The paracoronal (marginal) cells of fungiform papilla of Rana esculenta.

Paracoronal secretory cells can be observed outside the sensorial area of the fungiform papilla of Rana esculenta. The morphology of these cells, the type of secretion and their function have, to date, only been incidentally described. By scanning electron microscopy (SEM), the paracoronal cells appear as swallow's-nest-shaped formations with openings 10-15 microns in diameter. The walls of paracoronal cells are characterized by laminar processes subdividing the interior hollow. The cavity of these formations is occupied by amorphous material as demonstrated by light microscopy (LM) pictures. The secretory material fills 7/8 of the upper part of the cytoplasm and appears rather transparent. The secretory material is PAS-negative, unlike the secretory granules contained in laminar processes. By transmission electron microscopy (TEM), they appear as clear ovoid structures, the nucleus of which is situated in the deeper part of the cell, enveloped by a thin cytoplasmic layer and characterized by secretory apparatus and the presence of secretory granules of middle electron-opacity. The apical part of these cells presents large mucous droplets. These cells adhere both to ciliated and parietal cells. Following cytochalasin-B treatment, cells do not show any considerable ultrastructural modification, while after terbutaline treatment the profiles of secreted paracoronal cells increase greatly. Histochemical properties of their secretory products are similar to those of parietal cells and their particular anatomical localization may exclude the direct implication of these cells in taste transduction.