Development of order in the rat trigeminal system.

The trigeminal system of the rat is characterized by a high degree of order. The pattern of the distribution of vibrissae follicles on the face is replicated at each synaptic station between face and somatosensory cortex (Belford and Killackey, '80). The present study details the development of the trigeminal nerve, its intrinsic organization, and its relationship with its peripheral and central targets. We have observed that at early embryonic ages (E12 and E13) the trigeminal ganglion neurons grow out in straight lines without crossing, and the distance between these neurons and their peripheral and central targets is very short. We have found that fibers reach the periphery before follicle formation is first detectable (E14). At all ages, the trigeminal fibers show a marked tendency to fasciculate. After the development of the pattern of vibrissae follicles on the face, the pattern of fasciculation within the nerve can be clearly related to the rows of vibrissae and the buccal pad. This peripherally related order in the nerve was experimentally verified by injecting horseradish peroxidase into the follicles of individual rows and selectively sectioning portions of the nerve. Further, we provide evidence that the discrete brainstem pattern reflecting vibrissae distribution develops after organization is detectable in the nerve and in a temporal sequence from lateral to medial, which replicates the developmental sequence of vibrissae follicles from ocular to nasal on the face. This sequence is detectable in both the distribution of afferent terminals as measured with succinic dehydrogenase histochemistry and of horseradish peroxidase back-labeled trigeminothalamic relay cells. We interpret our results as suggesting that a number of factors may play a role in the establishment of specific neuronal topographies in the rodent trigeminal system.