In vivo observations of terminal Schwann cells at normal, denervated, and reinnervated mouse neuromuscular junctions

Abstract

We found a low-molecular-mass, fluorescent dye, Calcein blue am ester (CB), that labels terminal Schwann cells at neuromuscular junctions in vivo without damaging them. This dye was used to follow terminal Schwann cells at neuromuscular junctions in the mouse sternomastoid muscle over periods of days to months. Terminal Schwann cell bodies and processes were stable in their spatial distribution over these intervals, with processes that in most junctions were precisely aligned with motor nerve terminal branches. Three days after nerve cut, the extensive processes elaborated by terminal Schwann cells in denervated muscle were labeled by CB. The number and length of CB-labeled terminal Schwann cell processes decreased between 3 days and 1 month after denervation, suggesting that terminal Schwann cell processes are only transiently maintained in the absence of innervation. During reinnervation after nerve crush, however, terminal Schwann cell processes were extended in advance of axon sprouts, and these processes persisted until reinnervation was completed. By viewing the same junctions twice during reinnervation, we directly observed that axon sprouts used existing Schwann cell processes and chains of cell bodies as substrates for outgrowth. Thus, CB can be used to monitor the dynamic behavior of terminal Schwann cells, whose interactions with motor axons and their terminals are important for junction homeostasis and repair.

The role of perisynaptic Schwann cells in development of neuromuscular junctions in the frog (xenopus laevis)

“…Third, PSC processes were always longer than the nerve terminal, and nerve terminal processes were always closely associated with PSC sprouts. This result is similar to the finding after nerve damage, in which case PSC sprouts lead and guide the regeneration of motor axons and nerve terminals, both in mammals (Son and Thompson, 1995a;O'Malley et al, 1999) and frogs (Koirala et al, 2000). The present results raise the possibility that during development, PSCs also pioneer pathways for nerve terminal growth.…”

“…At the adult NMJ, PSC processes sprout profusely beyond the original synaptic site after nerve injury in mammalian muscles (Reynolds and Woolf, 1992;Son and Thompson, 1995a;O'Malley et al, 1999) and frog muscles (Astrow et al, 1998;Koirala et al, 2000). PSC sprouting is also induced by partial denervation or blockade of synaptic activity (Son and Thompson, 1995b).…”

Perisynaptic Schwann Cells at the Neuromuscular Synapse: Adaptable, Multitasking Glial Cells

“…2A,Be -g) (Son and Thompson 1995a,b;Love and Thompson 1999). The dynamic relationship between PSC and regenerating nerve terminals after nerve injury at NMJs has been examined with repeated in vivo observations of the same NMJs labeled with vital dyes (O'Malley et al 1999;Koirala et al 2000), or in transgenic mice that express GFP in Schwann cells and CYP in axons (Kang et al 2003). These in vivo studies further confirm that PSC sprouts guide regenerating nerve terminals following nerve injury.…”

The Pros and Cons of Growth Factors and Cytokines in Peripheral Axon Regeneration

“…Regarding the cellular correlates of muscle polyinnervation, it is well known that following nerve injury, denervated terminal Schwann cells (TSCs) enlarge and sprout processes that reach adjacent innervated motor endplates (O'Malley, Waran, & Balice- Gordon, 1999;Reynolds & Woolf, 1992). Using these bridges, TSCs reach, attract, and direct intramuscular axonal sprouts toward denervated endplates (Love & Thompson, 1999;Reddy, Koirala, Sugiura, Herrera, & Ko, 2003).…”