Dendritic retraction, but not atrophy, is consistent in amyotrophic lateral sclerosis-comparison between Onuf's neurons and other sacral motor neurons.

Background: Fundamental cytological changes of amyotrophic lateral sclerosis (ALS) were looked for by comparing relatively preserved Onuf's nucleus (ON) and severely affected neighboring motor neuron groups (dorsolateral alpha motoneurons (DL) and other anterior horn neurons (OAH)). The second sacral segments from 11 ALS patients and 5 controls were initially quadruple-labeled for phosphorylated and non-phosphorylated TAR DNA-binding protein of 43 kDa (TDP43), and p62 with DAPI to identify TDP43-related changes. After digital recording of these fluorescence data encompassing the entire specimen at a high resolution, the same sections were stained with Klüver-Barrera method to obtain their exact bright-field counterparts. This novel approach facilitated exact identification of ON. Furthermore, this cell to cell comparison enabled to correlate quantitative indices of the neuronal cell bodies: perimeter, area and circularity index (CI) i.e. the ratio of (perimeter/2π) divided by the square root of (area/π), which decreases with dendritic retraction, overall number of neurons and inclusions.

Results: In addition to known preservation of ON neuron number relative to DL and OAH, size reduction of ON neurons was not significant even in the advanced stage. Significant size reduction in DL was counteracted in the presence of TDP43-positive inclusions. Early increase of neuronal size in OAH was further enhanced by the presence of TDP43-positive inclusions. Even with these heterogeneous cytopathological changes, a decrease in CI was consistent in all groups at an early phase and was correlated with neuronal loss.

Conclusions: Among variable cytological changes of ALS, a decrease in CI is a consistent early feature shared between non-atrophic ON neurons and other anterior horn neurons with either decreased (DL) or even increased (OAH) size and profounder neuronal loss. This decrease in CI, representative of dendritic retraction, is fundamental to ALS pathogenesis, not necessarily linked to cell size and pathological inclusions.