Death in the taste bud: Morphological features of dying taste cells and engulfment by Type I cells.

Taste buds comprise 50-100 epithelial derived cells, which are renewed throughout the life of an organism. Immature cells enter the bud at its base, maturing into one of three distinct cell types. How taste cells die and/or exit the bud, however, remains unclear. Here we present morphological data obtained through Serial Blockface Scanning Electron Microscopy of murine circumvallate taste buds, revealing several taste cells at the end of their life (4-6 per bud). Cells we identify as dying share certain morphological 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. Based on these features, we divide the cells into "early" and "late" stage dying cells. 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 fibers often appear detached from the main trunk of an afferent nerve fiber. Dying cells, like mature Type II and Type III taste cells, are surrounded by Type I taste cells, the glial-like cells of the bud. In many instances Type I cells appear to be engulfing their dying neighbors, suggesting a novel, phagocytic role for Type I cells. Surprisingly, virtually no Type I cells, which have the shortest residence time in taste buds, display features of apoptosis. The ultimate fate of Type I cells therefore remains enigmatic. Our examination of serial EM sections through murine taste buds sheds light on the life cycle of taste cells-crucial components of our sense of taste. We find that dying taste cells exhibit features typical of programmed cell death, or apoptosis. Many dying cells retain contacts with nerve fibers, but those fibers are often disconnected from the nerve trunk, suggesting that they cannot signal to the brain. Interestingly, most dying cells are Type II cells, which detect bitter, sweet, or umami. Our data also suggest that glial-like Type I cells act as "undertakers" within taste buds, engulfing dying neighbors. Surprisingly, Type I cells, despite having the shortest lifespan, do not show signs of dying; their ultimate fate remains enigmatic.