( ( ( The base ) ( of ( the ( barrier cells ) ) ) ) ( ( are ( attached ( to ( ( ( the ( basement membrane ) ) , ) ( which ( is ( a ( dense ( extracellular matrix ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN base)) (PP (IN of) (NP (DT the) (NN barrier) (NNS cells)))) (VP (VBP are) (VP (VBN attached) (PP (TO to) (NP (NP (DT the) (NN basement) (NN membrane)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ is) (NP (DT a) (JJ dense) (NN extracellular) (NN matrix))))))))) (. .)))
The base of the barrier cells are attached to the basement membrane, which is a dense extracellular matrix.
(ROOT (S (NP (NP (DT A) (NN cell) (POS 's)) (NN membrane)) (VP (VBZ provides) (NP (DT a) (NN barrier) (S (VP (TO to) (VP (VB keep) (NP (NN extracellular) (NNS materials)) (PP (IN from) (S (VP (VBG mixing) (PP (IN with) (NP (PRP$ its) (JJ internal) (NNS components))))))))))) (. .)))