It is known that periods of constant darkness and constant light cause stimulation and inhibition of melatonin secretion from the pineal gland. In this study, we provide experimental evidence that changes in the rhythm of the photoperiod have considerable effects on thymic structure of the rats. Male albino rats were divided into 3 groups. Group I rats were exposed to a 12:12 h light: dark cycle, group II rats were kept in a dark vicinity, group III in a vicinity under a bright artificial light. AH animals were killed after 8 wk. In group II, thymus weight increased. Thymic cortical epithelial cells hypertrophied. In group III thymus weight decreased. Thymic cortical epithelial cells and lymphocytes had decreased, pyknotic lymhocyte nuclei were frequently in the cortex and medulla. It is concluded that constant darkness causes hypertrophy and increased cellularity of the thymus, while constant light causes involution of the thymus and death of lymphocytes. These changes possibly may be atributed the immunostimulatory effects of melatonin acting directly or indirectly, on the lymphocytes and epithelial cells.
Key words:Photoperiod, melatonin, thymus, microscopy
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