▁cells ▁form ▁pro tr us ions ▁( pse ud op odia ) ▁at ▁the ▁leading ▁edge , ▁use ▁integr ins ▁to ▁form ▁foc al ▁contacts ▁with ▁the ▁act in ▁c yt os ke leton , ▁and ▁perform ▁prote ol yt ic ▁de grad ation ▁of ▁the ▁ext rac ell ular ▁matrix , ▁creating ▁a ▁space ▁for ▁invasion ▁of ▁the ▁tum or ▁t issue ▁and ▁extens ively ▁involving ▁the ▁act in - my os in ▁contract ile ▁appar atus ▁in ▁the ▁process ▁to ▁ensure ▁successful ▁migration . <0x09> ▁ <0x09> ▁The ▁differences ▁in ▁the ▁polar ity ▁of ▁collect ively ▁migr ating ▁cell ▁groups ▁are ▁due ▁to ▁the ▁features ▁of ▁expression ▁of ▁surface ▁re cept ors , ▁such ▁as ▁C X CR 4 ▁and ▁C X CR 7 ▁chem ok ine ▁re cept ors , ▁in ▁the ▁“ le ader ” ▁cells . ▁The ▁growth ▁factors ▁and ▁chem ok ines ▁produced ▁by ▁st rom al ▁cells ▁and ▁a ▁diffusion ▁gradient ▁provide ▁ext rac ell ular ▁induction ▁of ▁cell ▁polar ization . ▁In vol vement ▁of ▁chem ok ines , ▁such ▁as ▁S DF 1 ▁( C X CL 1 2 ), ▁the ▁fib ro blast ▁growth ▁factor ▁( F G F ), ▁and ▁the ▁transform ing ▁growth ▁factor ▁ β ▁( T G F - β ), ▁in ▁these ▁processes ▁has ▁been ▁under ▁discussion . <0x09> ▁ <0x09> ▁Much ▁is ▁known ▁about ▁the ▁invol vement ▁of ▁T G F - β ▁in ▁car cin ogen esis , ▁with ▁its ▁role ▁being ▁two fold . ▁Taylor