Synaptic like microvesicles (SLMV) form a pleiomorphic population of v
esicles present in neuroendocrine cells of the pituitary and the adren
al medulla. Their composition and biogenesis is very similar to small
synaptic vesicles (SSV) present in nerve terminals. Membrane proteins
of SSV involved in fusion (synaptotagmin) and trafficking (Rab 3) toge
ther with synaptophysin have been identified on SLMV. The SLMV from pa
ncreatic beta cells store GABA, those present in chromaffin cells cate
cholamines and/or acetylcholine. We conclude that SLMV may participate
in regulated exocytosis based on (a) their localization near release
sites, (b) their content and (c) their membrane composition. The funct
ional meaning and the way this process is regulated remain to be eluci
dated.