Fundamental parameters of the developing thymic epithelium in the mouse.

The numbers of thymic epithelial cells (TECs) and thymocytes steadily increase during embryogenesis. To examine this dynamic, we generated several TEC-specific transgenic mouse lines, which express fluorescent proteins in the nucleus, the cytosol and in the membranes under the control of the Foxn1 promoter. These tools enabled us to determine TEC numbers in tissue sections by confocal fluorescent microscopy, and in the intact organ by light-sheet microscopy. Compared to histological procedures, flow cytometric analysis of thymic cellularity is shown to underestimate the numbers of TECs by one order of magnitude; using enzymatic digestion of thymic tissue, the loss of cortical TECs (cTECs) is several fold greater than that of medullary TECs (mTECs), although different cTEC subsets appear to be still present in the final preparation. Novel reporter lines driven by Psmb11 and Prss16 promoters revealed the trajectory of differentiation of cTEC-like cells, and, owing to the additional facility of conditional cell ablation, allowed us to follow the recovery of such cells after their depletion during embryogenesis. Multiparametric histological analyses indicate that the new transgenic reporter lines not only reveal the unique morphologies of different TEC subsets, but are also conducive to the analysis of the complex cellular interactions in the thymus.