The nerve terminals synapsed to muscle M12 are greatly reduced in size compared to controls in embryos expressing TlMhc.PR. The projection of other motor nerves (such as the ISN and SNa) appears normal.
The growth cone of the RP3 neuron often stalls near its target muscles in embryos expressing TlMhc.PR, failing to retract its filopodia and initiate synaptogenesis. The muscle 6/7 cleft is innervated in only 53% of hemisegments by the end of embryogenesis. Myopodia extending from muscle 6 are initially present at numbers similar to wild type (at 12 hours), but myopodia clustering can only be detected in 26% of hemisegments at 13 hours (compared to 46% of hemisegments in wild type).
The RP3 growth cone stalls underneath the muscle 6/7 cleft in 16 hour TlMhc.PR embryos (which ectopically express Tl in muscles 6 and 7) and synaptogenesis does not occur. In 87% of cases the RP3 growth cone remains stalled underneath the 6/7 cleft during hours 16-18. The cleft itself is wider than in wild type and is crowded with unlabelled axonal growth cones from the SNb/d motor pathway at 16 hours (in contrast to this stage in the wild type, where the 6/7 cleft is mainly occupied by the RP3 growth cone). Most neuromuscular innervation outside of the 6/7 cleft appears normal.
Embryos expressing TlMhc.PR in the muscles show no difference in the number or overall morphology of muscles. Embryos carrying TlMhc.PR show virtually wild-type-like overall development of the nervous system. However, the RP3 neuron often fails to reach the muscle 6/7 cleft. In most cases, the growth cone stalls just beneath and external to the cleft and fails to initiate synaptogenesis.
SNa growth cones develop normally, SNb growth cones look immature on muscle 6, 7 and 12. 6/7 cleft is often missing. RP3 growth cones reaches just below the correct target, cleft between muscle 6 and 7, but fails to innervate normally due to inhibition of synaptogenesis initiation.