NB 1-1 Details
Videos
NB 1-1 delaminates as an S1 NB.
In Grasshopper:
Goodman and colleagues began their studies of the developing grasshopper CNS by looking for cells that could be reproducibly located from ganglion to ganglion and from animal to animal, that were easy to see and that could be identified in relation to known morphological markers, such as neuropilar tracts. The cells they identified in these early studies included aCC, pCC, (see below) the RP motoneurons (see NB 3-1 and 4-2), and the U motoneurons (see NB 7-1).They then proceeded to identify the first neurons to extend axons into developing tracts within the CNS; these included the H cells ( see NB 7-4), the C, Q, A, P neurons (see NBs 7-2, 7-4 and 3-5), the progeny of MP2 and the MNB, among others.
Most of what we know about the cells in the NB 1-1 lineage derives from these studies of Schistocerca CNS development. In grasshoppers, the dividing NB remains dye-coupled to its GMC-daughters and neuronal-granddaughters and the entire lineage is encased in a glial sheath, so that determining the ancestry of an identified neuron is less difficult. Goodman et al (1982) observed that the first GMC from NB 1-1 generates two easily identified neurons, the aCC (anterior Corner Cell) and the pCC (posterior Corner Cell). aCC sits at the junction of the posterior commissure and the longitudinal connective. pCC is immediately adjacent to it, but is slightly ventral and posterior to aCC. aCC is a motoneuron that exits the CNS via the ISN and innervates muscle 1. pCC is an intersegmental interneuron that projects anteriorly (Goodman et al, 1982). These cells have been identified in Manduca and Drosophila as well, and appear identical to the grasshopper arrangement described by Goodman's group (Thomas et al, 1984).
Drosophila:
Subsequent studies of NB 1-1 in Drosophila revealed a complex pattern of molecular marker expression. To date five genes are known to be expressed in this NB: castor (cas) (Cui and Doe, 1992); mirror (mrr) (Broadus et al, 1995; McNeill et al, 1997); Klumpfuss (Klu) (Yang et al, 1997); zinc finger homeodomain protein-1 (zfh-1) (Fortini et al, 1991; Skeath et al, 1998); ventral nervous system defective (vnd) (White et al, 1983; McDonald et al, 1998).
odd-skipped (odd) is transiently expressed in NB 1-1; this expression appears to last only one cell cycle and is no longer detected in the lineage after S3 (Broadus et al, 1995). huckebein (hkb) is expressed at S2; castor (cas) is not expressed in NB 1-1 until S3, but its expression is detected throughout neurogenesis. These genes are therefore expressed as sub-lineage genes in NB 1-1.
We scored runt expression in S1, S2, and S3 neuroblast stages (but not S4 and S5), and found that NB 1-1 expresses runt (Doe, 1992) as it delaminates at S1. A recent paper reports NB 1-1 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.
The first GMC in the 1-1 lineage is vnd+, odd+, and mrr+. There is little or no information available about the expression of these molecular markers in the neuronal progeny of NB 1-1, with the exception of aCC and pCC, which are both derived from the first GMC in the lineage, GMC1-1a. Both aCC and pCC are eve+, while only pCC is vnd+ (Doe et al, 1988; McDonald et al, 1998).
The 1-1 lineage has been well described in Drosophila by DiI lineage analysis in several separate studies. Udolph et al (1993), Broadus et al (1995) and Bossing et al, 1995,1996 showed that the lineage is a mixed one, generating both neurons and glia in abominal segments, making NB 1-1 a neuroglioblast. Bossing et al (1995, 1996) described the lineage as consisting of aCC, pCC, and a cluster of interneurons in every segment; they also described a second motoneuron being derived from thoracic 1-1 clones and the SPGA and B glia being derived from abdominal clones.
A. Motoneurons:
aCC is a large round cell (6.9 microns in diameter before mid-stage 16, (n=4)) that enlarges with age (8.2 microns in diameter by mid stage 17, n=4). It sits at the dorsal surface of the CNS, just posterior to the junction of the posterior commissure and the longitudinal connective. Its sibling cell is the posterior corner cell (pCC), an intersegmental interneuron that is just posterior, medial and ventral to aCC. By stage 17, aCC has extended an axon to the dorsal midline of the embryo, synapsing on muscle 1 (see Fig 1-1, B). We also observed branches to neighboring muscles in the dorsal muscle group, muscles 2 and 9; see Fig 1-1). By late stage 16, aCC has a short contralaterally projecting neurite extending into the posterior commissure.
