Photomechanical responses in crustacean retinula cells: the role of microtubules.

Light and darkness promote opposite migrations of screening pigment granules in the retinal photoreceptors of the crayfish and other arthropods, and microtubules have been implicated in this part of the adaptation process. This report considers the topographic relationships of the migratory pigment granules with the microtubular system of the cells. Scanning electron microscopy was used to obtain a three-dimensional visualization of the internal organization of cells in different states of adaptation. Many pigment granules were found directly associated with the microtubular array, although most of them are not adjacent to microtubules irrespective of the adaptation conditions. However, the granules appear associated with each other and form extensive clusters or possibly a single conglomerate. Therefore, interaction of a limited number of granules with the microtubules could affect the movement of the whole granule population. When the pigment complex withdraws, many microtubules are still seen in the deserted regions. It is concluded that the microtubules might represent a supporting framework for the translocation of the pigment, which appears to move as a coherent mass rather than as independent particles.