Ultrastructure of the adrenal medulla of normal and insulin-treated hamsters.

Fine structural characteristics of the chromaffin cells both in normal and insulin-administered hamster adrenal gland were studied. Exocytosis occurs in 5 per cent of nonstimulated cells especially on the apical cell surfaces. At the same time the occurrence of a great number of closely attached secretory granules was conspicuous on the lateral plasma membrane in the untreated hamster adrenal medulla. Following insulin treatment (10 IU/100 g/body weight), characteristic was the development of large intercellular vacuoles between the lateral plasma membrane, in which electron-dense secretory material was frequently present. On the basis of this observation it is suggested that in the case of insulin-induced hormone secretion, exocytosis preferentially occurs on the lateral plasma membrane, and may play an important role in the discharge of secretory materials from the cells.