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.