FIGURE 1.

The types of extracellular vesicle release by photoreceptor cells. There are two types of extracellular vesicles–ectosomes and exosomes. Exosomes are formed by the fusion of a multivesicular body with the cell’s plasma membrane, releasing numerous exosomes together (blue arrow on left). Although there is evidence in support of exosome release by photoreceptors (Kalargyrou et al., 2021), this process has not been documented in vivo. Ectosomes originate by the outward budding of the cell’s plasma membrane, releasing a single vesicle directly to the extracellular space (red arrows). Left, ectosomes are often released from the photoreceptor’s inner segment plasma membrane [e.g., release driven by rhodopsin mislocalization (Li et al., 1996), see also Figure 2D], and sometimes contain pieces of mitochondria (Giarmarco et al., 2020). Middle, ectosomes have been observed budding from the newly forming disc membrane evaginations in Prcd^–/– mice (Spencer et al., 2019a) and in tunicamycin treated retinas (Fliesler et al., 1985; see also Figure 2C). Right, ectosomes are released from the ciliary plasma membrane of rds mice, which lack expression of peripherin-2 (Salinas et al., 2017; see also Figure 2A). In this mouse line, the photoreceptor cilia do not elaborate outer segments because all outer segment material is released in the form of ectosomes.