▁mur ine , ▁or ▁mouse , ▁and ▁x en opus ▁models . ▁Co - c ulture ▁of ▁neur ons ▁with ▁elect r ically ▁aligned ▁gli al ▁t issue ▁also ▁direct s ▁neur ite ▁out g row th , ▁as ▁it ▁is ▁rich ▁in ▁ne uro t roph ins ▁that ▁promote ▁n erve ▁growth ▁. ▁ ▁Est ab lish ing ▁polar ity ▁ ▁In ▁vit ro ▁ ▁An ▁und iffer enti ated ▁m amm al ian ▁neur on ▁placed ▁in ▁culture ▁will ▁ret ract ▁any ▁neur ites ▁that ▁it ▁has ▁already ▁grown . ▁ 0 . 5 ▁to ▁ 1 . 5 ▁days ▁after ▁being ▁pl ated ▁in ▁culture , ▁several ▁minor ▁neur ites ▁will ▁begin ▁to ▁pro tr ude ▁out ▁from ▁the ▁cell ▁body . ▁S omet ime ▁between ▁day ▁ 1 . 5 ▁and ▁day ▁ 3 , ▁one ▁of ▁the ▁minor ▁neur ites ▁begins ▁to ▁out g row ▁the ▁other ▁neur ites ▁significantly ▁. ▁This ▁neur ite ▁will ▁eventually ▁become ▁the ▁ax on . ▁On ▁days ▁ 4 - 7 , ▁the ▁remaining ▁minor ▁neur ites ▁will ▁begin ▁differenti ating ▁into ▁d end rit es . ▁By ▁day ▁ 7 , ▁the ▁neur on ▁should ▁be ▁completely ▁polar ized , ▁with ▁a ▁functional ▁d end rit es ▁and ▁an ▁ax on . ▁ ▁In ▁v ivo ▁ ▁As ▁previously ▁stated , ▁a ▁neur ite ▁growing ▁in ▁v ivo ▁is ▁surrounded ▁by ▁thousands ▁of ▁ext rac ell ular ▁signals ▁which ▁in ▁turn ▁can ▁be ▁mod ulated ▁by ▁hundreds ▁of ▁int rac ell