Fine ultrastructure of chromaffin granules in rat adrenal medulla indicative of a vesicle-mediated secretory process

Observation by transmission electron microscopy, coupled with morphometric analysis and estimation procedure, revealed unique ultrastructural features in 25.94% of noradrenaline (NA)- and 16.85% of adrenaline (A)-containing granules in the rat adrenal medulla. These consisted of evaginations of the granule limiting membrane to form budding structures having different morphology and extension. In 14.8% of NA-granules and 12.0% of A-granules, outpouches were relatively short, looked like small blebs emerging from the granule surface and generally contained electron-dense material. A proportion of 11.2% of NA-granules and 4.9% of A-granules revealed the most striking ultrastructural features. These secretory organelles presented thin, elongated, tail-like or stem-like appendages, which were variably filled by chromaffin substance and terminated with spherical expansions of different electron-density. A cohort of vesicles of variable size (30 to 150 nm in diameter) and content was found either close to them or in the intergranular cytosol. Examination of adrenal medullary cells fixed by osmium-zinc iodide (ZIO) revealed fine electron dense precipitates in chromaffin granules, budding structures as well as cytoplasmic vesicles. This data indicate that a common constituent is revealed by the ZIO histochemical reaction in chromaffin cells. As catecholic compounds are the main tissue target of ZIO complexes, catecholamines are good candidates to be responsible for the observed ZIO reactivity. This study adds further to the hypothesis that release of secretory material from chromaffin granules may be accomplished by a vesiclular transport mechanism typical of piecemeal degranulation.