To define the mechanism of immunoglobulin transport by mucous secretory cells, IgA, secretory component, IgG and J chain in human sublingual glands were localized by the peroxidase-labeled antibody method. The mucous cells examined here were classified into two groups according to the secretory cycle, namely comparatively immature cells and mature cells. IgA was localized to the basolateral plasma membrane and cytoplasmic vesicles of both immature and mature cells. In immature mucous cells, secretory component was localized to the perinuclear space, rough endoplasmic reticulum, Golgi apparatus, basolateral plasma membrane and cytoplasmic vesicles, and in mature cells faintly to the plasma membrane and cytoplasmic vesicles but not to the protein synthetic organelles. IgG was seen along the basal plasma membrane, but not in any other sites of both cells. In the sections treated with acid-urea, J chain was found on the plasma membrane of mucous cells of both types, whereas no J chain was identified in the untreated sections. These ultrastructural sites are analogous to those in intestine, where the mechanism of immunoglobulin transport has been defined in detail. These results indicate that the mucous cells as well as the human major salivary gland serous cells have the mechanism of immunoglobulin transport, and that the synthesis of secretory component by mucous cells is reduced as the secretory cycle progresses and the mature cells barely contribute to the immunoglobulin transport any longer.
