The human insula: Architectonic organization and postmortem MRI registration.

The human insula has been the focus of great attention in the last decade due to substantial progress in neuroimaging methodology and applications. Anatomical support for functional localization and interpretations, however, is still fragmented. The aim of the present study was to re-examine the microanatomical organization of the insula and relate cytoarchitectonic maps to major sulcal/gyral patterns by registration to high-resolution MR images of the same brains. The insula was divided into seven architectonic subdivisions (G, Ig, Id1-3, Ia1-2) that were charted on unfolded maps of the insula following a method used previously in monkeys. The results reveal overall similar patterns of Nissl, and to some extent also, myelin and parvalbumin (PV), as in monkeys, with a postero-dorsal to antero-ventral gradient of hypergranular to granular, dysgranular and agranular fields. Reversals occur ventrally along the inferior peri-insular sulcus (IPS), at the margin with the temporal operculum, and anteriorly at the limit with orbitofrontal cortex (OFC). A large portion of agranular cortex is characterized by a dense accumulation of the spindle-shaped von Economo neurons (VENs) in layer V. The distribution of VENs is not restricted to agranular insula but also extends into the anterior part of dysgranular fields. The patterns of intracortical myelin and of PV neuropil in the middle layers follow decreasing gradients from postero-dorsal granular to antero-ventral agranular insula, with particularly strong staining in posterior and dorsal insula. A separate PV enhanced area in the middle-dorsal insula corresponds in location to the presumed human gustatory area. Projections of the cytoarchitectonic maps onto high-resolution stereotactic MRI reveal a near concentric organization around the limen insula, with each cytoarchitectonic subdivision encompassing several major insular gyri/sulci. The dysgranular domain is the largest, taking up about half of the insula. The present study of the human insula provides a new anatomical basis for MR imaging and clinical applications.

The insula of Reil revisited: multiarchitectonic organization in macaque monkeys.

The insula of Reil represents a large cortical territory buried in the depth of the lateral sulcus and subdivided into 3 major cytoarchitectonic domains: agranular, dysgranular, and granular. The present study aimed at reinvestigating the architectonic organization of the monkey's insula using multiple immunohistochemical stainings (parvalbumin, PV; nonphosphorylated neurofilament protein, with SMI-32; acetylcholinesterase, AChE) in addition to Nissl and myelin. According to changes in density and laminar distributions of the neurochemical markers, several zones were defined and related to 8 cytoarchitectonic subdivisions (Ia1-Ia2/Id1-Id3/Ig1-Ig2/G). Comparison of the different patterns of staining on unfolded maps of the insula revealed: 1) parallel ventral to dorsal gradients of increasing myelin, PV- and AChE-containing fibers in middle layers, and of SMI-32 pyramidal neurons in supragranular layers, with merging of dorsal and ventral high-density bands in posterior insula, 2) definition of an insula "proper" restricted to two-thirds of the "morphological" insula (as bounded by the limiting sulcus) and characterized most notably by lower PV, and 3) the insula proper is bordered along its dorsal, posterodorsal, and posteroventral margin by a strip of cortex extending beyond the limits of the morphological insula and continuous architectonically with frontoparietal and temporal opercular areas related to gustatory, somatosensory, and auditory modalities.