Aggregating brain cell cultures are primary, three-dimensional cell cultures consisting of even-sized, spherical structures that are maintained in suspension by constant gyratory agitation. Because of the avidity of freshly dissociated embryonic cells to attach to their counterparts, cell aggregates form spontaneously and rapidly under appropriate culture conditions. The reaggregated cells are able to migrate within the formed structures, and to interact with each other by direct cell-cell contact, as well as through exchange of nutritional and signaling factors. This tissue-specific environment enables aggregating neural cells to differentiate, and to develop specialized structures (e.g., synapses, myelinated axons) resembling those of brain tissuein situ. Aggregating cell cultures are therefore classified as organotypic cultures (Doyle et al., 1994 ).
