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.

Whisker maps in marsupials: Nerve lesions and critical periods

“…It was then embedded in 4% agar and 100 m horizontal sections were cut though the muzzle and base of the skull using a vibratome. Sections (Leamey et al, 1996); start of afferent clustering ; well-developed afferent clustering ; and patterns with CO (Leamey et al, 1996). For the cortex: afferent arrival (Marotte et al, 1997); start of patterns with CO (Mark et al, 2002); well-developed afferent clustering (Marotte et al, 1997); and patterns with CO (Mark et al, 2002).…”

“…Figure 1 summarizes the development of whisker-related patterns in the wallaby. In the thalamus, afferents first arrive in the ventroposteromedial nucleus (VPM) at P15; patterns begin to develop at P50 and are well developed by P73 (Leamey et al, 1996. In the present experiments, with CO staining in the horizontal plane, the whiskerrelated patches in the thalamus were clearly delineated by P73 (Fig.…”

“…In the present experiments, with CO staining in the horizontal plane, the whiskerrelated patches in the thalamus were clearly delineated by P73 (Fig. 3a), confirming the results of Leamey et al (1996). They were still clearly visible at around P120 ( Fig.…”

“…4a and b), although they are less clear in the adult (Fig. 1 of Leamey et al, 1996). In the cortex, thalamic afferents first arrive at P15 but it is not until P75 that whiskerrelated patterns start to develop (Mark et al, 2002).…”

“…Despite this, the similarities in neural organization and development (Mark and Marotte, 1992), including trigeminal connectivity (Waite et al, 1998), are striking. Like rodents, wallabies have well-developed sinus hairs on the muzzle and whisker-related patterns in the brainstem (Waite et al, 1994), thalamus (Leamey et al, 1996), and cortex (Waite et al, 1991;Marotte et al, 1997). The major difference between wallabies and rats relates to the time that trigeminal pathway development lasts, spanning nearly 3 months in wallabies (Fig.…”