NGF synthesis and NGF receptor expression in the embryonic mouse trigeminal system.

1. The development of the mouse trigeminal system is outlined and its advantages for studying the synthesis of low-abundance regulatory proteins are described. 2. The onset of NGF gene expression and NGF synthesis in the cutaneous target field of the trigeminal ganglion coincide with the arrival of the earliest nerve fibres. 3. The distribution and magnitude of NGF synthesis within the target field are related to its final innervation density. 4. NGF receptors are first detected on trigeminal neurons when their peripheral axons reach the target field. 5. Neither NGF synthesis nor NGF receptor expression are dependent on nerve-target contact but appear to occur as part of an intrinsic developmental program in the target field and neurons, respectively. 6. The time-course of NGF synthesis and NGF receptor expression indicate that NGF does not play a role in guiding axons to their target fields in development.