NB 7-1 details
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NB 7-1 delaminates during S1 as the posterior-most NB of the medial column.
No information about the lineage derived from NB 7-1 is available from other insects.
NB 7-1 expresses gooseberry distal (gsb-d), engrailed (en), ventral nervous system defective (vnd), and achaete (ac) as it forms during S1 (Doe, 1992; Broadus et al, 1995; White et al, 1983; McDonald et al, 1998; Skeath et al, 1995; Skeath et al 1994). It adds seven-up-laZ (svp-lacZ) expression at S2; it loses ac expression by S3, adds Klumpfuss (Klu) expression at S3, and adds castor (cas) expression at S5 (Doe, 1992; Broadus et al, 1995; Skeath et al, 1994; Yang et al, 1997; Cui and Doe, 1992, 1995). The first GMC derived from NB 7-1 is eve+ and gsb+, as are its progeny. Together with four other eve+, gsb+ cells, it contributes to a cluster of six eve+ U-motoneurons (Doe, 1992; Goodman and Doe, 1993; Broadus et al, 1995; Bossing et al, 1996).
Bossing et al. (1996) described the NB 7-1 lineage in Drosophila as consisting of 16-22 neurons, including the even-skipped-positive U motoneurons that innervate dorsal muscles and a group of interneurons. Landgraf et al (1997) back-filled from NMJs and found six anterior-root ISN motoneurons by stage 16 as being derived from NB 7-1. These they observed to innervate muscles 9,10,3,4, by the ISN, and possibly 15,16 and 17 by a branch to SNd.
The NB 7-1 clone is the largest of all embryonic NB clones, generating 40 or more cells in thoracic segments.There is no segmental variation except there are always more cells in thoracic clones.
A. Motoneurons
There are two classes of motoneurons: a group of 4-6 that we always observe, and a pair of motoneurons that are infrequently observed (4/47 clones). The group of 4-6 motoneurons are large cells (6.4 um; n=24), positioned in the middle of the clone. They have complex ipsilateral projections: (1) out the anterior root of the ISN to muscles 2, 3, 4, 9, 10, 19, 20 and possibly 11 (dorsal muscle group); (2) into the SNb to muscles 6, 7 and possibly 13; and (3) into the SNd to muscles 15, 16 and 17 (Fig. 7-1C,). We agree with Bossing et al. (1996) that these are the previously described U motoneurons (Sink and Whitington, 1991a,b; Goodman and Doe, 1993; Van Vactor et al., 1993; Chiba and Keshishian, 1996), and we support the results of Landgraf et al. (1977) that there are six U motoneurons. Remarkably, nearly half of the larval body wall muscles (13 of 30) receive innervation from the U motoneurons (Fig. 7-1C).
Larval muscle 9 is innervated by one of the U motoneurons and gives rise to the adult DLMa and DLMb muscles; in the adult, DLMs are innervated by a motoneuron called MN5. Because the U and MN5 motoneurons have the same CNS position, axonal morphology, and muscle target (Ikeda and Koenig, 1988; Fernandes and Keshishian, 1998; this study), we suggest that an embryonic U motoneuron develops into the adult MN5 motoneuron, which would make it the first motoneuron to be identified throughout the Drosophila lifecycle.The rare pair of motoneurons were only observed in abdominal segments; they lie at the anterior border of the clone and project out the posterior root of the ISN by stage 16. They were never seen fully extended to their target (Fig. 7-1 inset).
Finally, motor innervation to muscles 6, 7, 12 and 13 has been a topic of study for a number of years (see NB 3-1). Jan and Jan, in 1976, recorded from these muscles in the third instar larva and observed that more than one motoneuron supplied innervation to these muscle targets, probably in precisely over-lapping synaptic patterns. Broadie and Bate in 1993 recorded from muscle 6 in the embryo and made the same observation, as did Vivian Budnik in 1996. We believe that this "hidden" motoneuron(s) innervating muscle 6 (and also possibly muscles 7 and 13) is derived from NB 7-1. The only other components of SNb are the AC motoneuron that derives from NB 5-2 and innervates muscle 12 exclusively, and the thoracic contribution to SNb from the CoA derived from NB 1-1, which innervates muscles 12 and 13 exclusively.
B. Interneurons
There are only local interneurons in this clone. The previously identified "friend of pCC" (fpCC) interneuron (Bossing et al., 1996; Broadus et al., 1995) is the most medial cell in the clone. It projects anteriorly in a medial fascicle of the ipsilateral connective, then across the anterior commissure, and ultimately extends anteriorly in the contralateral connective (Fig. 7-1, fpCC inset). Two other interneurons have a similar trajectory, except they take a more lateral fascicle in the ipsilateral connective before crossing the anterior commissure with the fpCC, and then one turns anterior and the other posterior. Finally, there are 2-6 axons that project across the posterior commissure before turning both anterior and posterior. The fpCC may act as a guidepost cell, because we observe interneuronal processes making intimate contact with this cell both dorsal and ventral to it (Fig. 7-1, fpCC inset). All of the contralaterally-projecting interneurons are large (5 um; n=90), but there is also a population of small axonless local interneurons (2.8 um; n=36) that may modulate U motoneuron function, as has been observed in other systems (Pearson and Fourtner, 1975).
C. Glia
A proximal nerve root glial cell is reproducibly observed (Fig. 7-1A), similar to the nerve root glia cell derived from NB 1-1, and either could be a "segment boundary cell" (Jacobs and Goodman, 1989a,b).
References:
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