The trigeminal system: an advantageous experimental model for studying neuronal development.

The trigeminal system is a well-characterized sensory system that has been studied extensively in mammals and birds. The clear definition and easy accessibility of the components of this system throughout development have facilitated investigation of several fundamental aspects of neural development. This has led to important advances in our understanding of the mechanism of axonal guidance, the physiology of neurotrophic factors and the establishment and refinement of neural connections. The first convincing evidence for axonal guidance by chemotropism has been obtained. The site and timing of nerve growth factor (NGF) synthesis and NGF receptor expression have been elucidated, thereby clarifying the role of this neurotrophic factor in development. A novel concept in neurotrophic factor physiology has emerged: the survival of neurones that innervate two separate target fields is regulated by two different neurotrophic factors derived from the respective target fields. The development of somatotopic maps of the periphery in the central nervous system (CNS) is dependent on spatial information provided by the periphery. The transfer of this information from the periphery to the CNS is not simply achieved by the ordered growth and arrangement of the intervening sensory nerve fibres.