THE MECHANISM OF SEGMENTATION IN THE LEECH CENTRAL NERVOUS SYSTEM (CNS): MORPHOGENETIC COMPARISONS OF CNS SEGMENTATION AMONG ANNELIDS
Shain, D. H.1; Bargiacchi, F.2 & Weisblat, David A.1
1Department of Molecular and Cell Biology, LSA 385, University of California, Berkeley, CA 94720-3200 USA
2Dept. Biological Sciences, California State University, Hayward, CA 94542-3090 USA.
Morphogenetic processes associated with ganglion formation in the glossiphoniid leech, Theromyzon rude were examined by microinjecting fluorescent lineage tracers into neurogenic stem cells (NL and NR teloblasts) and their progeny (blast cells). Cellular events in the N lineage were then characterized with reference to their clonal age, i.e. the time in hours (at 23šC) after the birth of each blast cell. Each N teloblast divided repeatedly, producing a column of several dozen blast cells that coalesced along the ventral midline (~40 h clonal age). Each column comprises two distinct classes of alternating blast cells, designated nf and ns. Shortly after coalescence, iterated bulges arose within each column, followed by the appearance of a distinct transverse cleft at the lateral edge of each bulge. The left and right clefts elongated rapidly and met at the ventral midline (~50 h), forming a fissure that subdivided the N-derived cells into prospective ganglia. Fissure formation appears to be autonomous within the N lineage since the independent ablation of mesoderm and the remaining ectoderm did not prevent ganglionic separation. By comparison, ablation of the nf or ns primary blast cells (within the N lineage) sometimes caused ganglionic fusions or slips (posterior displacement of ganglia), respectively. Injecting lineage tracer into individual blast cells and their progeny revealed that the fissure invariably formed between clones derived from nf.p cells (anterior to the fissure) and ns.a (posterior to the fissure). Later (~60 h), two laterally directed stripes of cells emerged from the posterior region of each nf clone, forming cellular bridges that transiently connected the ganglion with the ventral body wall. The orientation of the stripes and the time at which they appeared make them candidates for guiding the outgrowth of segmental nerves. In particular, we propose that the anterior stripe of cells, which arises from the nf.a clone and whose members express the leech engrailed-class gene, is required for the formation of the posterior segmental nerve. Finally, nerve cords from leech and other members of the annelid phylum were compared at an ultrastructural level in an effort to identify similarities and differences in their mechanisms of CNS segmentation.