The serous demilune of rat sublingual gland is an artificial structure produced by conventional fixation

The ultrastructure of the secretory end-piece of the rat sublingual gland w as examined in samples prepared by rapid freezing and freeze-substitution m ethod, and results were analyzed in combination with 3-D images reconstruct ed by computer graphics from light micrographs of serial sections. Fixation by rapid freezing followed by freeze-substitution preserved cellular ultra structures, especially the membrane structure, in perfect condition, and de monstrated the terminal portion of the sublingual gland to be a compound br anched tubulo-alveolar gland with serous cells distributed throughout the e nd-pieces. All the serous cells aligned with mucous cells to surround a com mon lumen, leaving no demilune structure. In contrast, samples fixed by the conventional immersion method showed distended mucous cells displacing the serous cells toward the basal portion of the acinus to form the demilune s tructure. The luminal space was also compressed and appeared disconnected f rom the serous cells. From these observations, the serous demilune that for more than 130 years has been believed to be an actual histological entity was proved to be an artificial structure produced through compression by th e hydrated and expanded mucous cells during immersion fixation.