The Biology of Nerve Growth Factor in Vivo

A major theme in developmental neurobiology is the role of target tissues in the maintenance of the neurons that innervate those targets. During the development of the nervous system, more neurons are produced and send axons to targets than survive in the mature animal. Typically about 50% of the neurons projecting to a given structure will die during a restricted period. This has been termed naturally occurring cell death. Several lines of evidence indicate that the size of the target field is a major determinant of this cell death: Removal of the target results in greatly increased cell death, while expansion of the target decreases cell death (for reviews, see Refs. 1 and 2).