[Function of myoepithelial cells in salivary secretion: reevaluation of the expulsion theory]

Although myoepithelial cells have been suggested to provide the expulsive force to secrete saliva into the mouth, direct evidence to support this idea has been lacking. We examined the structure and distribution of myoepithelial cells in rat parotid and sublingual glands, and observed whether contraction occurs in living acini during secretion in vitro. Scanning electron microscopy and the whole mount alkaline phosphatase histochemistry revealed well-developed myoepithelial cells around the sublingual acini. In the parotid gland, myoepithelial cells were abundant along the intercalated ducts but were sparse around the acini. A stress fiber-like distribution of actin fibers was demonstrated by rhodamine phalloidin staining in the myoepithelial cells of sublingual glands. Under a video-enhanced microscope, a rapid shrinkage of sublingual acini was observed within 2 min of secretory stimulation. However, the shrinkage was slight and transient, and thereafter the acinar shape was almost maintained over 30 min, during which time considerable amounts of secretory granules were released. It was suggested that the major function of myoepithelial cells in salivary secretion is the support for the glandular structure through isometric contraction. This conclusion is compatible with our previous morphometric analyses on fixed cells (Noriko Shoi: Kitasato Med 24: 432-434, 1994).