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.
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.