Worm Breeder's Gazette 10(3): 153

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.

A Glimpse at the Early Stages of Development of the Postembryonic Ventral Cord Motor Neurones

J.G. White, E. Southgate and J.N. Thomson

We have been interested for some time in the way in which the late 
developing ventral cord motor neurones wire themselves up, but, until 
recently, we have not been able to see these events take place at the 
EM level because we missed the narrow time window in which they occur. 
A late L1 lethargus reconstruction had a ventral cord that was 
virtually identical to the adult structure, whereas a prelethargus 
animal, which had a pro-metaphase P3aaa, had no significant process 
outgrowths from the nascent motor neurones (all reconstructions were 
done in the anterior cord and spanned VA2 to around DA3).
An animal was fixed at the time of the P12aaa metaphase in early L1 
lethargus and, in this case, all the nascent motor neurones (with the 
exception of VC1) had grown out processes.  All the process endings 
tapered to a point and did not have any structures that resembled 
'classical' growth cones.
The VA/VB sisters sent out two closely apposed processes from their 
cell bodies which split at the NMJ region of the cord, with the VA 
process running anteriorly and the VB process running posteriorly.  
These processes had grown to about 1/3 their estimated mature length.  
No NMJs were present on them, although gap junctions were seen between 
AVB interneurones and the cell bodies of VBs and between AVA 
interneurones and both the cell bodies and processes of VAs.  Both the 
VA and the VB neurones were monopolar, lacking the dendritic process 
that runs in the opposite direction to the process on which the NMJs 
are ultimately sited.
The two AS neurones in the region had sent out commissures from 
their cell bodies and made gap junctions to AVA interneurones (the 
characteristic chemical synapses from AVB were not present).
The two VD neurones in the region had sent out anteriorly directed 
processes which ended in a commissure.  There was no 'T' junction at 
the point where the process left the cord and consequently no process 
making gap junctions to the neighboring VD.  The VD3 neurone had an 
additional process emanating from its cell body which ran anteriorly, 
on top of the process bundle of the ventral cord.  Such a process has 
never been seen on a mature VD neurone, so it seems likely that it may 
have been an 'exuberant' process that had grown along an inappropriate 
tract and would ultimately have been retracted.  There were several 
small NMJs present on the VD processes and interestingly, several 
unambiguous synapses onto the process of the DD2 neurone, a feature 
that is not generally apparent in the mature structure.
The DD2 neurone had considerably fewer NMJs onto ventral muscles 
than has been seen on the same neurone in a mid L1 animal.  Several of 
the NMJs had no vesicles in the region of the pre-synaptic 
specialization.  It therefore seems likely that this neurone was 
loosing its ventral NMJs in the process of re-wiring.
None of the commissures from the postembryonically developing 
neurones had reached the dorsal cord.  The dorsal process of DD was 
post-synaptic to the DA and DB neurones in the region and there was no 
sign of any NMJ along its length.