Structural comparisons of human and mouse fungiform taste buds

Taste buds are commonly studied in rodent models, but some differences exist between mice and humans in terms of gustatory mechanisms and sensitivities. Whether these functional differences are reflected in structural differences between species is unclear. Using immunofluorescent image stacks, we compared the morphological and molecular characteristics of mouse and human fungiform taste buds. The results suggest that while the general features of fungiform taste buds are similar between mice and humans, several characteristics differ significantly. Human taste buds are larger and taller than those of mice, yet they contain similar numbers of taste cells. Taste buds in humans are more heavily innervated by gustatory nerve fibers expressing the purinergic receptor P2X3 showing a 40% higher innervation density than in mice. Like type II cells of mice, a subset (about 30%) of cells in human taste buds is immunoreactive for phospholipase C beta (PLCβ2). These PLCβ2-immunoreactive cells display calcium homeostasis modulator 1 (CALHM1)-immunoreactive puncta closely opposed to gustatory nerve fibers suggestive of channel-type synapses in type II cells in mice. These puncta, used as a measure of synaptic contact, are significantly larger in humans compared to mice suggesting a higher efflux of adenosine triphosphate (ATP) neurotransmitter in humans. Altogether these findings suggest that while many similarities exist in the organization of murine and human fungiform taste buds, significant differences do exist in taste bud size, innervation density, and size of synaptic contacts that may impact gustatory signal transmission.