OpenNLP Parser Tree Result

I am using OpenNLP to parse some medical report, but one of the results from the Parser tree catches my attention. The original line is as follows:

"They are replaced by tumor tissue that exhibits glandular differentiation."

The Parser tree looks like this:

(TOP (S (NP (PRP They)) (VP (VBP are) (VP (VBN replaced) (PP (IN by)
(NP (NP (JJ tumour) (NN tissue,)) (SBAR (WHNP (WDT which))
(S (VP (VBP show) (NP (JJ glandular))))))))) (. differentiation.) ))

      

        
        
        
      

    

What I don't get is differentiation. As I know "." is an average offer closer. But how can you see the word "differentiation" there, so what is the meaning of that? I know this is NN, but what can I do to ask OpenNLP to handle this? Do I need to provide my information and train the system for this?