Fig. 2.

The upper pictures illustrate the localization of TrkA-(left), TrkB-(middle) and p75^NTR (right)-positive cells in the rodent thymus; receptor-expressing cells are represented in black. Below are shown the corresponding TrkA (A), TrkB (B) and p75^NTR (C) immunostained sections. The cells expressing TrkA are subcapsular and medullar thymic epithelial cells (mouse); those showing TrkB immunoreactivity, concentrated at the cortico-medullary border (arrows), are macrophages (rat), and p75^NTR immunoreactivity is confined to a subpopulation of medullar thymic epithelial cells (arrows, rat). The lower images correspond to morphological aspects of functionally trkA- and trkB-deficient mice. The trkA-kinase –/– mouse thymus (D) is characterized by disorganization of the thymic architecture and the presence of medullar endodermic cysts containing amorphous material and cell debris, whereas the trkB-kinase –/– mouse (E) shows images of apoptotic lymphocyte death, especially in the cortex. c, cortical; m, medullar. Scale bar = 5 µm.