ABSTRACT
The discovery of nerve growth factor opened a new era of understanding of neurobiology. For example, Cajal (1928) had used histologic techniques to demonstrate the capacity of a severed axon to grow back toward a target, and the discovery of trophic factors explained these observations. Nerve growth factor (NGF) has become the model for studying trophic factor function in the central and peripheral nervous system. Growth factors are now known to be involved in every step of neuronal biology, from development to maturation and phenotype determination. Many trophic factors are target derived and are responsible for maintaining healthy innervation. Loss of target and subsequent loss of growth factor production result in the degeneration of innervation through apoptosis. This effect was elegantly demonstrated in Spoendlin’s studies of ototoxicity in the cat (Spoendlin, 1988).
