Active testis of a pigeon (HE)
The testes enlarge in the breeding season. The diameter of the seminiferous tubules increases, the epithelium heightens and is composed of more cell lines. Lots of spermatozoa pile into the lumen. The interstitial tissue is reduced to an extremely thin connective tissue layer.
In birds, spermatogenesis does not proceed in regular waves as in mammals but the germinal epithelium is arranged in columns that show different stages of sperm maturation. Consequently all cell types can be demonstrated on one histological section of the seminiferous tubules.