Fig. 7. Timing of MCK expression correlates with cessation of axon growth. In E14.5 and E16.5 Musk^-/-; MCK-Musk and MCK^-/-; MCK-MMT embryos, motor axon growth proceeds well beyond the normal termination zone, similar to Musk mutant embryos. Motor axon growth ceases and nerve terminals differentiate only after Musk, or MMT, is expressed from the MCK regulatory region, between E16.5 and E18.5. Wholemounts of diaphragm muscles from E14.5, E16.5 and E18.5 embryos were stained with antibodies to neurofilament (NF) and synaptophysin (Syn) to label axons and presynaptic nerve terminals, respectively. (B) Quantitation of axon growth shows that motor axons stop in Musk^-/-; MCK-Musk embryos at E16.5, when MCK expression begins. Likewise, in MCK^-/-; MCK-MMT embryos, exuberant motor axon growth reverts at E16.5, but this reversion is partial as motor axon growth at E18.5 is intermediate between that in wild-type and Musk mutant embryos.