Top-down  an apex of professors and doctors buttressed by the holders of college degrees (many of them lower-level administrators, teachers, and researchers within the system).
( ( ( Top-down ( an apex ) ) ( of ( ( professors and ) doctors ) ) ) ( ( ( buttressed ( by ( ( the holders ) ( of ( college degrees ) ) ) ) ) ( -LRB- ( ( ( ( many ( of ( ( ( ( ( them lower-level ) administrators ) , ) teachers ) , ) ) ) and ) ( researchers ( within ( the system ) ) ) ) -RRB- ) ) ) . ) )
(ROOT (S (NP (NP (JJ Top-down) (DT an) (NN apex)) (PP (IN of) (NP (NNS professors) (CC and) (NNS doctors)))) (VP (VBN buttressed) (PP (IN by) (NP (NP (DT the) (NNS holders)) (PP (IN of) (NP (NN college) (NNS degrees))))) (PRN (-LRB- -LRB-) (NP (NP (NP (JJ many)) (PP (IN of) (NP (NP (ADJP (NP (PRP them)) (JJ lower-level)) (NNS administrators)) (, ,) (NP (NNS teachers)) (, ,)))) (CC and) (NP (NP (NNS researchers)) (PP (IN within) (NP (DT the) (NN system))))) (-RRB- -RRB-))) (. .)))
There is no sort of appex of doctors and professors. 
( There ( ( is ( ( no sort ) ( of ( appex ( of ( ( doctors and ) professors ) ) ) ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBZ is) (NP (NP (DT no) (NN sort)) (PP (IN of) (NP (NP (NN appex)) (PP (IN of) (NP (NNS doctors) (CC and) (NNS professors))))))) (. .)))