Thymocytes not only receive signals from thymic epithelial cells but c
an also activate the latter, at least in the medulla. We have previous
ly reported tyrosine phosphorylation of medullary epithelial cell subs
trates, after co-culture with thymocytes, and identified a number of p
rotein tyrosine kinases in a line of thymic epithelial cells. We repor
t here the in situ localisation by immunohistochemistry of JAK2 in med
ullary epithelial cells, of PDGF-R in medullary vascular endothelium,
of FGF-R in Hassall's corpuscles, and the weak expression of JAK1 and
RYK throughout the thymus.