NB 4-1 Details
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NB 4-1 delaminates as an S3 NB.
In grasshoppers and locusts NB 4-1 generates intersegmental and local interneurons that project contralaterally in both anterior and posterior commissures; the interneurons are local, spiking and respond to sensory stimulation on the leg (Shepherd and Laurent, 1992; Leitch et al., 1992; Burrows (1996), for review). See interspecific comparison below.
In Drosophila NB 4-1 delaminates as an S3 NB and expresses seven-up-lacZ (svp-lacZ) and ventral nervous system defective (vnd) as it is forms (Broadus, et al, 1995; White et al, 1983; McDonald et al, 1998). By S5, it has added unplugged (upg), castor (cas), and Klumpfuss (Klu) to its expression profile (Chiang et al, 1995; Cui and Doe, 1992, 1995; Yang et al, 1997). We scored runt expression in S1, S2, and S3 neuroblast stages (but not S4 and S5), and found that NB 4-1 expresses runt (Doe, 1992) as it delaminates at S3. A recent paper reports NB 4-1 to be runt-negative as it delaminates, and throughout the rest of neurogenesis (Dormand and Brand, 1998). Each study used a different antibody, and thus different expression patterns may be due to each antibody recognizing different runt epitopes. Differences may also be due to a greater sensitivity of one antiserum compared to the other, or due to mistakes in scoring neuroblast identities.
A. Motoneurons:
Abdominal clones produce the TN motoneuron; thoracic clones do not have this motoneuron, nor do they have TNs (they are replaced by neurohaemal organs that appear to contain only neurosecretory cells (Gorczyca et al., 1994; Chiang et al., 1994; Taghert et al, 1988). The TN motoneuron is ovoid (6.5 x 4.2 um; n=3) and dorsal in the CNS, similar to the position of the TN motoneuron in Manduca (Carr and Taghert, 1988a,b); it migrates medially and posteriorly between stage 16 and 17 (Fig 4-1,F-H). Its axon is first detectable at stage 16 as it projects to the midline, and by stage 17 it bifurcates in the TN (Fig 4-1F-H, arrows) after traversing the median nerve (Fig 4-1 F,G asterisks). Previous studies identify a motoneuron with similar morphology that forms a synapse on muscle 25 (Gorczyca et al, 1994; Landgraf et al, 1997; Thor and Thomas, 1997); we did not image the ending of the motoneuron in our clone. Bossing et al., (1996) did not observe this motoneuron in the NB 4-1 clone, presumably because they did not assay beyond stage 15. Schmidt et al. (1997) attribute the TN motoneuron to "Clone Y," which they were unable to assign to a NB (we believe "Clone Y" is from NB 5-5 and generates one CCAP+ neurosecretory cell, see below).
B. Interneurons:
The interneurons are relatively large 6.0 um; n=72 cells) and form a tightly packed cluster spanning the dorsoventral (external/internal) extent of the CNS. At least eight axons cross the anterior commissure, with 2 axons turning anteriorly and the rest posteriorly. There are three separate fascicles in the posterior commissure; the most anterior ends abruptly in the contralateral connective, the middle one projects anteriorly a short distance, and the posterior one projects posteriorly a short distance. There are also ipsilateral projections extending anteriorly; a short projection in a lateral fascicle and a longer projection in a medial fascicle.
These interneurons are quite similar to the interneurons derived from NB 4-1 in Schistocerca, except the Drosophila interneurons are not purely local and some form ipsilateral arborizations. The NB 4-1 Lineage in Schistocerca Shepherd and Laurent found the sub-lineages of the 4-1 clone they labeled in Schistocerca to consist entirely of intersegmental and local interneurons. We find this to be true for the majority of the 4-1 clone in Drosophila as well and observed these interneuronal projections to extend both contralaterally and ipsilaterally to the anterior border of the next adjacent anterior segment. Posteriorly, only contralateral projections extend beyond the segmental border. Shepherd and Laurent did not detect the motoneuron of the TN in their lineage studies. They were never able to label NB 4-1 prior to its first division; when they injected NB 4-1 late in its lineage, the resultant labeled progeny were all local interneurons. Injecting at a slightly earlier timepoint generated intersegmental interneurons as well. Goodman et al (1980) observed that most NBs generate motoneurons first, followed by intersegmental interneurons and finally by local interneurons, a birth order consistent with the lineage findings of Shepherd and Laurent. This birth order would account for the missing motoneuron in the Schistocerca lineages.
References:
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