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Comments on Norris CR et al. (1998) East Coast Worm Meeting "Structure of the Developing Nerve Ring" (0)
Overview
Norris CR, Bazykina IA, Hedgecock EM, & Hall DH (1998). Structure of the Developing Nerve Ring presented in East Coast Worm Meeting. Unpublished information; cite only with author permission.
In order to understand the cellular interactions that are important for the formation of pathways into the nerve ring, we have been examining embryos at the EM level. To date, we have reconstructed two stages of development: one when the amphid commissures are beginning to form but before axons enter the nerve ring proper (350'/start of elongation), and the other when the nerve ring contains axons and the lateral pathways are just forming (430'/1 and 2/3-fold elongation). We have been focusing on the development of 3 pathways from the lateral ganglion to the nerve ring: the amphid commissures, the lateral pathway, and the subdorsal pathway. Amphid commissural axons are the first to extend in C. elegans. Commissural axons cross to the ventral ganglion between muscle basal lamina and neuron SIBD, which transiently insinuates between muscle and hypodermis, forming a bridge between the lateral and ventral ganglion. Presumably, SIBD is an attractive/permissive substrate, and perhaps amphid axons are unable to traverse directly between muscle and hypodermis. (Although later commissural axons, such as motorneurons, grow directly between muscle and hypodermis.) Axons that enter the nerve ring via the lateral and subdorsal routes extend later. Interestingly, the timing of their outgrowth correlates with the separation of muscles into dorsal and ventral quadrants. We hypothesize that muscles may block access to a chemoattractant from the nerve ring. We have observed that axons in all these pathways extend in waves. For example, by the 1 and 2/3-fold stage, 8 amphid axons have reached the nerve ring via the amphid commissures, but the remainder of amphid axons have not yet crossed to the ventral ganglion. Similarly, AVA and ADL, which grow directly to the ring via a lateral route, have reached the ring, when other neurons destined to extend laterally have not initiated growth cones. We are unsure of the significance of this temporally distributed axon outgrowth, but have several hypotheses. We are processing more embryos of intermediate stages to fill in the developmental gaps, and are using laser ablations to test our ideas about cellular interactions important for guiding axons to the ring.
Affiliation:
- Department of Biology, Johns Hopkins University, 3400 N. Charles, Baltimore, MD 21218 Albert Einstein College of Medicine, Bronx, NY 10461