Redefining the basilar pons: An ultra-high resolution MRI/DTI and tractography study of the postmortem human brainstem
Creators: name: Skliros, Christodoulos
name: Ashwell, Ken
name: G. Allan Johnson
name: Calabrese, Evan
name: Aggarwal, Manisha
name: Paxinos, George
Publisher: name: figshare
Year: 2019
Subjects: Neuroscience

description: The trajectories of the cortico-pontine, -bulbar, and -spinal tracts in the pons have traditionally been considered a melange. It is presented herein that there is a topography and discrete organisation of these descending fibre bundles. Through ultra-high field magnetic resonance imaging insights can be gained into the anatomy of the brainstem including its pathways, previously impossible using traditional techniques such as, histology, viral tracing, electrophysiological methods or brain ablation. The present study maps the topography and architecture of the crus cerebri and basilar pons in which the descending fibres course and shows that the anatomy of the basilar pons presents an image of phylogenetically preserved architecture and a well-defined morphological pattern. Thus, revealing the organisation of the crus cerebri and basilar pons into functional and topographic subdivisions. More specifically, in the present study, the crus cerebri has been subdivided into seven curved lamellae that give rise to the descending and transverse fibres of the pons (via relay in the pontine nuclei), with the cortico-pontine, -bulbar, and -spinal tracts into nine consistent fibre bundles within the human (n=4). Within the macaque four consistent fibre bundles are observed (n=1). For both species, all the subdivisions contain fibres that terminate at rostrocaudal levels of the pons, from the ventral to the dorsal part, implying a topographical representation within the descending fibres with the medial part of each fibre bundle as cortico-bulbar and cortico-spinal and the lateral and external parts as corticopontine. These cortico-pontine fibres separate from the rest of the bundles and bend towards the opposite hemipons. The cortico-cerebellar pathway is synaptically interrupted into the pontine nuclei. From there the transverse fibres of the pons are formed, which have also been observed to be discreet and have been subdivided into ten bundles within the human, and five within the macaque. The transverse fibre bundles commence in the rostral pons (rhombomere two) and they fan out completely at the level of the maximum width of the pons where they begin gradually to converge again. Combining directionally colorized fractional anisotropy (FAC) and T2<sup>*</sup>-weighted gradient echo (GRE) with tractography revealed that the pyramidal bundles as they descend they eventually converge, at the level of the pontomedullary junction to form the cervical, thoracic, lumbar and sacral parts of the pyramidal tract. In addition to the descending and transverse fibre bundles, it has been discovered a 20<sup>th</sup> fibre bundle within the pons, hitherto unknown, that runs in a ventro-dorsal orientation in the midline, named herein pontine stria that conveys information from the cortex to the pontine tegmentum. Finally, in this article, the internal capsule has been subdivided in four parts with the fibre bundles placed along its long axis. In addition, it has been identified a bundle that courses through the internal part of the internal capsule. This bundle is located behind the descending fibre bundles and along the long axis of the internal capsule. Given the topography and its course, this bundle may contain projections from the cortex (corticonigral pathway) combined with the efferent projections of the head of the caudate to the substantia nigra. These projection fibres terminate into the substantia nigra pars reticulata.