Glia fascicle leads principal neurons from the pallial-subpallial boundary into the developing human insula.
Date
2017Abstract
The human insular lobe, in the depth of the Sylvian fissure, displays three main
cytoarchitectonic divisions defined by the differentiation of granular layers II and IV.
These comprise a rostro-ventral agranular area, an intermediate dysgranular area, and
a dorso-caudal granular area. Immunohistochemistry in human embryos and fetuses
using antibodies against PCNA, Vimentin, Nestin, Tbr1, and Tb2 reveals that the insular
cortex is unique in that it develops far away from the ventricular zone (VZ), with most of
its principal neurons deriving from the subventricular zone (SVZ) of the pallial-subpallial
boundary (PSB). In human embryos (Carnegie stage 16/17), the rostro-ventral insula
is the first cortical region to develop; its Tbr1+ neurons migrate from the PSB along
the lateral cortical stream. From 10 gestational weeks (GW) onward, lateral ventricle,
ganglionic eminences, and PSB grow forming a C-shaped curvature. The SVZ of the
PSB gives rise to a distinct radial glia fiber fascicle (RGF), which courses lateral to the
putamen in the external capsule. In the RGF, four components can be established: PF,
descending from the prefrontal PSB to the anterior insula; FP, descending from the
fronto-parietal PSB toward the intermediate insula; PT, coursing from the PSB near the
parieto-temporal junction to the posterior insula, and T, ascending from the temporal
PSB and merging with components FP and PT. The RGF fans out at different dorsoventral and rostro-caudal levels of the insula, with descending fibers predominating
over ascending ones. The RGF guides migrating principal neurons toward the future
agranular, dysgranular, and granular insular areas, which show an adult-like definition
at 32 GW. Despite the narrow subplate, and the absence of an intermediate zone
except in the caudal insula, most insular subdivisions develop into a 6-layered isocortex,
possibly due to the well developed outer SVZ at the PSB, which is particularly prominent
at the level of the dorso-caudal insula. The small size of the initial PSB sector may,
however, determine the limited surface expansion of the insula, which is in contrast to
the exuberant growth of the opercula deriving from the adjacent frontal-parietal and
temporal VZ/SVZ