Release mechanism in mucous cells of the mouse sublingual salivary gland.

The release mechanism of mucous secretory product was studied in mouse sublingual glands after cholinergic stimulation both in vivo and in vitro. Secretory glycoproteins of the mucous sublingual gland cells are packaged into small secretory granules, which fuse intracellularly with each other to form larger ones. This intracellular fusion starts with close apposition of both granule membranes, followed by the formation of a five-layered membranous structure. The next step involves the intermixing of the central electron dense layer of this structure into two other electron dense layers, forming a three-layered structure. Finally this three-layered structure becomes thinner and disrupts. Release of secretory material occurs through gaps in the apical surface. After fusion of the granule membrane with the luminal plasma membrane the protruding mass of secretory material bulges into the acinar lumen, limited by a trilaminar membrane. Ultimately several ruptures arise in the bulging membrane and membrane fragments are released together with secretory products. The phospholipid composition of the released membrane fragments differs from that of the granular membranes of parotid and submandibular glands. Thus, compared to serous cells the mucous sublingual gland cells exhibit remarkable differences in the release mechanism which are obviously due to differences in physiological and chemical characteristics of the mucous secretory granule membrane.