Parallel organization of contralateral and ipsilateral prefrontal cortical projections in the rhesus monkey

Abstract

Background: The neocortical commissures have a fundamental role in functional integration across the cerebral hemispheres. We investigated whether commissural projections in prefrontal cortices are organized according to the same or different rules as those within the same hemisphere, by quantitatively comparing density, topography, and laminar origin of contralateral and ipsilateral projections, labeled after unilateral injection of retrograde tracers in prefrontal areas.

Results: Commissural projection neurons constituted less than one third of the ipsilateral. Nevertheless, projections from the two hemispheres were strongly correlated in topography and relative density. We investigated to what extent the distribution of contralateral projections depended on: (a) geographic proximity of projection areas to the area homotopic to the injection site; (b) the structural type of the linked areas, based on the number and neuronal density of their layers. Although both measures were good predictors, structural type was a comparatively stronger determinant of the relative distribution and density of projections. Ipsilateral projection neurons were distributed in the superficial (II-III) and deep (V-VI) layers, in proportions that varied across areas. In contrast, contralateral projection neurons were found mostly in the superficial layers, but still showed a gradient in their distribution within cortical layers that correlated significantly with cortical type, but not with geographic proximity to the homotopic area.

Conclusion: The organization of ipsilateral and contralateral prefrontal projections is similar in topography and relative density, differing only by higher overall density and more widespread laminar origin of ipsilateral than contralateral projections. The projections on both sides are highly correlated with the structural architecture of the linked areas, and their remarkable organization is likely established by punctuated development of distinct cortical types. The preponderance of contralateral projections from layer III may be traced to the late development of the callosal system, whose function may be compromised in diseases that have their root late in ontogeny.

Figure 1

Composite of injection sites shown on the medial (A), lateral (B) and orbital (C) surfaces of the right cerebral hemisphere. The injection sites are superimposed on an architectonic map of the prefrontal cortex [38]. Different cortical types are depicted in shades of grey as follows: 1 (darkest grey) agranular areas with three distinguishable layers; 2, dysgranular areas with four distinguishable layers, including a poorly developed layer IV; 3–5, eulaminate areas with increasing cellular density and thickness of layer IV from levels 3 to 5. In A-C, small dashed lines demarcate architectonic areas indicated by numbers; large dashed lines depict the cortex buried in sulci. MPAll, OPAll, OPro, OLF indicate architectonic areas. Letters before architectonic areas designated by letters or numbers denote: C, caudal; L, lateral; M, medial; O, orbital; R, rostral. Other letter combinations refer to cases. Abbreviations: A, arcuate sulcus; Cg, cingulate sulcus; LF, lateral fissure; ST, superior temporal sulcus.

Figure 2

Comparison of contralateral to ipsilateral projections. Cases on the abscissa are identified by the area of the injection of neural tracer. Multiple cases with injection of tracers in the same area are identified by the number in parenthesis after each case. (A) Ratios of contralateral to ipsilateral projection densities. Values were calculated as the total number of contralateral neurons retrogradely labeled by an injection, divided by all labeled neurons ipsilaterally. (B) Ratios of contralateral to ipsilateral projection frequencies. Values were determined as the total number of areas with projection neurons on the contralateral side, divided by all areas with projection neurons in the ipsilateral hemisphere. Projections were included independently of their density and matching origins in the two hemispheres. (C) Strength of correlation between contralateral and ipsilateral projection patterns.

Figure 3

Non-metric multidimensional scaling (NMDS), assessing the pairwise similarities of projection patterns among cases. Cases are identified by the area of tracer injection. Multiple cases with injection of tracer in the same area are identified by the number in parenthesis after each case. Since NMDS configurations are invariant to rotation, the coordinate axes provide a scale of relative similarity, but do not prescribe specific dimensions, and are left unlabeled. (A) ipsilateral cases; (B) contralateral cases. Similarity was defined as the correlation of relative retrograde projection patterns resulting from the injections. The two main projection systems apparent in the diagram consist of predominantly mediodorsal areas (to the left), and basoventral areas (to the right) for both ipsilateral (A) and contralateral (B) projections.

Figure 4

Regional average for ratios of contralateral to ipsilateral projection densities. The apparent trend of progressively smaller contralateral projection densities in medial prefrontal, orbital and lateral prefrontal cortices was significant (rank correlation ρ, p < 0.01).

