FIGURE 9.

Human fungiform papillae (FP) showing taste bud and basal stem cell layer. Immunofluorescence stainings are of Ki67 (green, a marker of proliferating cells) and dipeptidyl peptidase (DDP; red). DPP4, also known as CD26 and adenine deaminase complexing protein 2, is a multifunctional membrane-bound and soluble protein that is best known for its activity as an exopeptidase. It is also a viral receptor (for MERS-CoV) and is involved in the regulation of intracellular signal transduction of T lymphocytes and apoptosis. DPP4 staining of individual cells, mostly likely T lymphocytes, can be seen in the lamina propria of the fungiform papillae and in all the stem cells of the basal layer, including the few cells directly underneath the taste bud. This is the first time that DPP4 has been shown to be present on membranes of this stem cell layer. No DPP4 is present in taste buds, and therefore mature taste receptor cells lose this cell surface marker. The arrow indicates the position of a taste bud (outlined in black), which can be seen in the hematoxylin and eosin staining image. FP biopsy was obtained from a young male in the fasted state. Scale bar = 50 µm.