Fig. 1.

Schematics of rod and cone photoreceptor structure. A: Photoreceptor consists of outer segment, inner segment, nucleus, and synaptic vesicles. Inner segment (IS) consists of mitochondria-rich ellipsoid and ER/Golgi complex-rich myloid. Outer segment (OS) is rich in lipids and undergoes shedding daily. B: An electron micrograph of a partial human cone photoreceptor [reprinted with permission from (150)]. IS ellipsoid is densely packed with mitochondria. In OS, the stacked discs are clearly visible. A narrow gap is visible between the distal IS membrane and proximal discs of the OS (yellow arrow). If this gap is an artifact of tissue preparation for electron microscopy or whether it corresponds to a region of interstitial fluid that separates the IS and OS is unknown.