▁expos ure ▁to ▁imm une ▁system ▁cells . ▁Given ▁the ▁intr atum oral ▁heter ogene ity , ▁in ▁the ▁struggle ▁for ▁surv ival , ▁some ▁tum or ▁cells ▁may ▁be ▁subject ed ▁to ▁regression ▁and ▁death , ▁while ▁other ▁cells , ▁which ▁resist ▁powerful , ▁counter act ing ▁micro en ▁vir on ment al ▁factors , ▁gain ▁an ▁ag gress ive ▁phen ot ype ▁and ▁the ▁ability ▁of ▁met ast atic ▁pro gression ▁. ▁In vas ive ▁tum or ▁growth ▁is ▁enabled ▁by ▁the ▁det achment ▁of ▁m align ant ▁cells ▁from ▁the ▁tum or ▁mass ▁due ▁to ▁a ▁reduction ▁in ▁or ▁complete ▁loss ▁of ▁inter cell ular ▁ad hes ion ▁mole cules , ▁and , ▁therefore , ▁the ▁cells ▁gain ▁the ▁ability ▁of ▁anom al ously ▁high ▁mot ility ▁en abling ▁pen etr ation ▁through ▁the ▁st iff ▁struct ural ▁elements ▁of ▁the ▁surrounding ▁st rom a ▁. ▁In ▁this ▁case , ▁the ▁invasion ▁process ▁extens ively ▁involves ▁various ▁mole cular ▁and ▁cell ular ▁mechan isms ▁that , ▁according ▁to ▁published ▁data , ▁depend ▁directly ▁on ▁another ▁bi ological ▁phenomen on ▁– ▁the ▁ep ith el ial - mes en ch ym al ▁transformation , ▁which ▁was ▁first ▁described ▁by ▁E . D . ▁Hay ▁in ▁ 1 9 9 5 . ▁Later , ▁the ▁term ▁“ ep ith el ial - mes en ch ym al ▁transition ” ▁( E MT ) ▁was ▁put ▁to ▁use ▁to ▁clarify ▁the ▁revers ibility ▁of ▁this ▁process ▁. ▁Currently , ▁E MT ▁is ▁known