WM/GM ratio of mammalian spinal cord: insights from African hedgehog (Atelerix albiventris)

Introduction. The mammalian spinal cord consists of myelinated axons forming the white matter (WM) and neurons forming the gray matter (GM). The ratio of WM to GM areas (WM/GM) is a well-known macroanatomical characteristic of the spinal cord. This ratio is lower within cervical and lumbar enlargements and higher between them (in the thoracic region). It also varies significantly across species: for example, in the upper lumbar segments, it is lower in rats but higher in horses. We hypothesized that this species-dependent variation might be (at least partially) explained by the degree of spinal cord ascension.

Methods. We assessed the level of SC ascension and WM/GM ratio of the spinal cord segments in the African hedgehog. Based on the literature data from other mammals (goat, European hedgehog, horse, human, mouse, opossum, pig, rabbit, rat, rhesus macaque, and mole), we performed phylogenetic correlations between the WM/GM ratio of the L1–L5 lumbar segments and the level of ascension, defined as the number of the vertebra where the 29th spinal cord segment was located.

Results. We found that in African hedgehog, similarly to European hedgehog, the spinal cord is highly ascended, and the WM/GM ratio in all lumbar segments is less than those in other mammalian species. The phylogenetic correlation was significant for all lumbar segments studied (L1 and L5: p < 0.05; L2–L4: p < 0.01).

Conclusion. The WM/GM ratio of the lumbar spinal cord segments is significantly correlated with the degree of spinal cord ascension: the higher the ascension, the lower the WM/GM ratio. This relationship may be explained by the fact that a longer spinal cord requires a relatively larger volume of conducting pathways to exchange the same amount of information with the brain.