Distribution patterns of stromal eosinophil cells in chick thymus during postnatal development

Eosinophils are a type of thymic stromal cell that are present in the thymus of both humans and mice. They participate in regulating T-cell development under non-pathological conditions. However, studies are scarce regarding the role of eosinophils in the development of the thymus in chickens. Therefore, this study investigated the distribution of eosinophils in normal chicken thymi at different stages of development. Seven thymi were obtained from chickens at days 1, 21 and 35 of development. The distribution of eosinophils in the thymi was analyzed by histological and immunohistochemical techniques using Lendrum's chromotrope 2R method and an antibody against eosinophilic cationic protein (ECP), respectively. Eosinophils were constitutively located in the chick thymus. They were mainly distributed in the thymic corticomedullary junction and medulla, especially around vessels and Hassall's corpuscles, and only a few were in the trabeculae among thymic lobules and around vessels. There were none in the cortex. The number of thymic eosinophils decreased with increasing age (P < 0.01). These results indicated that eosinophils comprise a type of thymic stromal cells in the chick, which may regulate thymic development, especially during the early stages of development.