• tropical rats;
  • pineal gland;
  • ultrastructure;
  • seasonal changes;
  • extracellular accumulations

The ultrastructure of the superficial pineal gland of the yellow-bellied country rat (Rattus losea Swinhoe), living under natural climatic conditions at the margin of the tropical region, was stuthed with special reference to the seasonal changes. In the rainy season with average monthly rainfall of 400 mm (light phase of 13.5 h, and temperature of 28°C) the gland was characterized by the presence of frequent large vacuoles containing flocculent material (LVFs), large condensed inclusions (LCIs) in the pinealocytes, and extensive accumulations of presumptive secretion in the extracellular compartment, suggesting an enhanced secretory activity. In the dry season with average monthly rainfall of 13 mm (light phase of 11 h, and temperature of 19°C), however, the pinealocytes were characterized by the presence of numerous clear spindles, which exhibited acid phosphatase activity; at this time there were few LVFs and LCIs and rare extracellular accumulations, suggesting a declined glandular activity. We conclude that the cellular activity of the pineal gland in R. losea, living under tropical climatic conditions, is synchronized with the annual changes of rainfall, which is apparently more important than the annual changes of photoperiod and temperature.