It is known that periods of constant darkness and constant light cause stim-ulation and inhibition of melatonin secretion from the pineal gland. It is also suggested that neuroendocrine responses to environmental stimuli, such as light, can influence immune responses through the pineal gland. For these reasons, in this study the effects of the alterations in the photoperiod rhythm on the pineal gland and thymus morphology were experimentally investigated. 30 Swiss albino rats, aged 4 wk, were divided into 3 groups. Group I rats were kept in a dark room, group II in a room under a bright artificial light and group III (control) animals were exposed to a 12:12 h light: dark cycle. All animals were killed after 5 wk. Overall body weight was not affected by the dark or light periods. In group I, the thymus weight increased by 38%, the increase in the thymus cortex thickness was 34.5%. In group II, the thymus weight decreased by 21% and the decrease in the thymus cortex thickness was 18%. The differences among groups in the thymus weight and the thymus cortex thickness were found statistically significant. In differences in the diameter of the pineal gland and the pineal cell number of the darkness group and the light group according to the control group was not found statistically significant. However, the differences in he diameter of he pinealocyte nuclei among the groups were statistically significant. These findings point out that the changes in the periods of darkness and light have effects on the pineal gland and immune system.

It is known that periods of constant darkness and constant light cause stim-ulation and inhibition of melatonin secretion from the pineal gland. It is also suggested that neuroendocrine responses to environmental stimuli, such as light, can influence immune responses through the pineal gland. For these reasons, in this study the effects of the alterations in the photoperiod rhythm on the pineal gland and thymus morphology were experimentally investigated. 30 Swiss albino rats, aged 4 wk, were divided into 3 groups. Group I rats were kept in a dark room, group II in a room under a bright artificial light and group III (control) animals were exposed to a 12:12 h light: dark cycle. All animals were killed after 5 wk. Overall body weight was not affected by the dark or light periods. In group I, the thymus weight increased by 38%, the increase in the thymus cortex thickness was 34.5%. In group II, the thymus weight decreased by 21% and the decrease in the thymus cortex thickness was 18%. The differences among groups in the thymus weight and the thymus cortex thickness were found statistically significant. In differences in the diameter of the pineal gland and the pineal cell number of the darkness group and the light group according to the control group was not found statistically significant. However, the differences in he diameter of he pinealocyte nuclei among the groups were statistically significant. These findings point out that the changes in the periods of darkness and light have effects on the pineal gland and immune system.