Architecture and connectivity of the human angular gyrus and of its homolog region in the macaque brain

Abstract

The angular gyrus roughly corresponds to Brodmann's area 39, which is a multimodal association brain region located in the posterior apex of the human inferior parietal lobe, at its interface with the temporal and occipital lobes. It encompasses two cyto- and receptor architectonically distinct areas: caudal PGp and rostral PGa. The macaque brain does not present an angular gyrus in the strict sense, and the establishment of homologies was further hindered by the fact that Brodmann defined a single cytoarchitectonic area covering the entire guenon inferior parietal lobule in the monkey brain, i.e. area 7. Latter architectonic studies revealed the existence of 6 architectonically distinct areas within macaque area 7, further connectivity and functional imaging studies supported the hypothesis that the most posterior of these macaque areas, namely Opt and PG, may constitute the homologs of human areas PGp and PGa, respectively. The present review provides an overview of the cyto-, myelo and receptor architecture of human areas PGp and PGa, as well as of their counterparts in the macaque brain, and summarizes current knowledge on the connectivity of these brain areas. Finally, the present study elaborates on the rationale behind the definition of these homologies and their importance in translational studies.

Keywords: Angular gyrus; Connectivity; Cytoarchitecture; Homologies; Myeloarchitecture; Parcellation; Posterior inferior parietal lobe; Receptorarchitecture.

Fig. 1

3D reconstruction of the right hemisphere (in lateral view) of human and macaque brains showing the location and extent of the areas that occupy the human angular gyrus and their macaque counterparts. Abbreviations: pcs post-central sulcus, as angular sulcus, ips intraparietal sulcus, lf lateral fissure, sts superior temporal sulcus, its inferior temporal sulcus, aos anterior occipital sulcus, los lateral occipital sulcus, tos transverse occipital sulcus, cs central sulcus, pos parietal-occipital sulcus

Fig. 2

High-resolution photomicrographs of representative cytoarchitectonic fields through human areas PGp and PGa, and macaque areas Opt and PG. Scale bar, 300 μm. Roman numerals indicate cytoarchitectonic layers

Fig. 3

High-resolution photomicrographs of representative myeloarchitectonic fields through human areas PGp and PGa, and macaque areas Opt and PG. Scale bar, 300 μm. Arabic numerals indicate myeloarchitectonic layers

Fig. 4

Laminar distribution of receptors for glutamate (AMPA, kainate, NMDA), GABA (GABA_A, GABA_B, GABA_A/BZ), acetylcholine (M₁, M₂, M₃), norepinephrine $({\alpha }_1,{\alpha }_2)$, serotonin (5-HT_1A, 5-HT₂), and dopamine (D₁) in human areas PGp and PGa, and macaque areas Opt and PG. Color coding indicates receptor densities in fmol/mg protein. Blue tones indicate low densities and red tones high densities. For numeric information concerning receptor densities, see Table 2