In this study effects of seasonal variation on the cellular response of bursa and testes in Japanese quails (Coturnix japonica) have been studied.  Thirty mature male Japanese quails, ten birds in each reproductive phase, namely reproductive active phase (June, July), regressive phase (September, October) and non active phase (January, February) were sacrificed. Blood samples were taken and total leukocyte and lymphocyte count were performed. The serum was separated and serum testosterone levels were measured. The testes and bursa of Fabricius from each bird were collected and the macroscopic and microscopic dimensions of testes and bursa of Fabricius were measured. Sections were therefore cut at 5-7 μm and stained by hematoxylin and eosin (H&E). Automated computer software Image J^® was used for histometric analysis.
The results revealed that morphometrical parameters such as the length, circumference, thickness, and width of the testis showed a variable pattern in different seasonal phases e.g. in the active phase they were significantly (P<0.01) larger and decreased at regular pattern in the regressive and the non active phase. In contrast the immune parameters were significantly (P<0.01) at higher levels during the non active phase and the regressive phase and decreased at regular pattern in the active phase. The serum testosterone levels were also high during the active phase and decreased at regular pattern in the regressive and the non active phase. The histometric analysis revealed similar results such as diameters of seminiferous tubules showing a significant (P<0.01) rise during the active phase as compared with the regressive and the non active phase, and a significant declined was record during the regressive and the non active phase. In conclusion, the increased reproductive activity in Japanese quails coincided with enhanced steroid hormone production during summer which suppressed the immune response. Moreover, during the winter decreased gonadal activity resulted in elevated immune function that helped to combat the harsh environment.

Key words: Bursa of Fabricius, Japanese quails, Serum testosterone, Testes, Total leukocyte count, Lymphocyte count