Distribution and ultrastructure of taste buds in the oropharyngeal cavity of the rainbow trout, Salmo gairdneri Richardson

The distribution, external surface morphology and ultrastructure of taste buds in the oropharyngeal cavity of the rainbow trout, Salmo gairdneri Richardson, were studied using scanning and transmission electron microscopes (SEM and TEM). The SEM revealed three taste bud types, varying only in their degree of elevation from the general level of the epithelium. Types I and II were located on elevated papillae associated with teeth on the dentary, maxilla, palate, tongue and pharyngeal pads while the unelevated Type III were mainly found in the anterior (branchial) pharynx.
Each taste bud was composed of four cell types: basal, dark, intermediate and light cells, the apical processes of the last three filling the taste pores. The intermediate and light cells appeared similar in ultrastructure, varying only in the amount and organization of smooth endoplasmic reticulum (SER) in their cytoplasm. In addition to its contacts with the processes of intragemmal nerves distally, the basal cells established independent contacts with processes of extragemmal nerves basally. It is suggested that the distribution of the taste buds and their close association with teeth are adaptations to the predatory feeding habit of the rainbow trout. Age differences may account for the existence of two types of gustatory cells and the manner of innervation of the taste bud suggests the existence of two pathways for the transmission of gustatory sensation to the central nervous system (CNS).