Soluble factors from primary cultures are insufficient in promoting ISL1. A, B, Condition-medium (CM) assay: neurosphere-expanded LGE cells express ISL1 when in contact with primary LGE cells during differentiation (control), but when differentiated in CM obtained from the same primary cells, no expression of ISL1 could be detected in the expanded cells. C, D, Shared medium assay: when primary LGE cells and neurosphere-expanded LGE cells were differentiated in separate wells but with shared medium, ISL could be detected in the primary cells, but not in the expanded cells. Scale bar, 50 μm.