Lysozyme expression in the rat parotid gland: light and electron microscopic immunogold studies

Lysozyme (muramidase) is capable of direct bacteriolytic action by hydrolyzing glycosidic bonds in bacterial cell walls. Although it is broadly distributed in vertebrate tissues and secretions, the cellular and subcellular localizations of the enzyme are still not well known. The present study examines the distribution of lysozyme expression in the various cell types of LR gold-embedded rat parotid gland, applying a postembedding immunogold-silver staining technique for light microscopy. Simultaneously, a postembedding immunogold method for electron microscopy was used to determine the cellular compartments engaged in the biosynthesis and exocytosis of lysozyme. Silver-amplified immunogold staining for lysozyme demonstrated identical localization in both paraffin and semithin LR-gold sections: in the supranuclear parts of acinar and intercalated duct cells. Staining intensity varied even between adjacent cells. In the electron microscope, immunogold labeling was detected over the cell compartments associated with protein synthesis and exocytosis in acinar and intercalated duct cells. Lysozyme antigenic sites were visible over endoplasmic reticulum and throughout the Golgi apparatus, being intense over the trans-Golgi network, but even stronger in the condensing vacuoles and most prominent over secretory granules in both cell types. The findings provide the first immunocytochemical evidence of the synthesis and secretion of lysozyme in parotid acinar and intercalated duct cells.