Initial formation and secondary condensation of nerve pathways in the medicinal leech

Abstract

Invertebrates have proved to be important experimental systems for examining questions related to growth cone navigation and nerve formation, in large part because of their simpler nervous systems. However, such apparent simplicity can be deceiving because the final stereotyped patterns may be the result of multiple developmental mechanisms and not necessarily the sole consequence of the pathway choices of individual growth cones. We have examined the normal sequence of events that are involved in the formation of the major peripheral nerves in leech embryos by employing (1) an antibody directed against acetylated tubulin to label neurons growing out from the central nervous system, (2) the Lan3-2 antibody to label a specific population of peripheral neurons growing into the central nervous system, and (3) intracellular dye filling of single cells. We found that the mature pattern of nerves was characterized by a pair of large nerve roots, each of which branched into two major tracts. The earliest axonal projections did not, however, establish this pattern definitively. Rather, each of the four nerves initially formed as discrete, roughly parallel tracts without bifurcation, with the final branching pattern of the nerve roots being generated by a secondary condensation. In addition, we found that some of the nerves were pioneered in different ways and by different groups of neurons. One of the nerves was established by central neurons growing peripherally, another by peripheral neurons growing centrally. These results suggest that the formation of common nerves and neuronal pathfinding in the leech involves multiple sets of growth cone guidance strategies and morphogenetic mechanisms that belie its apparent simplicity.

Cell interactions that affect axonogenesis in the leechTheromyzon rude

“…The MA axon fiber(s) originated from a peripheral group of neurons corresponding in position to the P-derived pz8/cf3 cell cluster (Braun and Stent, 1989a). Pioneering of the MA nerve from the periphery is consistent with what has been observed in Hirudo, although these fibers are not ACT-positive in Hirudo (Jellies et al, 1996). The pioneering MA fibers in T. rude Fig.…”

“…In addition, cell-dependant neuronal migrations within the CNS are required for normal axon patterning in a range of organisms, including vertebrates (Hatten, 2002). Our results identify several commonalities with nervous system development in the medicinal leech Hirudo medicinalis (Jellies et al, 1996): the initial axon fibers appearing along the ventral midline of each leech pioneer Faivre's nerve; and intraganglionic commissures and bilateral connectives appearing prior to segmental nerves. Hirudo and Theromyzon differ, however, in the number, order of appearance and secondary condensation of their segmental nerves.…”

“…Previous studies demonstrate that ACT antibody preferentially labels axonal processes in leech (Jellies et al, 1996). Because leech embryos generate homonymous segments sequentially from a posterior growth zone (see Fig.…”

“…In contrast to Hirudo (Jellies et al, 1996), there was no condensation of the AA and MA nerves, thus the three segmental nerves remained distinct at the edge of the ganglion; also by this time, more than 10 well-defined commissures were evident across the midline. The longitudinal connectives and Faivre's nerve were considerably larger in diameter than at earlier stages, and ganglia had become encapsulated by connective tissue sheaths.…”

“…Developmentally, these arise as four discrete, roughly parallel tracts, which later condense to form the anterior and posterior nerves (Jellies et al, 1996). In glossiphoniid species, by contrast, three major nerve roots (AA, MA and PP), rather than two, project laterally from each hemiganglion (Kramer and Goldman, 1981) (Fig.…”