Death in the taste bud: engulfment of dying taste receptor cells by glial-like Type I cells

Taste buds comprise 50-100 epithelial derived cells, including glial-like cells (Type I) and two types of receptor cells (Types II and III). All of these taste cells are renewed throughout the life of an organism from a pool of uncommitted basal cells. Immature cells enter the bud at its base, maturing into one of the three mature cell types. How taste cells die and/or exit the bud, as well as the role of the glial-like cells in this process, remains unclear. Here we present morphological data obtained through Serial Blockface Scanning Electron Microscopy of murine circumvallate taste buds, revealing taste cells at the end of their life. Cells we identify as dying share morpholowgical features typical of apoptosis: swollen endoplasmic reticulum, large lysosomes, degrading organelles, distended outer nuclear membranes, heterochromatin reorganization, cell shrinkage, and cell and/or nuclear fragmentation. Most early stage dying cells have Type II cell morphologies, while a few display Type III cell features. Many dying cells maintain contacts with nerve fibers, but those postsynaptic fibers often appear to be detached from the main trunk of an afferent nerve. Dying cells, like mature Type II and III taste cells, are surrounded by glial-like Type I cells. In many instances Type I cells appear to be engulfing their dying neighbors, suggesting for them a novel, phagocytic role. Surprisingly, virtually no Type I cells display features of apoptosis, although they reportedly have the shortest residence time in taste buds. The ultimate fate of Type I cells therefore remains unknown. Table of Contents EntryO_ST_ABSMain PointsC_ST_ABSO_LIDying taste cells display morphologies consistent with apoptosis. C_LIO_LIGlial-like Type I cells engulf dying neighbors, perhaps acting as "undertakers." C_LIO_LIWe find no dying Type I cells--all early dying cells are taste transducing Type II or III cells. C_LI