Selective loss of proximal motor neurons.

<div><p>(<b>A</b>) 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). </p> <p>(<b>B</b>) Quantitative analysis of mean areas of the 3 MN pools in SMA <i>vs</i> 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 <i>vs</i> 526.5 ±15 µm² in WT, ^**p < 0.01, t-test; right, P13: 378 ± 13 µm² in SMA <i>vs</i> 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). (<b>C</b>) The analysis of soma size distribution of MNs innervating proximal muscles revealed a significant shift to lower size in SMA <i>vs</i> 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.</p>