Timecourse of development of the wallaby trigeminal pathway. II. Brainstem to thalamus and the emergence of cellular aggregations.

Abstract

This paper is the second in a series which makes use of the protracted postnatal maturation of the wallaby to study the development of the trigeminal sensory system. Previous work has established similarities in the organisation of the trigeminal sensory system in the wallaby and in rodents. This study describes the structure and development of the ventroposteromedial nucleus in the wallaby in relationship to the arrival of afferents from the trigeminal nuclei, the formation of neuronal aggregations and naturally occurring cell death. Enzyme histochemistry, Nissl and myelin stains were used. Pathway development was followed using carbocyanine dyes. In the adult wallaby the nucleus demonstrates evidence of a parcellated organisation. Cells are arranged in dorsoventrally aligned bands resembling fingers. In the horizontal plane, these appear as circular clusters which are encircled by fine myelinated bundles. The clusters of cells are believed to correspond to the mystacial vibrissae. The first afferents from the principal trigeminal nucleus arrive between 10 and 15 days postnatal. This is more than two weeks prior to the time at which the borders of the nucleus can be discerned cytoarchitecturally. The first hints of segmentation are visible around day 50, and discrete aggregations form over the ensuing 3-4 weeks. Coincident with the aggregation of the neurons is an increase in their level of reactivity for acetylcholinesterase. A high level of acetylcholinesterase reactivity is maintained for at least 4 months, but has disappeared in adult animals. The peak of cell death occurs subsequent to the appearance of aggregations in the thalamus, but coincident with the appearance of vibrissae related patches in the cortex at day 85 (Waite et al. [1991] Dev. Brain Res. 58:35-41). The timing of the appearance of the neuronal aggregations supports the hypothesis that pattern formation occurs sequentially at successive levels of the pathway, and suggests the importance of target maturation in pattern formation.