▁different ▁secret ed ▁and ▁contact - dependent ▁guidance ▁c ues ; ▁it ▁navig ates ▁through ▁the ▁developing ▁nerv ous ▁system ▁in ▁search ▁of ▁its ▁target . ▁The ▁migration ▁of ▁the ▁growth ▁cone ▁is ▁medi ated ▁through ▁the ▁interaction ▁of ▁numerous ▁tro ph ic ▁and ▁trop ic ▁factors ; ▁net r ins , ▁sl its , ▁e phr ins ▁and ▁sem ap hor ins ▁are ▁four ▁well - stud ied ▁trop ic ▁c ues ▁( Fig . 1 ). ▁The ▁growth ▁cone ▁is ▁capable ▁of ▁modifying ▁its ▁sens itivity ▁to ▁these ▁guidance ▁mole cules ▁as ▁it ▁migr ates ▁to ▁its ▁target ; ▁this ▁sens itivity ▁reg ulation ▁is ▁an ▁important ▁theme ▁seen ▁throughout ▁development . ▁ ▁Net r ins ▁▁ ▁Net r ins ▁are ▁diffus ible ▁chem o att ract ive ▁mole cules ▁that ▁guide ▁comm iss ural ▁ax ons ▁across ▁the ▁mid line ; ▁they ▁are ▁secret ed ▁by ▁floor ▁plate ▁cells ▁at ▁vent ral ▁mid line ▁of ▁the ▁sp inal ▁cord . ▁Net r ins ▁establish ▁a ▁gradient ▁to ▁direct ▁comm iss ural ▁ax ons ▁at ▁a ▁distance ; ▁Net rin - 2 ▁is ▁expressed ▁broad ly ▁in ▁the ▁vent ral ▁two ▁third s ▁of ▁the ▁sp inal ▁cord , ▁but ▁not ▁in ▁the ▁floor ▁plate . ▁M ice ▁with ▁net rin - 1 ▁loss - of - function ▁exhib it ▁severe ▁dis ruption ▁in ▁comm iss ural ▁ax on ▁migration ; ▁this ▁experiment ▁established ▁the ▁importance ▁of ▁Net rin - 1 ▁in ▁guidance ▁dec isions . ▁ ▁Net rin - 1