Sequential activation of butyrylcholinesterase in rostral half somites and acetylcholinesterase in motoneurones and myotomes preceding growth of motor axons.

By applying double-staining procedures that combine cholinesterase histochemistry (acetyl- and butyrylcholinesterase, respectively) as indicators of neuronal and myotomal tissue differentiation on longitudinal sections, together with detection of motor axons with antibodies to G4 antigen, we here describe the spatiotemporal expression of all components of the segmental motor units along the trunk of chicken embryos between stages 16-20. In particular, BChE expression is spatially elevated on the rostral part of the differentiating somite. About 2-3 somites more rostrally (and thus developmentally later), AChE is expressed almost simultaneously in a nonsegmented fashion in neuronal cell bodies of the ventral horn and in the corresponding dermomyotomes. There it is first detectable in a rostromedial sector. With a delay (4-6 somites compared with AChE in motoneurones), motor axons begin to grow exclusively through the BChE-rich sclerotomal space towards the AChE-activated myotome anlage. On motor axons, AChE detection is significantly retarded. We conclude that the rostrocaudal segmental asymmetry is not restricted to the sclerotomes (which other authors have described before by using different markers), but it extends into the dermomyotome, in which cholinesterases introduce an early subdivision. Hence, the entire process of first myotome differentiation, motor axon growth and establishment of first target contacts are taking place within the rostral half somite. We suggest that both cholinesterases might be involved in processes of motor unit differentiation and fibre guidance.