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Functional interactions of neurotrophins and neurotrophin receptors.

Hypothesis Programmed neuronal cell death is a prominent feature of vertebrate embryonic development. Timing and extent of cell death are dependent on the influence of the targets those neurons innervate, The number of neurons in the adult nervous system and the density of innervation of their targets are determined by a competitive process in which tissues produce, in limiting quantities, trophic factors that support neuronal survival. Neurons actively compete to establish optimum contact with the target, and those making the least effective contacts with the target are eliminated by cell dea th. If each target of inner­ vation elaborated a unique trophic factor, the resulting mechanism would provide a means of enforcing the correct patterns of connectivity between