/min) was used to document dynamic behavior of ONL microglia in rd10 retinal explants at ages P21–24.

1. Infiltrating microglia established rapid and transient physical contacts with nearby photoreceptor somata (red and yellow circles) via their processes; these processes often terminated in a “cup”-like structure (arrow) that upon soma contact proceeded to extend across the entire soma (arrowhead, and red circle and *) to engulf it completely. The majority of microglia–photoreceptor contacts were transient with the microglial processes dynamically contacting and releasing photoreceptor somata in repetitive cycles (e.g. yellow circle and *) (period ≅10–15 min).
2. Example in which the engulfment of photoreceptor somata by microglial phagocytic cup was followed up by actual phagocytosis in which the engulfed cell (red *) is translocated intracellularly within the microglial cell toward the cell body. Phagocytosis of soma occurred simultaneously with “probing” of other somata by processes of the same microglial cell (yellow *).
3. Phagocytosis of photoreceptor somata occurred via flattened microglial lamellipodial microglial processes (arrow) that extended across somata (yellow *) to engulf them. Nuclei of phagocytosed photoreceptors within microglial phagosomes occasionally developed staining for propidium iodide (red arrow); these subsequently faded and disappeared over ≅10 min, possibly representing intraphagosomal breakdown.
4. Amoeboid infiltrating microglia lacking extended processes were also observed to phagocytose photoreceptors via phagocytic “cups” formed at their cell body (red *).