UNRAVELLING THE FINE STRUCTURE OF THE NEWLY IDENTIFIED TUFT THYMIC EPITHELIAL CELLS USING CORRELATIVE LIGHT-ELECTRON MICROSCOPY

Tuft cells are epithelial chemosensory cells known to reside in various mucosal tissues such as the intestine, gallbladder or respiratory tract. Importantly, the fact that these organs are exposed to the external environment determines their polarized structure with an apical and basal directionality. Correspondingly, tuft cells residing in these organs were shown to have a unique morphology, with microvilli extensions towards their apical side, supported by actin filaments. ‘Tasting’ the external environment, diverse receptors reside on these microvilli including succinate receptors and taste receptors amongst others. In addition, tuft cells have a signature elongated morphology, as well as lateral extensions protruding into the nuclei of neighboring cells. Interestingly, we and others recently discovered tuft cells to be present also in the thymus, providing the first evidence for tuft cells in non-mucosal tissue. However, the tuft cell morphology in the thymus remains unknown. In contrast to the organs mentioned above, the thymus is not exposed to the external environment nor does it have an apical-basal polarity. The thymus is a primary immune organ that is crucial for establishment of adaptive immunity. In the thymus, developing T cells go through processes of positive and negative selection, ensuring that only T cells that will not attack ‘self’ targets will fully mature and take part in the immune system, thus establishing self-tolerance. The key players in these selection processes are the thymic epithelial cells (TECs), and tuft cells are a subset of this vastly heterogeneous cell population. It is puzzling whether tuft thymic epithelial cells (tTECs) share the common morphological features of their counterparts in other organs, or perhaps they bear unique morphological features of their own. To address these puzzling questions we sought to unravel tTECs fine structure. However, given the extreme rarity of tTECs in the thymus, this goal poses a major technical challenge. Therefore, we combined unique features of fluorescent and electron microscopy and employed correlative light-electron microscopy (CLEM). Indeed, using CLEM we have successfully identified tTECs and were able to unravel their ultra-structure. Specifically we found that tTECs possess an elongated shape and are surrounded by structural filaments and putative microvilli. In addition, our analyses highlighted a very complex cytosolic structure characterized by dozens of fine vesicles. These findings may provide important insights about the still elusive role of these cells in the thymus.