Which region of the photoreceptor connects the outer and inner segments?

• A. Myoid
• B. Ellipsoid
• C. Cilium
• D. Soma

Answer: (C)

Photoreceptors are organized with an outer segment that houses the photopigments and an inner segment that contains the cellular machinery. The connecting cilium is the narrow bridge that links these two parts, forming the essential conduit for transporting proteins and membrane components from the inner segment to the outer segment. This trafficking is crucial for maintaining and renewing the outer segment discs where light detection occurs. The inner segment contains two regions—the ellipsoid, packed with mitochondria for energy, and the myoid, which houses components involved in protein synthesis and processing—but neither of these forms the bridge to the outer segment. The soma is simply the cell body. So, the connecting cilium is the region that connects the outer and inner segments.