Molecular determinants of the face map development in the trigeminal brainstem.

The perception of external sensory information by the brain requires highly ordered synaptic connectivity between peripheral sensory neurons and their targets in the central nervous system. Since the discovery of the whisker-related barrel patterns in the mouse cortex, the trigeminal system has become a favorite model for study of how its connectivity and somatotopic maps are established during development. The trigeminal brainstem nuclei are the first CNS regions where whisker-specific neural patterns are set up by the trigeminal afferents that innervate the whiskers. In particular, barrelette patterns in the principal sensory nucleus of the trigeminal nerve provide the template for similar patterns in the face representation areas of the thalamus and subsequently in the primary somatosensory cortex. Here, we describe and review studies of neurotrophins, multiple axon guidance molecules, transcription factors, and glutamate receptors during early development of trigeminal connections between the whiskers and the brainstem that lead to emergence of patterned face maps. Studies from our laboratories and others' showed that developing trigeminal ganglion cells and their axons depend on a variety of molecular signals that cooperatively direct them to proper peripheral and central targets and sculpt their synaptic terminal fields into patterns that replicate the organization of the whiskers on the muzzle. Similar mechanisms may also be used by trigeminothalamic and thalamocortical projections in establishing patterned neural modules upstream from the trigeminal brainstem.