Normal and abnormal development of an identified leech motor neuron

In embryonic and mature leeches, the identified L motor neuron, which innervates the longitudinal muscles of the contralateral half body segment, can be identified by the location and relatively large size of its cell body. Here the morphological and physiological development of the L motor neuron has been investigated by intracellular recording and dye-filling techniques in normal and abnormal embryonic leeches. Normally the L motor neuron growth cone projects from the cell body at about the same time as from many other neurons located in the lateral part of the ganglion, including the P mechanosensory neurons. The L motor axon, like many other leech axons, projects directly into the appropriate pathway. The L motor neuron does not initially extend an excessive number of axons followed by elimination of the inappropriate ones. Its growth cone is tapered and relatively free of filopodia and grows out of the ganglion in the contralateral posterior nerve behind the growth cone of the primary peripheral axon of the dorsal P mechanosensory cell, which is one of the earliest axons in the posterior root. Occasionally the bilateral halves of the germinal plate fail to fuse resulting in an embryo with separated but intact half ganglia, body wall, and skin. In such embryos the L motor neuron axons cannot grow out the contralateral posterior nerve since it is not available. Instead they grow out a variety of ipsilateral nerves and/or connective tracts. The P mechanosensory cells, which normally grow out of the ganglion in specific ipsilateral nerves, extend their axons along their normal pathways. In these abnormal embryos the L motor neurons did not preferentially grow into the ipsilateral posterior nerve, normally the pathway taken by the bilateral homologue and the nerve most similar to the L motor neuron's normal pathway. The failure of these L neurons to either consistently choose or avoid the ipsilateral posterior root suggests that the bilateral homologues ignore one another's pathfinding cues or that such cues are missing or changed in these embryos. The axons of the P neurons, however, appear to require no cues or interactions with contralateral structures or cells for normal development.