Configuration of myoepithelial cells in various exocrine glands of guinea pigs.

To study the configuration of myoepithelial cells, we isolated glandular endpieces of various guinea pig glands by collagenase, and visualized the myoepithelial cells by immunohistochemistry for actin, or by Bodipy-phallacidin, under both a light microscope and laser scanning confocal microscopes. In parotid and mandibular glands, the glandular acini were small (about 20-30 microns diameter) and spherical, and each acinus had one or two myoepithelial cells attached that were stellate in shape (central cell body and four to six thin processes). Most of the basal surface of the glandular cells was not covered by myoepithelial cells, and processes often extended to the neighboring acinus. The tubular glandular endpieces of the major sublingual gland, which secretes a mucous substance, were almost fully encircled by band-like myoepithelial cells (about 3-6 microns wide). Although there were many differences between the lacrimal gland and the Harderian gland (e.g., the secretory product of the lacrimal gland was mucous, and glandular lumina were narrow; the Harderian gland secreted lipids and showed wide lumina), the outer contours of both glandular endpieces were the same (about 50-100 microns diameter, ellipsoid or spherical in shape). In both glands, 5-20 stellate myoepithelial cells were attached onto a glandular endpiece, and their arrangement had a lacy appearance. Actin filaments in myoepithelial cells aggregated and formed bundles in the broad processes and cell bodies. The bundles ran across the cell body, but there was no point where the bundles converged. In the arborization, some distal processes reversed their direction. We conclude that the configuration of myoepithelial cells depends on the outer contour of the glandular endpieces rather than on the secretory material or luminal width. The variety of myoepithelial cell configurations in the different exocrine glands we examined suggests that it is quite difficult to assign to myoepithelial cells the general role of expelling secretory products from glandular lumina. These cells seem to maintain the contour of the glandular endpieces, serving as the exoskeleton of the endpieces.