Figure 4. Selective loss of proximal motor neurons.

(A) Representative ChAT reacted coronal sections of the cervical spinal cord illustrating 3 different motor neuron (MN) pools (hatched lines) in WT at P13: phrenic MNs at C3 level (a1), lateral MNs at C5-C6 innervating forelimb proximal muscles (a2), and lateral MNs at C8 innervating forelimb distal muscles (a3).

(B) Quantitative analysis of mean areas of the 3 MN pools in SMA vs WT mice revealed a significant soma size reduction only in the MN pool innervating proximal forelimb muscles (left, P4: 386.3 ± 22.5 µm² in SMA vs 526.5 ±15 µm² in WT, ^**p < 0.01, t-test; right, P13: 378 ± 13 µm² in SMA vs 599.7 ± 20.7 µm² in WT, ^***p < 0.001). Mean area were unchanged in phrenic MNs (left) and not significantly reduced in MNs innervating distal muscles (right). (C) The analysis of soma size distribution of MNs innervating proximal muscles revealed a significant shift to lower size in SMA vs WT mice at P4 (left) and even more at P13 (right) (^***p<0.001, chi-square test between WT and SMA curves). Values are expressed as mean ± SEM. Scale bar: 100 µm.