Fig. 6. Light micrograph (A) and electron micrographs (B–E) of pineal photoreceptor cells. (A) Pineal photoreceptor cells exist in the dorsal (arrowheads) and ventral region (arrows). (B) The outer segment of lamprey parapinopsin-containing cells (electron dense) is in the dorsal region. It is similar in shape to that of cone-type photoreceptor cells. (C) The outer segment of pineal photoreceptor cells, which is lamprey parapinopsin immunonegative (electron lucent), is in the ventral region. (D,E) Synaptic ribbons in the basal processes of dorsal (D, arrowheads) and ventral (E, arrowheads) photoreceptor cells. l, lumen. Scale bars, 10 µm in (A), 1 µm in (B,C) and 0.5 µm in (D,E).