Anterograde labeling of ventrolateral funiculus pathways with spinal enlargement connections in the adult rat spinal cord

Abstract

The ventrolateral funiculus in the spinal cord has been identified as containing important ascending and descending pathways related to locomotion and interlimb coordination. The purpose of this descriptive study was to investigate the patterns of axon termination of long ascending and descending ventrolateral pathways within the cervical and lumbar enlargements of the adult rat spinal cord. To accomplish this, we made discrete unilateral injections of the tracer biotinylated dextran-amine (BDA) into the ventrolateral white matter at T9. Although some BDA-labeled axons with varicosities were found bilaterally at all cervical levels, particularly dense BDA labeling was observed in laminae VIII and IX ipsilaterally at the C6 and C8 levels. In the same animals, dense terminal labeling was found in the lumbar enlargement in medial lamina VII and ventromedial laminae VIII and IX contralaterally. This labeling was most apparent in the more rostral lumbar segments. These observations continue the characterization of inter-enlargement (long propriospinal) pathways, illustrating a substantial and largely reciprocal inter-enlargement network with large numbers of both ascending and descending ventrolateral commissural neurons. These pathways are anatomically well-suited to the task of interlimb coordination and to participate in the remarkable recovery of locomotor function seen in the rat following thoracic spinal cord injuries that spare as little as 20% of the total white matter cross sectional area.

Figure 1

(A) Shown is an example BDA-labeled axons with varicosities (arrowhead), from ipsilateral lamina VII, at C6, which were magnified and traced as well as axons without varicosities (arrow) which were not traced in an animal that received a 0.18µl injection of BDA into the VLF at T9. Shown in (B) is an example of BDA-labeled axons with varicosities in ipsilateral laminae VIII and IX in close proximity to motoneurons in an animal that received a 0.33µl injection of BDA into the VLF.

Figure 2

Shown are sections taken from the T9 VLF injection site (upper left, asterisk; 0.33µl; BDA 4) and corresponding camera lucida drawings of BDA-labeled axons in the white matter at C8 and BDA-labeled axons with varicosities from five superimposed, 30µm sections, 150µm apart, taken from each of the cervical segmental levels examined (C2,C4,C6,C8).

Figure 3

Shown are sections taken from the T9 VLF injection site (upper left, asterisk; 0.18µl; BDA 15) and corresponding camera lucida drawings of BDA-labeled axons in the white matter at C8 and BDA-labeled axons with varicosities from five superimposed, 30µm sections, 150µm apart, taken from each of the cervical segmental levels examined (C2,C4,C6,C8).

Figure 4

A. Camera lucida drawings of BDA-labeled axons with varicosities from five sections (150µm apart) superimposed from each of the lumbar segmental levels (L1–L6) from the same preparation (0.18µl; BDA 15) shown in Fig. 3. Shown on the left are camera lucida drawings (five sections superimposed) of BDA-labeled fibers in the ventrolateral white matter at the L2 level. B. Cameral lucida drawings of BDA-labeled axons with varicosities from five sections (150µm apart) superimposed from L2 and L4 from the same preparation (0.33µl; BDA 4) show in Fig. 2. Shown on the left are camera lucida drawings (five sections superimposed) of BDA-labeled fibers in the ventrolateral white matter at the L2 level.

Figure 5

Shown are summary diagrams illustrating the primary findings of this work based on the preparations that received 0.18µl injections of BDA into the VLF at T9. The shaded areas represent BDA filled axons with terminals, and the segments shown are those for which the pattern is most representative. Shown in A are the patterns of descending axons or collaterals. Shown in B are the patterns of ascending axons or collaterals. The cell body locations are taken from our previously published work using retrograde tracing (Reed et al., 2006). The question marks (?) indicate non-enlargement sources which would include reticulospinal, long-propriospinal and others.