Populations of “synaptic” ribbons (SR) in pinealocytes of the rat pineal gland at various ages from the neonatal period through early adulthood were analyzed morphometrically. Diurnal and nocturnal numbers of SR were small on day one but had increased dramatically by the end of the first week and beginning of the second week. By day 10, numbers of SR had peaked. Thereafter nocturnal numbers of SR demonstrated no significant increases or decreases. However, between days 10 and 19, diurnal numbers of SR decreased precipitously, appearing to have leveled off at the onset of puberty to levels observed in adults. SR were never scarce or absent in any age group studied. The neonatal appearance of large numbers of SR coincides developmentally with the initial sympathetic innervation of pineal parenchyma. Circadian rhythm in numbers of SR begins shortly thereafter. It is suggested (1) that circadian rhythm in numbers of SR is initiated and regulated or modified by sympathetic innervation of pinealocytes containing SR, and (2) that SR formation may be related to the intracellular flow of membrane relative to the transport and release of specific pineal products.
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