Reorganization of Corticospinal Projections after Prominent Recovery of Finger Dexterity from Partial Spinal Cord Injury in Macaque Monkeys

Injection sites of BDA and distribution of the BDA-labeled axons at different segments from C2 to Th1. A, Left, Example of injection sites in M1; bulk of BDA-reaction product at the injection site in the gray matter of the rostral bank of the central sulcus (CS). Center, A high-magnification view of BDA-labeled pyramidal neurons in Layer V at the injection site in Mo Fu. Scale bars in left and center panel indicate 2 mm and 50 μm, respectively. Right, Schematic distribution of injection tracks of BDA (green area). Detailed location of injection sites is shown in Table 1. Arc, arcuate sulcus; Arsp, arcuate sulcus spur. B, Definition of the laminae of Rexed in the C3 and C8 spinal segments (Rexed, 1952). Scale bars = 1 mm. C, Distribution of BDA-labeled axons in the C2, C3, C4, C7, C8, and Th1 spinal segments originating from the right M1 in the intact monkey [Mo-4 from Yoshino-Saito et al., 2010; C(i)], and from the contralesional (right) M1 in Monkey To [C(ii)] and from the ipsilesional (left) M1 in Monkey Fu [C(iii)]. The black shaded area in the panels of C4 in C(ii) and C(iii) indicates the lesion extent at the border of C4 and C5 in these animals Scale bar = 1 mm.

A, CST axons descending from the contralesional M1 in Monkey To at the C8 segment [compare Fig. 2C(ii)]. Photomicrograph in the left and camera lucida tracing of the BDA-labeled axons (in magenta) in the right panel. D, dorsal; V, ventral; L, left; R, right. Scale bar = 200 μm. B, CST axons descending from the ipsilesional M1 in Monkey Fu at C8 segment [compare Fig. 2C(iii)]. Photomicrograph in the left and camera lucida tracing of the BDA-labeled axons (in green) in the right panel. Scale bar = 200 μm. Note that the CST axons in A are mostly short and penetrate the section perpendicularly, while those in B are traversing the section horizontally for a long distance.

Collateral branching from CST axons originating from the contralesional M1 (coM1) or ipsilesional M1 (ipM1) at the C3 (A–I) and C8 segment (J, K). A, An example trace of CST axons in an axial section showing transverse CST axons entering the gray matter from the ipsilesional dorsolateral funiculus (DLF) at C3 in a monkey with BDA injection into the coM1 [Monkey To, the same as Fig.2C(ii), C3]. R, right; L, left. Scale bar = 1 mm. B, A high magnification view of the square in A. The dashed line indicates the border between the gray matter and white matter (DLF). D, dorsal; L, left. Scale bar = 50 μm. C, Bar graphs indicate the percentage of transverse axons among the labeled axons in ipsilesional DLF at C3. Data were obtained by averaging the number from three axial sections from each monkey. Error bars indicate standard deviation. Intact monkeys (Mo-1, Mo-2, Mo-3, Mo-4); lesioned monkeys (Monkeys To and Di). D, An example trace of longitudinal section of the C3 segment showing transverse axons (black arrow) entering the gray matter from the ipsilesional DLF at C3 in a monkey with BDA injection into the coM1 (Monkey Gr). Descending axons are indicated with black arrowheads. A dashed line indicates the border between gray matter and white matter. R, rostral; L, left. Scale bar = 1 mm. E, High-magnification view of the square in D. The dashed line indicates the border between gray matter and white matter. R, rostral; L, left. Scale bar = 100 μm. F, Percentage of transverse axons among the labeled axons in contralesional DLF at C3 in monkeys with BDA injection into the coM1 (Monkeys Al and Gr). G, An example trace of CST axons in an axial section showing transverse CST axons entering the gray matter from the contralesional DLF at C3 in the monkey with BDA injection into the ipM1 [Monkey Fu, the same as Fig.2C(iii), C3]. Scale bar = 1 mm. H, High magnification of the square in G. D, dorsal; R, right. Scale bar = 100 μm. I, The percentage of transverse axons out of the total number of labeled axons in the contralesional DLF at C3. Data were obtained from the axial sections of Monkeys Fu, As, and De. J, An example trace of CST axons in an axial section showing transverse CST axons entering the gray matter from the contralesional DLF at C8 in a monkey with BDA injection into the ipsilesional M1 [Monkey Fu, the same as Fig.2C(iii), C8]. Scale bar = 1 mm. K, Percentage of transverse axons among the labeled axons in contralesional DLF at C8 in monkeys with BDA injection into the ipM1 (Monkeys De, As, and Fu).

