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 from the 18th day of gestation (E18) to the 15th postnatal day (P15). Nerve processes were seen in the epithelium of E18 rats before taste buds were obvious. At E20, early taste buds were visible, but were embedded within the epithelium, i.e., their cells were shielded from the oral cavity by overlying squamous epithelium. At this stage, the epithelium 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 processes. In postnatal animals, keratinized epithelial cells were seen overlying taste buds, but taste pores were not observed until P10. How, then, do stimuli reach the taste cells and elicit physiological and behavioral responses as reported by others? The keratinized epithelium overlying 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 epithelium overlying the bud, they apparently were not released into the intercellular space. These lipid-rich lamellated bodies are thought to represent the water barrier of the epithelium, i.e., the barrier which prevents aqueous solutions from passing through the epithelium. We determined that the keratinized epithelium overlying the taste bud was permeable to a tracer, lanthanum nitrate, whereas that on the lateral surface was not. Lanthanum was visualized around taste cells and around nerve profiles within and near the taste bud. We propose that the absence of an aqueous permeability barrier in the epithelium overlying taste buds likely explains the ability of tastants to reach the taste bud cells and nerves in the developmental period before pore formation.