Fig. 1. NO induced cGMP-immunoreactivity of MG neurons during midgut plexus development. Guts were incubated with SNP and then immunostained with an anti-cGMP antiserum. (A) Images were drawn from individual preparations. Each panel shows a dorsal view of the embryonic gut: ingluvial ganglion (ig), the midgut is marked in gray; mg indicates midgut neurons. For the sake of clarity, the caeca are not shown. Scale bar: 200 µm. (B) Appearance of NO-induced cGMP-IR in MG neurons. The total number of MG neurons was calculated from anti-acetylated -tubulin staining which labels all MG neurons. The percentages of cGMP-positive MG neurons during different embryonic stages are shown. The mean values and s.e.m. were calculated for each developmental stage (n=5). At 55% E, almost no cGMP-IR was found in the premigratory population of MG neurons. When migration started at about 60% E, all migrating MG neurons showed strong levels of anti-cGMP staining. The MG neurons exhibited cGMP-IR throughout the phase of migration (60-70% E) and continued to show high levels of anti-cGMP staining in the phase of lateral axon branching and the formation of terminal processes on the midgut musculature (70-85% E). When the midgut plexus acquired a mature innervation pattern (90 and 95% E), there was a rapid decrease of cGMP-IR.