Synaptic vesicle proteins and target-dependent morphogenesis of calyx-like contacts in vitro

Chick Edinger-Westphal neurons form large, calyciform synaptic terminals, also known as calyces, on a subpopulation of ciliary ganglion neurons in vivo. This distinctive morphology is mimicked by contacts formed between Edinger-Westphal and ciliary ganglion neurons in culture. The work reported here finds that calyx-like contacts in culture are immunoreactive for the two vesicle proteins SV-2 and synaptotagmin, supporting the hypothesis that these contacts are synaptic terminals developing in vitro. When Edinger-Westphal neurons are cultured with ciliary ganglion neurons, calyx-like contacts are three times more frequent than when Edinger-Westphal neurons are cultured inappropriately with sympathetic neurons. Furthermore, calyx-like contacts are rare in cultures of ciliary ganglion neurons alone, even though ciliary ganglion neurons are known to functionally innervate themselves in culture. These results suggest that calyx morphogenesis is favored when Edinger-Westphal neurons interact specifically with ciliary ganglion neurons and discouraged when they interact with inappropriate neurons. That this specificity is expressed in culture, in the absence of normal tissue topography and patterned activity, adds to the evidence that direct surface interactions between target cells and growing neurites are involved in the morphogenesis of synaptic terminals in vivo.