Review article fine structure of the exocrine cells of rat sublingual gland revealed by rapid freezing and freeze substitution method

The ultrastructure of mucous cells of rat sublingual gland processed by rapid freezing, followed by freeze substitution, was compared with that obtained by the standard chemical fixation technique. The rapid freezing method gave a very good preservation of membrane structure with round and discrete mucous droplets (granules) not showing any sign of coalescence. The cisterns of the Golgi apparatus and the trans Golgi network also were well preserved. Upon secretory stimulation by pilocarpine, mucous droplets were discharged by the usual mechanism of exocytosis. From all these findings it emerged that mucous cells had the same structural characteristics as serous cells. In the endpieces of rat sublingual gland prepared by the rapid freezing method, serous cells aligned with mucous cells around the central lumen, and no cap-like arrangement of serous cells (demilunes) was observed. Furthermore, computer reconstruction of stereo images from serial section light micrographs prepared by the rapid freezing method showed that, within a given endpiece, all serous cells had direct access to the lumen and that they were disseminated throughout it and not only in its fundus. From our observations it seems very likely that, at least in rat sublingual gland, serous demilunes are an artificial product caused by the compression exerted on serous cells by the mucous cells distended during the conventional fixation procedure.