Myosin VIIA is a marker for the cone accessory outer segment in zebrafish

The accessory outer segment, a cytoplasmic structure running alongside the photoreceptor outer segment, has been described in teleost fishes, excluding the model organism zebrafish. So far, the function of the accessory outer segment is unknown. Here, we describe the ultrastructure of the zebrafish cone accessory outer segment by electron microscopy. Starting at the connecting cilium, the accessory outer segment runs parallel alongside the cone outer segment (COS). A thin plasma bridge connects the outer segment with the accessory outer segment, whose surface is enlarged by foldings and invaginations. Beside the morphological descriptions, we demonstrate that the Usher protein myosin VIIa (Myo7a) is a specific marker for the zebrafish cone accessory outer segment. Zebrafish cone photoreceptors possess a large and well-differentiated accessory outer segment, in which the unconventional motor protein Myo7a is highly enriched. The direct cytoplasmic contact with the COS as well as the surface enlargement of the accessory outer segment suggests an important role of this structure in transport and exchange of metabolites between the COS and the surrounding retinal pigment epithelium. In future studies of the outer retina, more attention should be paid to this often neglected structure.