title: The craniosacral progression of muscle development influences the emergence of neuromuscular junction alterations in a severe murine model for spinal muscular atrophy.
abstract: OBJECTIVE
  As 4-day-old mice of the severe spinal muscular atrophy (SMA) model (dying at 5-8 days) display pronounced neuromuscular changes in the diaphragm but not the soleus muscle, we wanted to gain more insight into the relationship between muscle development and the emergence of pathological changes and additionally to analyse intercostal muscles which are affected in human SMA.
  
  
  METHODS
  Structures of muscle fibres and neuromuscular junctions (NMJs) of the diaphragm, intercostal and calf muscles of prenatal (E21) and postnatal (P0 and P4) healthy and SMA mice were analysed by light and transmission electron microscopy. NMJ innervation was studied by whole mount immunofluorescence in diaphragms of P4 mice.
  
  
  RESULTS
  During this period, the investigated muscles still show a significant neck-to-tail developmental gradient. The diaphragm and calf muscles are most and least advanced, respectively, with respect to muscle fibre fusion and differentiation. The number and depth of subsynaptic folds increases, and perisynaptic Schwann cells (PSCs) acquire a basal lamina on their outer surface. Subsynaptic folds are connected to an extensive network of tubules and beaded caveolae, reminiscent of the T system in adult muscle. Interestingly, intercostal muscles from P4 SMA mice show weaker pathological involvement (that is, vacuolization of PSCs and perineurial cells) than those previously described by us for the diaphragm, whereas calf muscles show no pathological changes.
  
  
  CONCLUSIONS
  SMA-related alterations appear to occur only when the muscles have reached a certain developmental maturity. Moreover, glial cells, in particular PSCs, play an important role in SMA pathogenesis.
authors.affiliation: Institute of Anatomy, University of Bern, Bern, Switzerland.
authors.forename: Tilman
authors.initials: T
authors.lastname: Voigt
authors.forename: Anuja
authors.initials: A
authors.lastname: Neve
authors.forename: Daniel
authors.initials: D
authors.lastname: Schümperli
mesh_terms: D000818:Animals; D003964:Diaphragm / Q000648:ultrastructure; D004195:Disease Models, Animal; D018450:Disease Progression; D005260:Female; D007366:Intercostal Muscles / Q000648:ultrastructure; D051379:Mice; D024510:Muscle Development; D018482:Muscle, Skeletal / Q000648:ultrastructure*; D009134:Muscular Atrophy, Spinal / Q000473:pathology*; D009469:Neuromuscular Junction / Q000648:ultrastructure*
keywords: development; neuromuscular junctions; perineurial cells; perisynaptic Schwann cells; skeletal muscle; spinal muscular atrophy