Figure 3. A high level of ectopic MuSK promotes motor axon outgrowth.
In wild-type mice (A, B), motor axons are fasciculated near the middle of the muscle (arrow), whereas the axons of sensory and/or autonomic neurons (arrowheads) are found at the periphery of the muscle. In wild-type mice, motor axons branch and terminate adjacent to the main intramuscular nerve (A), and AChRs are clustered selectively at synaptic sites (B). In mice carrying the MuSK-H transgene (C, D), the main intramuscular nerve is positioned correctly, but motor axons fail to stop and instead grow throughout the muscle (C). In contrast, in mice carrying the MuSK-L transgene (E, F), the pattern of motor axon outgrowth is indistinguishable from wild-type mice (E). (G) Quantitation of motor axon growth (see Experimental Procedures). Motor axons in wild-type and MuSK-L mice extend over 5–10% of the muscle, whereas motor axons in MuSK-H mice grow over 90% of the muscle (m ± S.E.M., n = 4 mice for each genotype). The expanded zone of axon growth found in P0 MuSK-H mice does not broaden further during postnatal development (Figure S1). Scale bar = 200 μm.