Which region of the photoreceptor inner segment is primarily responsible for protein synthesis?

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

Answer: (A)

The myoid region of the photoreceptor inner segment is the area where protein synthesis mainly takes place. It contains the machinery needed to make proteins and phospholipids, such as ribosomes and rough endoplasmic reticulum, which are essential for producing components that renew the outer segment membranes. In contrast, the ellipsoid is packed with mitochondria and provides energy (ATP) for cellular functions, including outer segment maintenance, but it is not the primary site of protein synthesis. The connecting cilium (the channel between inner and outer segments) serves as a transport route for proteins, not a synthesis site, and the nucleus lies in the cell body within the outer nuclear layer, not inside the inner segment.