A scanning and transmission electron microscope study of the premetamorphic papillae: possible chemoreceptive organs in the oral cavity of an anuran tadpole (Rana japonica).

Premetamorphic papillae of an anuran tadpole (Rana japonica) were studied by scanning and transmission electron microscopy. Premetamorphic papillae of several shapes are seen in and around the oral cavities of tadpoles during late larval and early metamorphic stages. These papillae are composed of three parts: the apical cellular part, the underlying connective tissue core and the epithelial covering. In the apical cellular part, two populations of cells are observed: apical and basal cells. The apical cell has a slender cytoplasmic process whose apical surface is exposed in the oral cavity. The basal cell is located at the basal portion of the apical cellular part of the papilla without researching the oral cavity. A long solitary microvillus and a bunch of short microvilli are seen on the apical surface of each apical cell. The apical cells contain dense-cored vesicles of 100 nm diameter and make synaptic contacts at their basal membranes with terminals of nerve fibers. Possible chemoreceptive function of the premetamorphic papilla is discussed.