Application of serial sectioning and three‐dimensional reconstruction to the study of taste bud ultrastructure and organization

Abstract

The lingual taste buds of mammals are complex organs containing dozens of cells of varying morphology and numerous nerve fibers that are intermingled among the cellular processes. Some of the taste bud cells form synaptic contacts with these nerve fibers. Important questions remain to be answered regarding the structure and function of the cells of various types within taste buds and the means by which responses to gustatory stimuli are transmitted to the nerve fibers that communicate with the brain. Using both conventional and high voltage electron microscopy, we have examined serially sectioned taste buds from the tongues of mice and rabbits in order to address these issues and to obtain more complete information than that available from sampling of sections. The technique of computer-assisted 3-D reconstruction was used to generate models of whole taste buds and individual cellular and neural elements within taste buds from the serial sections. Analysis of serially sectioned taste buds from mice and rabbits has revealed that in both of these species relatively few (30% or less) of the cells within the taste buds form synaptic contacts with nerve fibers. In the foliate taste buds of rabbits, all of the cells that are presynaptic to nerve fibers are of a single morphological type (type III). The cells that are presynaptic to nerve fibers within the taste buds of mice are morphologically diverse. A pattern of synaptic connectivity exists within murine taste buds such that a given nerve fiber receives synaptic input only from taste cells that are ultrastructurally similar. In the taste buds of both mice and rabbits, we have observed both divergence and convergence of synaptic input from the putative taste receptor cells onto nerve fibers, suggesting that at the level of the taste bud there is some integration of the information generated by individual receptor cells. In addition to typical chemical synapses, other cytoplasmic specializations (such as subsurface cisternae and atypical mitochondria) may be involved in interactions between taste bud cells and nerve fibers.