Developmental pattern of axonal pathways in the house shrew maxillary nerve

Abstract

The topographic patterns of peripheral receptors and effectors seem to contribute to the construction of the neuronal circuit in the central nervous system (CNS) in mammals. Many patterns replicating those of the periphery have been found in the CNS, and fasciculation has been regarded as having a central role in the pattern replication. The house shrew, Suncus murinus, is an excellent species in which to study this topic because it has a vibrissae system arranged in a single ordered fashion and extraordinarily well-developed trigeminal spinal tracts. Using immunostaining and retrograde-tracing techniques, we examined the developmental pattern of the maxillary nervous system in the house shrew. The results indicate that the basic pattern of axonal extension reiterates with a parallel arrangement throughout the course of development except at a site in the brainstem where the central processes bifurcate into ascending and descending branches. Dorsoventral inversion of the peripheral pattern in the spinal tract occurs with this dualleveled bifurcation in association with the mediolaterally ordered entry of the central processes into the brainstem. The basic pattern of the central processes is established prior to the appearance of the vibrissae, indicating that the basic topographic pattern of the maxillary nerve is not related to the vibrissae system. The fasciculation pattern does not correspond to the overall layout of the arrays of vibrissae, and there are frequent exchanges of axons between fascicles both in the periphery and centrally. The parallel organization of the majority of the processes, together with the free exchange of processes between fascicles, suggests that these processes have an important role in the formation of the fasciculation and somatotopic patterns.