Mechanism of Photoreceptor Outer Segment Tip Ingestion: Evidence of Trogocytosis.

The ingestion of the photoreceptor outer segment (OS) tips by the retinal pigment epithelium (RPE) cells is an essential process for maintaining photoreceptor health and thus vision. However, it has not been clear whether this process occurs by the RPE directly biting off the OS tips or whether the tips of the OSs are shed into the subretinal space prior to their engulfment by the RPE cells. Here, we discuss the lack of evidence of shed OS tips, and point to new live-cell evidence that shows that the RPE cells actively nibble the OS tips, indicating that OS tip ingestion occurs through a mechanism known as trogocytosis. The distinction between whole particle engulfment and trogocytosis is more than semantic since trogocytosis includes the rupture of the target cell's membrane.