IMMUNO-ELECTRON-MICROSCOPIC LOCALIZATION OF ENKEPHALIN IN THE SECRETORY GRANULES OF C-CELLS IN THE CHICKEN ULTIMOBRANCHIAL GLANDS

In the chicken, enkephalin-immunoreactive cells and nerve fibers are d istributed in the ultimobranchial glands, which consist of C-cell grou ps and cyst structures. Ultrastructural features of the enkephalin cel ls and nerve fibers were examined by immuno-electron microscopy using both the streptavidin-biotin-peroxidase method and the protein A-collo idal gold method. Immunoreactivity for enkephalin was located on the s ecretory 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 ce lls 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). Ty pe-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 c ontacted other C cells. Enkephalin-immunoactivity was revealed by dens e deposits of immunogold particles on the secretory granules of type-I I and type-III cells. There were only a few type-I cells showing immun oreactivity for enkephalin. A double immunogold labeling procedure dem onstrated that calcitonin and enkephalin were colocalized in the same secretory granules of type-I and type-II cells. Type-III cells were de void of immunoreactivity for calcitonin. Enkephalin-immunoreactive ner ve 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 fiber s may be related to the regulation of C cell activities, i.e., synthes is and secretion of hormones and catecholamines.