EVIDENCE FOR STIMULUS ACCESS TO TASTE CELLS AND NERVES DURING DEVELOPMENT - AN ELECTRON-MICROSCOPIC STUDY

We have examined developing taste buds in fungiform papillae of rats f rom the 18th day of gestation (E18) to the 15th postnatal day (P15). N erve processes were seen in the epithelium of E18 rats before taste bu ds were obvious. At E20, early taste buds were visible, but were embed ded within the epithelium, i.e., their cells were shielded from the or al cavity by overlying squamous epithelium. At this stage, the epithel ium on the lateral aspects of the fungiform papillae was keratinized, but that overlying the taste bud was not. Some taste bud cells at E20 contained synapse-like structures near their contacts with nerve proce sses. In postnatal animals, keratinized epithelial cells were seen ove rlying taste buds, but taste pores were not observed until P10. How, t hen, do stimuli reach the taste cells and elicit physiological and beh avioral responses as reported by others? The keratinized epithelium ov erlying the buds was unlike that on the lateral aspect of the papilla in at least one significant way. Few lamellated bodies were present in intercellular spaces beneath the stratum corneum, whereas these were abundant in the corresponding location within epithelium on the slope of the papilla. Although some were present within the squamous epithel ium overlying the bud, they apparently were not released into the inte rcellular space. These lipid-rich lamellated bodies are thought to rep resent the water barrier of the epithelium, i.e., the barrier which pr events aqueous solutions from passing through the epithelium. We deter mined that the keratinized epithelium overlying the taste bud was perm eable to a tracer, lanthanum nitrate, whereas that on the lateral surf ace was not. Lanthanum was visualized around taste cells and around ne rve profiles within and near the taste bud. We propose that the absenc e of an aqueous permeability barrier in the epithelium overlying taste buds likely explains the ability of tastants to reach the taste bud c ells and nerves in the developmental period before pore formation. (C) 1993 Wiley-Liss, Inc.