▁own ▁research , ▁breast ▁tum ors ▁containing ▁both ▁al ve olar ▁and ▁trab ec ular ▁structures , ▁as ▁well ▁as ▁demonstr ating ▁significant ▁morph ological ▁divers ity , ▁are ▁character ized ▁by ▁increased ▁drug ▁resistance . ▁Interest ingly , ▁the ▁contribution ▁of ▁the ▁trab ec ular ▁structures ▁to ▁chem ores istance ▁is ▁probably ▁explained ▁by ▁the ▁high ▁activity ▁of ▁ABC ▁trans por ters ▁in ▁tum or ▁cells ▁of ▁a ▁given ▁morph ological ▁variant . ▁In ▁contrast , ▁resistance ▁of ▁breast ▁tum ors ▁containing ▁the ▁al ve olar ▁structure ▁is ▁explained ▁by ▁other , ▁yet ▁un ident ified , ▁causes . <0x09> ▁ <0x09> ▁In vas ive ▁growth ▁and ▁its ▁phen ot yp ic ▁divers ity ▁are ▁associated , ▁both ▁directly ▁and ▁through ▁the ▁development ▁of ▁drug ▁resistance , ▁with ▁met ast asis . ▁Cir cul ating ▁tum or ▁cells , ▁which ▁are ▁responsible ▁for ▁the ▁development ▁of ▁future ▁met ast ases , ▁are ▁a ▁result ▁of ▁the ▁invasion ▁and ▁subsequent ▁pen etr ation ▁of ▁tum or ▁cells ▁into ▁l ym ph atic ▁or ▁blood ▁vessels . ▁Not ▁only ▁single ▁migr ating ▁tum or ▁cells , ▁but ▁also ▁cell ▁groups ▁can ▁have ▁the ▁int ra vas ation ▁ability . ▁There ▁is ▁an ▁assumption ▁that ▁collect ive ▁migration ▁much ▁more ▁often ▁leads ▁to ▁met ast asis ▁compared ▁to ▁individual ▁migration . ▁P ione ering ▁studies ▁in ▁animal ▁models ▁have ▁demonstrated ▁that ▁met ast ases ▁more ▁often ▁form ▁after ▁int ra ven ous ▁injection ▁of ▁tum or ▁clusters ▁rather ▁than ▁single ▁tum or ▁cells . ▁Furthermore