Heterogeneity of fish taste bud ultrastructure as demonstrated in the holosteans Amia calva and Lepisosteus oculatus

Taste buds are the peripheral sensory organs of the gustatory system. They occur in all taxa of vertebrates and are pear-shaped intra-epithelial organs of about 80 mu m height and 50 mu m width. Taste buds mainly consist of specialized epithelial cells, which synapse at their bases and therefore are secondary sensory cells. Taste buds have been described based on studies of teleostean species, bur it turned out that the ultrastructure of teleostean taste buds may differ between distinct systematic groups and that this description is not representative of those taste buds in other main taxa of fishes, such as selachians, holosteans and dipnoans. Furthermore, it is not known how variable the micromorphologies of non-teleostean taste buds are. For this reason the taste buds of two holosteans, Lepisosteus oculatus and Amia calva, were investigated and compared. While in both species the taste buds are of the same shapes and sizes, the cellular components of their sensory epithelia differ: in Lepisosteus taste buds comprise two types of elongated light cells and one type of dark cells. In contrast, Amia taste buds contain only one type of light, but two types of dark elongated cells. Afferent synapses are common in the buds of both species, efferent synapses occur only in Lepisosteus taste buds. These differences show that even in the small group of holostean fishes the taste buds are differently organized. Consequently, a representative type of fish taste buds does not exist.