Light-dependent pigment migration in blowfly photoreceptors studied by in vivo CLSM

The light-dependent migration of pigment granules in the soma of fly photoreceptors has been studied in vivo with a fast confocal laser scanning microscope. Images as well as photometric measurements were obtained in the reflection and fluorescence modes. Measurements at the single cell level were performed by using water immersion. The illumination of dark adapted photoreceptors causes a rapid increase in reflectance due to the migration of light scattering pigment granules toward the rhabdomeres. In the steady-state, the reflection signal strongly fluctuates, indicating that the pigment granules undergo a very rapid fluctuating movements. A major part of the reflection signal is due to light back-scattered by the pigment granules and channeled through the light guiding rhabdomeres. The optical axes of the rhabdomeres can thus be directly traced and appear to be directed toward the optical centre of the corresponding facet lens. Simultaneous with the reflection increase, the fluorescence of the photoreceptors decreases, because the pigment granules accumulating near the rhabdomeres act as a light-controlling pupil. Broad-band, white light filtered by the predominantly blue absorbing pupil causes an increased fraction of the visual pigment in the rhodopsin state.