Classification of the frog lingual gland cells and their exocytotic features

We examined under light and electron microscopes the frog lingual gland before and after acetylcholine-stimulation. The lingual gland is composed of five kinds of secretory cells (type 1-5) and ciliated cells. Each secretory cell had numerous cell-type characteristic secretory granules, probably in which both serous and mucous secretory products were contained. The ciliated cells had lysosomes, cytoplasmic filaments and on their apical surface microvilli in addition to cilia. Two kinds of exocytotic features were demonstrated in the secretory cells. One of them, termed multigranular exocytosis, was whole-cell discharge of secretory products in which intracellular huge vacuoles continuous to the cell exterior were formed as the result of multiple granule-to-granule fusion as well as granule-to-plasma membrane fusion. The other type was local exocytotic release of a single or a few secretory granules near the plasma membrane. The multigranular exocytosis was restricted to type 4 and 5 secretory cells while single or a few granular exocytosis was seen in type 1, 2, 4 and 5 secretory cells. This may be because secretory granules are larger and more closely packed in type 4 and 5 secretory cells than in other types of secretory cells. Type 3 secretory cells contained abundant cytoplasmic filaments, and showed no exocytotic feature, suggesting transitional cells between secretory cells and ciliated cells.