Longitudinal sections of the cervical spinal cord showing CST axons from the contralesional M1 (coM1) that descended in the ipsilesional gray matter. A, Upper panel, The slice obtained at 6.75 mm dorsal from the ventral end of the spinal cord sections (dashed line). Lower panel, A photomicrograph of a longitudinal section spanning C3–C6 segments showing the BDA-labeled CST axons. Horizontal arrows at C4–C5 indicate the levels of rostral and caudal borders of the C4/C5 lesion. The black square corresponds to the area of B. The dashed line indicates the rostral and caudal ends of C. The black line indicates the direction of the central canal, which corresponds to the R-C axis in C. Scale bar = 500 μm. B, A high magnification view of the square in A. Arrowheads show a BDA-labeled CST axon that ran horizontally and entered the gray matter from the DLF. Scale bar = 100 μm. C, Traced axons in the gray matter at the C5 segment of a lesioned monkey [(i); Monkey Gr] and intact monkey [(ii); Mo-5]. Scale bars = 500 μm. D, Direction (see the inset) and length of individual axons in C(i) and C(ii) are indicated with the distribution of vectors in the left and right panels, respectively. Scale bar = 50 μm. E, Relationship between length of each axon piece and its direction shown in D in three sections from Monkey Gr (i) and Mo5 (ii; different color assignment), respectively. F, Density map of the axon pieces with different length (horizontal axis) and different direction (vertical axis). Data obtained from E. The values were normalized by the overall number of the labeled axons in each condition. Encased bins indicate the baseline for the normalization in each panel. G, Photomicrograph of the C6 segment. Magnified view of the square is indicated in H, where the axons are terminated on a large, presumptive motoneuron. The black arrowhead indicates a presumptive bouton-like swelling terminating on the motoneuron. R, rostral; C, caudal; L, lateral; M, medial; DLF, dorsolateral funiculus; GM, gray matter; DF, dorsal funiculus. Scale bars in G and H indicate 500 and 20 μm, respectively.

Comparison of the number of CST axons and boutons originating from the contralesional M1 between the intact (white columns) and lesioned (magenta columns) animals. A, Histograms indicate the number of axons in each lamina of the ipsilesional (i) and contralesional C3 (ii), divided by the total number of the axons in the DLF on both sides at C3. Insets in each panel indicate the p-value of t test. B, The number of bouton-like swellings in each lamina of the ipsilesional (i) and contralesional C3 (ii). C, The number of axons in each lamina of the ipsilesional (i) and contralesional C8 (ii). D, The number of bouton-like swellings in each lamina of the ipsilesional (i) and contralesional C8 (ii). *p < 0.05/7. Red letters in the insets indicate statistically significant values. Three sections are obtained from each monkey. The number of intact and lesioned monkey are four and two, respectively.

Comparison of the number of CST axons and boutons originating from the ipsilesional M1 between the intact (white columns) and lesioned (green columns) animals. A, Histograms indicate the number of axons in each lamina of the ipsilesional (i) and contralesional C3 (ii), divided by the total number of the axons in the DLF on both sides at C3. Insets in each panel indicate the p-value of t test. B, The number of bouton-like swellings in each lamina of the ipsilesional (i) and contralesional C3 (ii). C, The number of axons in each lamina of the ipsilesional (i) and contralesional C8 (ii). D, The number of bouton-like swellings in each lamina of the ipsilesional (i) and contralesional C8 (ii). *p < 0.05/7. Red letters in the insets indicate statistically significant values. Three sections are obtained from each monkey.