The origin of small vesicles in neurosecretory axons.

The mechanism of release of neurosecretory products from the corpus cardiacum of the stick insect Carausius morosus is discussed. Results of experiments using ferritin as an exogenous marker show a) direct communication between the cavity of the larger (2000 A diameter) vesicles and extracellular space at the axon periphery, and b) ferritin also occurs in smaller vesicles (300 A diameter) derived, by a process similar to pinocytosis, from the axon surface. These findings are discussed in relation to exocytosis and the implications of this mode of release in terms of membrane turnover. It is suggested that neurosecretory products are released into the haemolymph by fusion between the limiting membrane of the neurosecretory droplet and the axon membrane. Small vesicles may be involved in retrieval of membrane material, countering that added in the above way.