How well do we understand neurotrophic factors and the control of CNS neuronal growth?

The review of Hefti et al. (7) presents current theories and concepts about the functions and activities of neurotrophic factors (NTF). The validity of several components and assumptions may be questionable. Should NTFs be divided into small diffusible survival-promoting factors and large, insoluble neurite-promoting factors? How are NTFs and neurotransmitters related? Do neurons need one or several NTFs? Can NTFs be multifunctional trophic factors? Are there endogenous inhibitory modulators of neuronal growth? Answers to these questions may have significant impact on the design of therapies using NTFs.