Changing myoepithelial cell distribution during regeneration of rat parotid glands

Abstract

The distribution of the myoepithelial cells during regeneration of the rat parotid gland after atrophy induced by one week of parotid duct ligation was investigated by immunohistochemistry for actin and transmission electron microscopy (TEM). Immunohistochemically, residual ducts were surrounded by actin-positive cells when clips were removed from the duct. Three days later, most of the newly formed acini originating from the residual ducts were also embraced by actin-positive cells. After 10 days, actin-positivity tended to be seen as dots around acini that decreased in number day by day. On day 21 actin-positive cells mainly surrounded intercalated ducts with only a few positive reactions identified at the acinar periphery. Electron microscopically, residual ducts and newly formed acini were peripherally embraced by myoepithelial cells before day 5. After day 7, shift of myoepithelial cells from the periphery of acini to the duct-acinar junctional region was identified. Then few myoepithelial cells were identified at the periphery of acini. These observations indicate that myoepithelial cells migrate from the acinar periphery to the duct-acinar junctional region during rat parotid regeneration, and that such behaviour is closely related to that seen during rat parotid development.

Figure 1

Actin immunohistochemistry of a, the control and b–e, experimental glands. a, Actin-positive reaction is observed at the periphery of the intercalated ducts (arrows) and a few acini (arrowheads). Blood vessels are also positive. b, Day 0. Actin-positive reactions are identified around many residual ducts. c, Day 3. Newly formed acini are embraced by actin-positive cells (arrowheads). d, Day 10. Actin-positive reactions tend to be dots (arrowheads) around acini. e, Day 21. The positivity is observed at the periphery of intercalated ducts (arrows) and a few acini (arrowheads) (bar = 50 μm for all parts).

Figure 2

Changes in percentage of acini with actin-positive reactions at the periphery in experimental parotid glands after removal of clips. The highest is 78.4% on day 5 and the lowest is 12.5% on day 17 and 21. The mean in controls is 9.3% (not shown in the graph). (n = 4 for experimental animals at each time point; results expressed as mean ± SD)

Figure 3

TEM of parotid glands from experimental groups. Throughout M, myoepithelial cells; A, acinar cells; D, duct cells. a, Day 0. Flat myoepithelial cell with dense bodies (arrowheads) is located at periphery of residual duct. b, Day 5. Myoepithelial cell located basal to newly formed acinar cells, one of which is undergoing mitosis. c, Day 10. Myoepithelial cell partially located between acinar cells. d, Day 10. Myoepithelial cell extending thin process to basement membrane, mainly situated among acinar cells and touching duct cell. e, Higher magnification of myoepithelial cell process in (d). Abundant microfilaments sometimes with dense bodies (arrows) and caveolae (arrowheads) along the basal plasma membrane are seen. f, Day 14. Myoepithelial cell is located between acinar cells and duct cells. Inset, Higher magnification of area adjacent nucleus of myoepithelial cell. Microfilaments with dense bodies (arrowheads) are seen. × 7400. Bar = 2 μm (a-d,f); bar = 1 μm (e).

Figure 4

TEM of parotid gland 10 days after removal of clips. The epithelial cell contains dense body like structures (arrowheads) and secretory granules (arrows). Bar = 1 μm.