▁an ▁important ▁role ▁in ▁top ographic ▁mapping ▁in ▁several ▁other ▁regions ▁of ▁the ▁developing ▁nerv ous ▁system ; ▁the ▁bid irection al ▁signal ing ▁illustr ates ▁some ▁of ▁the ▁complex ▁regul atory ▁mechan isms ▁involved ▁in ▁growth ▁cone ▁guidance ▁and ▁target ▁selection . ▁ ▁Sem ap hor ins ▁ ▁Sem ap hor ins ▁are ▁a ▁family ▁of ▁chemical ▁signal ing ▁mole cules ▁involved ▁in ▁ax onal ▁target ing ▁and ▁guidance . ▁S ema 3 ▁was ▁the ▁first ▁verte br al ▁Sem ap hor in ▁discovered , ▁and ▁since ▁then ▁Sem ap hor ins ▁have ▁been ▁shown ▁to ▁el ic it ▁both ▁att raction ▁and ▁rep uls ive ▁responses ▁in ▁comm iss ural ▁ax ons ; ▁addition ally , ▁Sem ap hor ins ▁can ▁function ▁as ▁a ▁secret ed ▁or ▁contact - dependent ▁guidance ▁c ue ▁( Fig . 5 ). ▁ ▁Sem ap hor ins ▁have ▁also ▁been ▁shown ▁to ▁medi ate ▁other ▁neur onal ▁processes ▁besides ▁target ing ▁such ▁as : ▁apo pt osis , ▁cell ▁migration , ▁ax on ▁pr uning , ▁syn apt ic ▁transmission , ▁and ▁ax onal ▁transport . ▁Sem ap hor ins ▁are ▁the ▁main ▁lig ands ▁for ▁the ▁Ne urop il in ▁ 1 ▁( N pn 1 ) ▁re ceptor ; ▁this ▁re ceptor ▁is ▁typically ▁located ▁in ▁the ▁med ial ▁and ▁later al ▁port ions ▁of ▁the ▁later al ▁motor ▁column ▁during ▁the ▁early ▁emb ry onic ▁period ▁of ▁motor ▁neur on ▁development . ▁Upon ▁binding ▁Sem ap hor ins , ▁the ▁N pn