Relation of koniocellular layers of dorsal lateral geniculate to inferior pulvinar nuclei in common marmosets

Abstract

Traditionally, the dorsal lateral geniculate nucleus (LGN) and the inferior pulvinar (IPul) nucleus are considered as anatomically and functionally distinct thalamic nuclei. However, in several primate species it has also been established that the koniocellular (K) layers of LGN and parts of the IPul have a shared pattern of immunoreactivity for the calcium-binding protein calbindin. These calbindin-rich cells constitute a thalamic matrix system which is implicated in thalamocortical synchronisation. Further, the K layers and IPul are both involved in visual processing and have similar connections with retina and superior colliculus. Here, we confirmed the continuity between calbindin-rich cells in LGN K layers and the central lateral division of IPul (IPulCL) in marmoset monkeys. By employing a high-throughput neuronal tracing method, we found that both the K layers and IPulCL form comparable patterns of connections with striate and extrastriate cortices; these connections are largely different to those of the parvocellular and magnocellular laminae of LGN. Retrograde tracer-labelled cells and anterograde tracer-labelled axon terminals merged seamlessly from IPulCL into LGN K layers. These results support continuity between LGN K layers and IPulCL, providing an anatomical basis for functional congruity of this region of the dorsal thalamic matrix and calling into question the traditional segregation between LGN and the inferior pulvinar nucleus.

Keywords: lateral geniculate; pulvinar; thalamic matrix; visual pathway.

Figure 1.

A-B. Photomicrographs of coronal sections through LGN and pulvinar in the left hemisphere. Case MY154. Soma stained for Nissl substance using NeuroTrace blue (NTb) are shown in green. Soma stained for calbindin (CaBP) are shown in magenta. C-D. Drawings outlining the LGN laminae and IPul sub-regions (central lateral inferior pulvinar – IPulCL; central medial inferior pulvinar – IPulCM; middle inferior pulvinar – IPulM). The brachium of the superior colliculus (bsc) is also indicated. The blue dashed line indicates the traditional border between LGN and IPul. Neurons with CaBP labelling are shown as magenta dots. E. Photomicrograph of a Nissl stained coronal section through LGN and pulvinar in a different preparation (case MY156), with LGN and IPul sub-regions indicated. Rectangles outline regions shown in higher magnification in F and G. Scale bar = 200 μm. F. High-power magnification of the upper rectangle in E, showing labelling for CaBP. Neurons labelled for CaBP indicated with white arrows. The border between IPulCM and IPulM is shown in white. Note absence of CaBP labelling in IPulM. G. As in F, a high-power view of the lower rectangle in E indicating CaBP labelling in IPulCL. Scale bar = 100 μm.

Figure 2.

Surface reconstructions of all injection site locations, transposed onto a marmoset brain atlas (Woodward et al., 2018) in lateral (top row) and dorsal view (bottom row). A. Locations of 9 anterograde tracer injections in V1, V2 and DM. Top panel shows a magnified lateral view from right hemisphere (left inset); and bottom panel magnified dorsal view (left bottom inset). The center of each injection is shown as a dot. Animal ID and tracer information for each case are shown in the lateral view. Dot diameter is proportional to the extent of the injection, determined via 3D reconstruction of the fluorescent sections. Dotted lines indicate the bounds of tracer spread. B. Locations of 7 retrograde tracer injections in V1, V2 and DM. Injection site information is represented as in A. Scale bar = 5 mm.

Figure 3.

Maximum intensity projection of fluorescent signals onto right hemisphere shown in lateral view from a whole-brain reconstruction (left column), and series of coronal sections through the LGN and IPul (right 4 columns) of case M820 (A), M919 (F) and M1144 (K) showing tracer injections in V1, V2 and DM. A. Lateral view from right hemisphere of case M820. Green label shows AAV-GFP injection in V1. Blue label shows FB injection in V1. Scale bar: 5 mm. B-E Sequential coronal sections through LGN and IPul from posterior (B) to anterior (E). Contours of regions are shown in light grey. Blue arrow indicates examples of FB-labelled cells. Green arrow indicates a region containing AAV-GFP-labelled axon terminals. F. Lateral view of right hemisphere of case M919. Red label shows the AAV-tdTom injection in V2; green label shows the AAV-GFP injection location in DM; blue label shows FB injection location in V2. Note that the injection of AAV-GFP leaked into white matter (Table 2). G-J coronal sections from the animal in F, labelled as in B-E. K. Lateral view case M1144. Red label shows the AAV-tdTom injection in V2; green label shows the AAV-GFP injection in V1; blue label shows the FB injection in DM. L-O coronal sections from this animal, labelled as in B-E. Note that one section was lost between L and M. Scale bar: 1 mm. Abbreviations: K1, K3: LGN koniocellular layers 1 and 3.

Figure 4.

A-E. Drawings of fluorescent coronal sections through LGN and IPul from posterior (A) to anterior (E) for the case of M822 with FB injection in V2. Detected FB-labelled cells are represented as dots. LGN lamina and IPul subdivisions taken from the adjacent Nissl sections and established IPul segmentation (as in Figure 1). Abbreviations as Figure 1. AP and d_ip (distance from most anterior section containing IPul) values are shown beneath each section. F. Frequency histogram of retrogradely labelled cell soma radii as given by automatic cell detection. G. Frequency histogram of estimated cell soma radii after profile-to-cell correction. H. Diameter of CaBP positive cells as measured in regions of interest in either K1, IPulCL or IPulCM I-K. Bar graphs showing the corrected cell counts for retrograde tracer-labelled neurons in IPulCL, K layers, M layers and P layers of LGN after injections in V1 (I), V2 (J), and DM (K). NB: cell counts for the M and P layers in I and K layers in J are <10. Bar heights represent average number of cells across different cases; error bars show the standard deviation.

Figure 5.

A-C. Grids representing each experiment where retrograde tracer was injected into either V1 (A), V2 (B) or DM (C). The number of retrogradely labelled cells counted in each coronal section (shown in green) are organised based on their location in IPul or K layers (grid columns) and the d_ip value of the section (grid rows). Empty white grids indicate a lack of labelling. Grey grids indicate where data was unavailable, either due to missing sections or absence of IPul in the section. D-E. Grids representing each experiment where anterograde tracer was injected into either V1 (D), V2 (E) or DM (F). Blue squares indicate where axon terminals where observed in each coronal section. Grids are organised as in A-C.