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