, 2012). The VENs and fork cells are concentrated within a small region in the agranular anterior insula that is located, as in humans, just anterior to the limen insula and medial to the superior limiting sulcus (Figure 1F). (In order to avoid the impression that we infer a clear anatomical and functional homology with the human AIC, in the macaque we use the term “agranular anterior insula” [AAI] to designate the agranular portion of the insula that lies anterior to the limen. We ascribe Tyrosine Kinase Inhibitor Library the concentration of VENs to a small region within the ventral AAI.) A preliminary examination of the cytoarchitecture of this
region in Nissl preparations does not reveal major interspecies differences and suggests that it constitutes a distinct selleck compound architectonic area. In this area, the small granule cells of layer 4 are absent; layer 2 is thin but darkly stained; layer 3 is distinctly sublaminated; and layer 5 is wide with a clear sublamination into a thin superficial layer 5a and a large layer 5b, which contains larger neurons including VENs and fork cells (Figure 1C). VENs and fork cells are also found in the ACC, particularly in area 24b (Figures S1B and S1C), but these are scarcer and more dispersed than those
in ventral AAI. Isolated VENs are infrequently found in other regions of the prefrontal cortex including areas 14 and 10. The present study focuses on the concentration of VENs in the ventral AAI, consistent with the overall aim of our research on primate insula. Other than frequency and number, there are no obvious differences in morphology, size, or laminar distribution between the insular and cingulate VENs. Details on the VEN distribution in ACC will be reported separately. A stereological estimate of the number of VENs in ventral AAI with the optical fractionator (West
et al., 1991) indicates an average of ∼1,500 and ∼2,000 VENs per hemisphere in M. fascicularis and almost M. mulatta, respectively ( Figures 1G and 1H; see Table S1 for details). This number is, as expected, much lower than the numbers reported previously in humans and the average number in great apes, but it is within the lower range of VEN numbers in some individual great apes ( Allman et al., 2010). Macaque VENs represent 2%–3% of the total number of neurons in layer 5, which fits the trend for progressively higher VEN/pyramidal ratio from human to phylogenetically more distant primate species ( Allman et al., 2010). Finally, a comparison of the number of VENs in the left and right AAI reveals a significantly higher number of VENs (F1,5 = 100.358; p = 0.0002) and a nonsignificant trend for a higher VEN/pyramidal ratio (F1,6 = 4.213; p = 0.0953) in the right AAI, also in accordance with the asymmetry reported in hominids ( Allman et al., 2010).