HVEM serial-section analysis of rabbit foliate taste buds: I. Type III cells and their synapses.

Serially sectioned rabbit foliate taste buds were examined with high voltage electron microscopy (HVEM) and computer-assisted, three-dimensional reconstruction. This report focuses on the ultrastructure of the type III cells and their synapses with sensory nerve fibers. Type III cells have previously been proposed to be the primary gustatory receptor cells in taste buds of rabbits and other mammals. Within rabbit foliate taste buds, type III cells constitute a well-defined, easily recognizable class and are the only taste bud cells observed to form synapses with intragemmal nerve fibers. Among 18 type III cells reconstructed from serial sections, 11 formed from 1 to 6 synapses each with nerve fibers; 7 reconstructed type III cells formed no synapses. Examples of both convergence and divergence of synaptic input from type III cells onto nerve fibers were observed. The sizes of the active zones of the synapses and numbers of vesicles associated with the presynaptic membrane specializations were highly variable. Dense-cored vesicles 80-140 nm in diameter were often found among the 40-60 nm clear vesicles clustered at presynaptic sites. At some synapses, these large dense-cored vesicles appeared to be the predominant vesicle type. This observation suggests that there may be functionally different types of synapses in taste buds, distinguished by the prevalence of either clear or dense-cored vesicles. Previous investigations have indicated that the dense-cored vesicles in type III cells may be storage sites for biogenic amines.