CONTINUOUS DARKNESS AND CONTINUOUS LIGHT INDUCE STRUCTURAL-CHANGES INTHE RAT THYMUS

It is known that neuroendocrine responses to environmental stimuli, su ch as light, can influence immune responses through the pineal gland. It is also known that periods of constant darkness and constant light cause stimulation and inhibition of melatonin secretion from the pinea l gland, successively. In this study, we provide experimental evidence that changes in the rhythm of the photoperiod have considerable effec ts on thymic structure of the rat. Male albino Wistar rats were divide d into 3 groups. Group 1 rats were kept in a dark room, group 2 in a r oom under a bright artificial light and group 3 (control) animals were exposed to a 12:12 h light:dark cycle. All animals were killed after 4 wk. In group 1, thymus weight increased by 315%, the increase in vol ume affecting the medulla (cortex 190%, medulla 655%). The absolute nu mber of epithelial cells and lymphocytes increased both in the cortex and medulla. Thymic cortical epithelial cells were hypertrophied and c ontained numerous large clear vesicles. Perivascular spaces were enlar ged. In group 2 thymus weight decreased by 53%, the reduction in volum e affecting mainly the cortex (cortex 61%, medulla 27%). The absolute numbers of cortical epithelial cells and lymphocytes were decreased, a nd pyknotic lymphocyte nuclei were frequent both in the cortex and med ulla. It is concluded that constant darkness causes hypertrophy and in creased cellularity of the thymus, while constant light causes involut ion of the thymus and death of lymphocytes. These changes possibly ref lect the well known immunostimulatory effects of melatonin acting dire ctly or indirectly, on the thymic lymphocytes and epithelial cells.