Which of the following is included in the inner segment of photoreceptors?

• A. Photopigments
• B. Synaptic endings
• C. Outer segment discs
• D. Organelles (including nuclei)

Answer: (D)

The inner segment is where the photoreceptor cell’s metabolic machinery sits. It’s the part of the cell that produces energy and builds the proteins and membranes the cell needs, so it contains the organelles—mitochondria for energy, rough endoplasmic reticulum and ribosomes for protein synthesis, Golgi for processing and shipping materials, and other subcellular components. This is why the idea of “organelles” fits the inner segment best. The photopigments themselves are in the outer segment discs, which are the light-detecting part, and the synaptic endings that connect to bipolar cells lie at the base of the cell, near the boundary with other retinal layers. The nucleus, while essential to the cell, is located in the cell body within the outer nuclear layer, not inside the inner segment, so the emphasis is on the organelles and metabolic machinery housed there.