Vertebral formula variability is coupled with spinal cord anatomy changes in the spiny mice

The spiny mouse is a promising model for central nervous system regeneration. Intraspecific lumbosacral vertebral variation was revealed in spiny mice: two main formulae were described (VL5-VS5 and VL6-VS4). At the same time, no data about the possible link between the vertebrae formulae and the spinal cord organization exist.

The present study compares the spine and spinal cord in spiny mice with two lumbosacral vertebrae formulae using morphometric analysis, axonal tracing, immunohistochemical protocol, and Hu invariant moments analysis.

With the total number of lumbosacral vertebrae remaining unchanged, altering the standard VL5-VS5 formula by transforming the first sacral vertebra into a sixth lumbar vertebra (VL6-VS4) leads to a cascade of alterations in the spinal cord. Despite similar gross anatomy, precise morphometric differences were found. Segment L6 in the VL6-VS4-group exhibited intermediate characteristics similar to segments L5 and S1 of the VL5-VS5-group, while being significantly distinct from the adjacent segments L5 and S1 within its own group. Gray matter contour analysis using Hu invariant moments confirmed a smoother morphological transition in the lumbosacral region of VL6-VS4-animals. The distribution of borders for visceral nuclei and motoneuronal columns, revealed a clear caudal shift in the structures located at the lumbosacral transition in VL6-VS4 animals, coupled with a significant redistribution of cells within motoneuronal pools, demonstrating a functional reorganization of the spinal cord internal structure.

The different numbers of vertebrae in the lumbar and sacral spine regions significantly impact the internal organization of the spinal cord. This should be considered for experimental design, data interpretation, and standardization of spiny mice as a spinal cord research model.