Daily rhythm of cell proliferation in the teleost retina

To determine whether the number of cell divisions in the teleost retina exhibited a regular daily variation, we labeled dividing cells with an antibody to proliferating cell nuclear antigen. The number of dividing rod precursor cells in the outer nuclear layer of the retina were counted in retinas from the telost fish Haplochromis burtoni, sacrificed at 4-h intervals during a standard light-dark cycle and in constant darkness. These rod precursor cells exhibited a striking rhythm of cell division. The highest number of cell divisions (acrophase) was found to occur at night when it was approximately 3 times higher than during the day. The observed rhythm persisted in animals held in constant darkness. We suggest that this endogenous 24-h rhythm of rod precursor cell division may be controlled by a circadian clock. Although there are several examples of continuously proliferating cell populations which exhibit circadian or diurnal rhythms, this appears to be the first documentation of a rhythm of division in cells destined to become neurons.