Immuno-electron-microscopic localization of enkephalin in the secretory granules of C cells in the chicken ultimobranchial glands

Abstract

In the chicken, enkephalin-immunoreactive cells and nerve fibers are distributed in the ultimobranchial glands, which consist of C-cell groups and cyst structures. Ultrastructural features of the enkephalin cells and nerve fibers were examined by immuno-electron microscopy using both the streptavidin-biotin-peroxidase method and the protein A-colloidal gold method. Immunoreactivity for enkephalin was located on the secretory granules of C cells. In 1-day-old chickens, three types of C cells were distinguished on the basis of their granule size. Type-I cells were filled with large secretory granules (200–600 nm in diameter). These elements represented a majority of the C-cell population. Type-II cells contained medium-sized granules (100–280 nm in diameter). Type-III cells displayed small secretory granules (60–200 nm in diameter). The latter cells were elongate or irregular in shape and frequently extended cytoplasmic processes into the connective tissue stroma or contacted other C cells. Enkephalin-immunoactivity was revealed by dense deposits of immunogold particles on the secretory granules of type-II and type-III cells. There were only a few type-I cells showing immunoreactivity for enkephalin. A double immunogold labeling procedure demonstrated that calcitonin and enkephalin were colocalized in the same secretory granules of type-I and type-II cells. Type-III cells were devoid of immunoreactivity for calcitonin. Enkephalin-immunoreactive nerve fibers were characterized by the presence of granular vesicles, 60–160 nm in diameter, and frequently established direct contact with the surface of C cells. It is considered that enkephalinergic nerve fibers may be related to the regulation of C cell activities, i.e., synthesis and secretion of hormones and catecholamines.