Innervation of melanocytes in human iris. An electron microscopic study.

Contacts between melanocytes and nerve terminals in human iris stroma were investigated by means of electron microscopy. Four distinct types of nerve terminal making synaptic contacts with melanocytes can be distinguished: (1) nerve terminals containing only empty synaptic vesicles (400-800 A in diameter) and with intervening basement membrane material (separation 1000 A); (2) nerve terminals with both empty vesicles and granulated vesicles (600-1100 A in diameter) also with intervening basement membrane material (separation 750 A); (3) nerve terminals containing empty vesicles and forming close contacts with melanocytes (distance about 400 A) without interposed basement membrane material, and (4) nerve terminals containg both empty and granulated vesicles (750 A in diameter) which make closer contacts with melanocytes (distance 270 A) without interposed basement membrane material. Type (1) and type (3) nerve terminals are thought to be cholinergic; type (2) and type (4) nerve terminals appear to be adrenergic. Taking into consideration these morphologic findings and previously reported data, it seems justified to postulate the presence of trophic innervation of melanocytes in the human iris.