Immunohistochemical study of tyrosine phosphorylation signaling in Hassall's corpuscles of the human thymus.

Tyrosine phosphorylation signaling has been reported to play a key role in thymocyte development. However, the physiological role of signaling in thymus stroma is poorly understood, and there is lack of information on the in situ localization of elements of the signaling pathway in thymus stroma. In the present study, we have found by immunohistochemical analysis that tyrosine-phosphorylated proteins are present in high amounts in Hassall's corpuscles of the thymus medulla. Hassall's corpuscles represent end stages of maturation of thymic medullary epithelium. We have also investigated the localization of the src family that is involved in tyrosine phosphorylation signaling in Hassall's corpuscles. A member of the src family protein tyrosine kinases, p59fyn, was shown to be abundantly expressed in the outer layer of Hassall's corpuscles. Another member of the family, p60c-src, was highly expressed in the entire Hassall's corpuscles. Furthermore, p50csk and p130cas, both of which are involved in the pathway, were shown to be preferably expressed in the outer layer of Hassall's corpuscles. These findings suggest that tyrosine phosphorylation signaling may play a role in thymic medullary epithelial maturation and that the src family is involved in the process.