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 rap id freezing, followed by freeze substitution, was compared with that obtain ed by the standard chemical fixation technique. The rapid freezing method g ave a very good preservation of membrane structure with round and discrete mucous droplets (granules) not showing any sign of coalescence. The cistern s of the Golgi apparatus and the trans Golgi network also were well preserv ed. Upon secretory stimulation by pilocarpine, mucous droplets were dischar ged by the usual mechanism of exocytosis. From all these findings it emerge d that mucous cells had the same structural characteristics as serous cells . in the endpieces of rat sublingual gland prepared by the rapid freezing m ethod, serous cells aligned with mucous cells around the central lumen, and no cap-like arrangement of serous cells (demilunes) was observed. Furtherm ore, computer reconstruction of stereo images from serial section light mic rographs prepared by the rapid freezing method showed that, within a given endpiece, all serous cells had direct access to the lumen and that they wer e disseminated throughout it and not only in its fundus. From our observati ons it seems very likely that, at least in rat sublingual gland, serous dem ilunes are an artificial product caused by the compression exerted on serou s cells by the mucous cells distended during the conventional fixation proc edure.