For over 60 years, scientists have debated a basic structural question about cone photoreceptors, the cells that let vertebrates see color: are their light-sensing disk membranes closed off inside the cell, or are they open to the extracellular space? In non-mammalian species, the answer is clear — cone disk membranes typically stay continuous with the surrounding plasma membrane and are therefore open. Whether the same holds true in mammals, and especially primates, has remained unresolved.
A new study, led by Dr. David S. Williams, professor of ophthalmology and neurobiology at the David Geffen School of Medicine at UCLA, in collaboration with researchers from UCSB and Harvard, suggests that the disks in primate cones are closed, independent structures. Researchers measured the disk membranes in the retinal tissue near the fovea from Rhesus monkeys ( Macaca mulatta) using two different approaches: electron tomography and electrophysiology. This was the first quantitative assessment of the relative proportion of open and closed disk membranes in a mammalian cone. Through both approaches, researchers found that the majority of the disk membranes are closed, like those in rod photoreceptors.
This information now allows for enhanced understanding of the relationship between structure and function of cone photoreceptors. The findings may have important implications for cell biological processes (e.g. disk membrane renewal) and the physiology of phototransduction, offering new understanding on how primate, and thus human, vision works.
Read the study in the Journal of Neuroscience .
JNeurosci
10.1523/JNEUROSCI.0245-26.2026
Imaging analysis
Animal tissue samples
Ultrastructural and Electrophysiological Determination of the Patency of Monkey Cone Outer Segment Membranes
9-Sep-2026
The authors declare no competing financial interests