[Qualitative and quantitative electron-microscopic studies on the postnatal development of pinealocytes and sympathetic nerves in the pineal gland of the Chinese hamster, Cricetulus griseus].

: Qualitative and quantitative electron-microscopic observations were made on pinealocytes and sympathetic nerves in the superficial pineal gland of Chinese hamsters (Cricetulus griseus) at 10 to 40 days of age. Nuclear and cytoplasmic volumes of pinealocytes in hamsters at 10 days were smaller during the first half of the light period and larger during the first half of the dark period. At 20 to 40 days, however, there were 24-h variations in these values with their increase and decrease occurring during the first half of the light period and the first half of the dark period, respectively. Thus, the reversal of these rhythms occurred between 10 and 20 days. The 24-h rhythm in the amount of condensed chromatin appeared at 20 days. At 10 days, rodshaped synaptic ribbons (SRs) often appeared as SR fields composed of more than two long SRs. By contrast, rod-shaped SRs were shorter in length and usually occurred singly at 20 to 40 days. A determination of the quantity of SRs, expressed as SR index, revealed that the above mentioned morphological changes of rod-shaped SRs seen between 10 and 20 days were followed by a decrease in the quantity of rod-shaped SRs and an appearance of its 24-h rhythm. Round SRs first appeared at 20 days and increased in number thereafter. Ring-shaped SRs occurred at 20 days and, at the same time, their quantity showed a 24-h change. In the Chinese hamster, the change of the 24-h rhythms in the nuclear and cytoplasmic volumes of pinealocytes observed between 10 and 20 days and the appearance of the 24-h variations in the amount of condensed chromatin and the quantity of rod- and ring-shaped SRs in pinealocytes during these postnatal days may be related to the development of pineal sympathetic nerves.