Worm Breeder's Gazette 14(5): 37 (February 1, 1997)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
|1||Johns Hopkins Univ., Baltimore 21218|
|2||Albert Einstein College of Medicine, Bronx 10461|
We have begun reconstructing serially-sectioned embryos in order to understand how the nerve ring is formed, in particular how the orderly relationship between the GLRs, muscle arms, and axons is established. We have focused on two embryos: one at a stage before axons enter the nerve ring proper (350') and the other when the nerve ring contains many processes (430'). In both embryos, we have identified all of the non-neuronal cells outside the pharynx. And in the 430 minute embryo, we have identified all neurons except those in the anterior ganglion, primarily using Fig. 8 of Sulston et al., 1983. This later stage is useful as a reference for cellular relations in the relatively mature nerve ring and as an aid in identifying cells in the younger embryo. In the 350' embryo, the first axons are establishing the amphid commissures and the sublateral pathway. We are beginning to understand how head and neck muscles connect to the nerve ring. In mature worms, it appears as if muscles must extend long processes from their somas, which are next to the hypodermis, to the inner nerve ring (see White et. al., 1986). However, early in development, head and neck muscles directly surround the pharynx. During development then, muscle cell bodies themselves must migrate peripherally to the hypodermis leaving behind a process (arm) next to the pharynx. In addition, the processes left behind from the neck muscles must grow anteriorly to reach the nerve ring, since the neck muscles are posterior to the GLRs in these young embryos. Therefore, the formation of muscle arm connections to the nerve ring may be quite different from the extension of muscle arms from body wall muscles to the ventral and dorsal nerve cords. The relation between muscles and the developing amphid commissural pathway is also interesting (see diagram below). In the 350' embryo, muscles do not separate lateral ganglia from the ventral ganglion where the commissures are forming. Instead muscles are separated from the hypodermis by a large neuron (spanning 90 thin sections) forming a substrate for early sublateral and amphid commissural axons (neuron 122 on LHS and neuron 113 on RHS in section #402). Anterior (e.g. secn #386) and posterior (e.g. secn# 522) to these nascent pathays, muscles are attached to the hypodermis. We do not know whether earlier in development, muscles were in contact with the hypodermis along the anterior-posterior length and the neuron migrated between the tissues or whether the muscles differentially set down upon the hypodermis. Our tentative identification of the neuron that provides the substrate for the early commissures is SMBD (neurons 122 and113 in digram below). We are beginning to process more embryos of intermediate stages to fill in the developmental gaps, and are hoping that by working our way back from the older 430 ' embryo, we can become more certain of cell identifications.