In many vertebrate retinas the outer segments of rod
photoreceptors have multiple incisures, that is, there
are numerous indentations in the highly curved membrane
forming the edge of their disks and in the plasma membrane
enclosing the entire stack of disks. Immunofluorescent
localization of tubulin in amphibian photoreceptors yielded
a novel series of thin, parallel, fluorescent lines in
rod outer segments that extended their full length and
coincided with their multiple incisures. Electron-microscopic
examination of amphibian retinas revealed the structures
responsible for this fluorescence: longitudinally oriented
microtubules were associated with incisures at heights
throughout rod outer segments. These microtubules were
located between the disk rims and the overlying plasma
membrane, in the small cytoplasmic compartment at the mouth
of incisures; the microtubules and membranes were separated
from each other by distances that were uniform, as though
interconnected by filaments described in other studies.
Thus, in amphibian rod outer segments the incisures mark
the site of a cytoskeletal system containing longitudinal
microtubules distinct from those of the ciliary axoneme,
linked by filaments to the adjacent membranes. This cytoskeleton
is expected to be important for the normal structure, function,
and renewal of rod outer segments. In amphibian cone outer
segments, which do not have incisures, the only anti-tubulin
immunofluorescence and the only microtubules were at the
axoneme. These findings may help elucidate the diverse
properties of rods and cones in many vertebrate retinas
and could prove relevant for human retinal degenerations.
NEW EVIDENCE SUPPORTING THE LINKAGE TO EXTRACELLULAR SPACE OF OUTER SEGMENT SACCULES OF FROG CONES BUT NOT RODS