Figure 5

Bilateral distribution of projections neurons directed to orbitofrontal cortices. Coronal sections through the prefrontal cortex showing labeled neurons in ipsilateral (ipsi) and contralateral (contra) hemispheres after infection of tracers in three cases. Rostral to caudal sections are shown from left to right (case AF), and top to bottom (cases BCb and AM). (Top) The injection of HRP was in area OPro (A, B, black area) and labeled neurons are seen in layers II-III (blue dots) and layers V and VI (red dots); (Left) Injection of fast blue was in area OPro (C), blue area). (Right) Injection of HRP was in the rostral part of area 11 (not shown). Dotted lines through the cortex in the coronal sections show the bottom of layer IV. Architectonic areas indicated by letters include: OPAll, orbital periallocortex; OPro, orbital proisocortex. Other architectonic areas are indicated by numbers. Abbreviations: A, arcuate sulcus; Cg, cingulate sulcus; LF, lateral fissure; LO, lateral orbital sulcus; MO, medial orbital sulcus; P, principal sulcus.

Figure 6

Bilateral distribution of projection neurons directed to medial prefrontal and lateral prefrontal cortices. Coronal sections through the prefrontal cortex showing labeled neurons in ipsilateral (ipsi) and contralateral (contra) hemispheres after infection of tracers in three cases. Labeled neurons were found in layers II-III (blue dots) and in layers V-VI (red dots). In all cases rostral sections are shown on the left, and caudal on the right. (Top) Injection of HRP-WGA (A, black area) was in medial area 32. (Center) Injection of HRP-WGA was in medial area 9 (not shown). (Bottom) Injection of fast blue was in dorsal area 46 (not shown). Dotted lines through the cortex in the coronal sections show the bottom of layer IV. Abbreviations as in Figure 5.

Figure 7

Comparison of border distance and structural type (Δ) as determinants of the pattern of contralateral projections. (A) Frequency of contralateral projections depending on border distance, defined as the number of borders between the contralateral area of projection origin and the area homotopic to the injection site. (B) The relationship of the density of contralateral projections to border distance (Spearman's ρ = -0.87; p (two-tailed) < 0.0002). (C) Frequency of contralateral projections as a function of the absolute structural type difference of the linked areas, delta (|Δ|). (D) The relationship of the density of contralateral projections to the type similarity between the contralateral area of origin and the ipsilateral target area, |Δ| (Spearman's ρ = -0.96; p (two-tailed) < 0.0005). Note that in panels A and C data were pooled over adjacent intervals to avoid artificial variance from sparsely filled categories. In these cases, a distance or Δ of '2', for instance, included data for distance or Δ for 2 proper as well as for 2.5.

Figure 8

Average proportion of laminar origins in superficial layers II-III for ipsilateral and contralateral projections. Data from all cases were included, except for instances where a projection resulted in fewer than 20 neurons. The black and white bars, respectively, add up to 1.0.

Figure 9

Comparison of the relative origin of projection neurons in superficial cortical layers II-III in ipsilateral and contralateral areas. Relative laminar origin of: (A) ipsilateral; (B) contralateral projections. The x-axis shows the proportion of projection neurons found in the upper cortical layers II-III, plotted into intervals of 10% (e.g., in the interval '.8,.9', 80%–90% of projection neurons were located in layers II-III, and the remaining 10%–20% were found in the deep layers, V-VI). Projections originating predominantly from layers V-VI are shown on the left, and projections originating in layers II-III are shown on the right. Projection neurons found in approximately equal proportions in superficial and deep layers are shown in the center. Ipsilateral projections (A) showed a broad Gaussian distribution of laminar origin patterns. By contrast, the distribution of contralateral projection origins (B) was more restricted and skewed towards the superficial layers.

Figure 10

Comparison of the relationship of laminar origin of contralateral projection neurons to structural type and border distance. (A) The x-axis represents the type level difference, Δ), calculated as level (projection origin) – level (projection target, that is, injection site). Normalized origins of contralateral projection neurons varied significantly with Δ (Spearman's ρ, = 0.70; p < 0.04). (B) The x-axis represents the border distance from the injection site on the contralateral side (Spearman's ρ, = -0.04, p > 0.91).

Figure 11

Comparison of the distribution of projection neurons in the supragranular layers II-III in matched areas on the contralateral and ipsilateral hemispheres for individual cases. (A-C) Areal distribution of projection neurons in cases with injection of tracer in orbitofrontal areas (area OPro, case ALy; area 13, case ALb; area 11, case AM); (D) In a case with injection of tracer in medial area 9 (case AO); (E) In a case with injection in dorsal area 46 (case BFb). In all cases the prevalence of projection neurons in layers II-III in most areas is higher on the contralateral side (silhouette bars) than on the ipsilateral side (black bars).