Development of day‐night rhythmicity in “synaptic” ribbon numbers in the pinealocytes of posthatch chicks kept under either natural photoperiodic conditions or continuous illumination

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Abstract
Pineal synaptic ribbons (SR) undergo characteristic changes over a period of 24 hr under natural photoperiodic conditions in various vertebrates, being low in number during daytime and elevated at night. During posthatch development of chicks, the rhythmicity of SR numbers is reported to appear at the age of about 2 weeks. Because the influence of external light during the growth phase of chicks on the development of day-night rhythmicity in SR numbers is unknown, we studied day-night differences in SR numbers in the pinealocytes of chicks at the posthatch ages of 15, 17, and 19 days; chicks had previously been kept under natural photoperiodic conditions or continuous illumination. Under natural photoperiodic conditions a statistically significant nocturnal (midnight) rise in SR numbers over the value of midday was seen in the pineal of 17- and 19-day-old chicks, but not in 15-day-old chicks. SR numbers in the pinealocytes of continuously illuminated chicks did not show any day-night rhythmicity on days either 15 or 17, but exhibited significant day-night differences on day 19 posthatch. These findings suggest that continuous illumination, which is known to dampen circadian rhythmicity of melatonin secretion in the chick pineal, causes a delay, but not a total suppression of the mechanism involved in the ontogenic development of diurnal rhythmicity in SR numbers in the pinealocytes of chicks.