The interactions between follower and pioneer axons can be of two types: isotypic, when the axons are from the same neuronal type; and heterotypic, when the axons are from different neuronal types and have different origins and targets. In the development of complex vertebrate nervous systems, axons usually have the same origin and target, so they can be guided by isotypic interactions.[4] It has been assumed that pioneer neurons have an important role in guiding follower axons, being essential for them to reach their targets. However, the results about this role of pioneer neurons in specifying the projection path of follower axons are conflicting, once that some studies have been showing that pioneer neurons are not required or only facilitate the extending along follower neurons’ normal path.[2][4][5][6] For example, studies in C. elegans and Drosophila developing embryos showed that the correct outgrowth of follower neurons is not dependent on pioneer neurons, instead they probably only give further cues to make sure that each axon is in the right axon tract.[3][6][7] There is even a study in zebrafish with results showing that, when pioneer axons are eliminated, they can be replaced by follower axons, which since then behave like pioneer ones.[8] On the other hand, in invertebrates, pioneer neurons seem to have a central role in guiding follower neurons to their targets, because their removal leads to errors in follower neurons’ pathfinding.[4][8]