▁a ▁part ▁of ▁the ▁tum or ▁mass ▁or ▁det aching ▁from ▁it ▁in ▁the ▁form ▁of ▁mult ic ell ular ▁groups , ▁pen et rate ▁into ▁the ▁surrounding ▁t issues ▁and ▁form ▁thin ▁short ▁ch ords , ▁clusters , ▁stri pes ▁and ▁wide ▁fields , ▁as ▁well ▁as ▁structures ▁with ▁l umen , ▁that ▁indicate ▁a ▁wide ▁variety ▁of ▁struct ural ▁elements ▁involved ▁in ▁tum or ▁invasion . <0x09> ▁ <0x09> ▁As ▁already ▁mentioned , ▁collect ive ▁migration ▁is ▁character ized ▁by ▁the ▁migration ▁of ▁whole ▁cell ▁groups ▁inter connected ▁by ▁cad her ins ▁and ▁inter cell ular ▁gap ▁j unction s . ▁A ▁moving ▁cell ▁group ▁has ▁a ▁“ leading ▁edge ” ▁or ▁“ leading ▁front ” ▁that ▁uses ▁integr ins ▁and ▁prote ases ▁( Fig . ▁ 1 ). ▁Research ers ▁indicate ▁clear ▁differences ▁in ▁the ▁expression ▁of ▁gen es ▁and ▁the ▁morph ology ▁between ▁the ▁“ le ader ” ▁cells ▁forming ▁the ▁leading ▁edge ▁and ▁the ▁“ follow er ” ▁cells ▁that ▁are ▁located ▁behind ▁them , ▁at ▁the ▁“ tra iling ▁edge .” ▁The ▁“ le aders ” ▁in ▁the ▁cell ▁shape ▁often ▁res emble ▁mes en ch ym al ▁cells ▁and ▁are ▁character ized ▁by ▁a ▁less ▁pron ounced ▁ordering ▁and ▁struct ural ▁organization , ▁while ▁the ▁“ follow ers ” ▁tend ▁to ▁form ▁more ▁tight ly ▁pack ed , ▁ro set te - like ▁tub ular ▁structures ▁with ▁tight ▁inter cell ular ▁contacts . <0x09> ▁ <0x09> ▁In ▁the ▁case ▁of ▁collect ive ▁migration , ▁tum or