In thoracic segments, we find a second motoneuron ventral, posterior and lateral to aCC and pCC. We refer to this cell as the Cousin of aCC (CoA). It is typically 4.4 microns long (anterior to posterior) and 7.0 microns wide (medial to lateral) and extends a motoraxon via SNb to muscles 12 and 13 (n=2, see Fig 1-1a). Thus the 1-1 lineage joins the lineages from 3-1, 5-2 and 7-1 in generating SNb branches to muscles 12 and 13. (See NB 5-2 for discussion of muscle 12 innervation).
B. Interneurons:
There is only one intersegmental interneuron in this lineage, the pCC neuron. We find pCC to be a large round cell, typically 6.8 microns in diameter by stage 17 (n=5), and capable of extending farther than 2 segments in the anterior direction (N=2, see movies). The remaining cells of the clone appear to be local interneurons. They fasciculate loosely in the ipsilateral longitudinal connective and extend approximately 40 microns posteriorly by stage 17 (N=5 embryos). Their cell bodies fall into 2 size populations: 50% at all stages are very small (less than 4 microns in diameter) and the remaining 50% are significantly larger, i.e. 5.2 microns in diameter (n=72 cells measured in 12 embryos). We observed that thoracic clones always produced at least two (and once, three) large cell bodies adjacent to aCC and pCC. They were usually the same size as the aCC and pCC in the clone, but more medially positioned. Because they are large cells, we speculate that they are either intersegmental interneurons that extend axons late in the lineage (as observed for the intersegmental interneurons derived from NB 2-2) or will develop into thoracic motoneurons, possibly involved in innervating the leg musculature (see NB 2-3).
C. Glia:
Insect glia have been well described using a number of techniques (Hoyle, 1986; reviewed by Carlson and Saint Marie, 1990), but in Drosophila, the most thorough study of glial cells is that of Ito et al, 1995. The Ito study used a number of glia-specific antibodies, glia-specific enhancer trap lines and computer reconstructions to develop a comprehensive overview of the vnc glia. They determined that glia in the embryonic and first instar larval nerve cord are variable in number and position, but can generally be considered to be of three types (and 6 sub-categories) based on the molecular marker expression patterns and morphologies. They found surface associated glia (including subperineurial cells and channel glia), cortex-associated glia (including cell body glial cells) and neuropile associated glia (including nerve root glia, interface glia and midline glia). We describe glia in a much more limited way here. We describe only sub-perineurial glial cells (such as those derived from NB 1-1 and 2-2), nerve root glial cells (such as those derived from NB 1-1, 2-2, 3-5 and 7-1) nd the lateral subperineurial glial cells derived from NBs 5-6 and 6-4. We found these glia to be reproducibly present in every clone and we found their morphology to be distinctive such that their identification was clear and straightforward.
NB 1-1 generates glia in the abdomen only (Udolph et al, 1993 and this study). We observed two types of glial cells in abdominal 1-1 clones. We observed a large dorsal sub-perineurial glial cell that exactly overlaid the cell bodies of aCC and pCC (see Fig 1-1). This cell was typically 27 microns wide at its greatest diameter and 39.5 microns long from anterior tip to posterior tip (N=3). It had a vast membraneous surface that significantly diluted the DiI initially delivered to its cell membrane, making it appear shroud-like (see especially rotational movies).
We also observed a segmental nerve root glial cell which may be the Segment Boundary Cell originally defined by Jacobs and Goodman (1989 a,b) to be the first of a long chain of nerve root glia that stretch end to end along the peripheral nerves and ensheathe their axons (see Fig 1-1). The Segment Boundary Cell (SBC) is believed to be important in pioneering the ISN and SN. Jacobs and Goodman report that in grasshopper, the UÕs pioneer the ISN by making contact with the SBC; aCC then follows the UÕs. In Drosophila, the aCC and UÕs encounter one another in the more confined spaces of the Drosophila cortex, prior to any encounter with the SBC. aCC then goes on to pioneer the ISN after contacting the SBC. Interestingly, we observe this putative SBC to be very similar in size and position to an almost identical glial cell derived from the NB 7-1 lineage (see Fig 7-1), the clonal source of the UÕs. It is intriguing that the same lineages that generate these important pioneering motoneurons also generate the glial cells they appear to use as guideposts. A likely mechanism that could account for this arrangment would be that motoneurons utilize a mechanism of lineage self recognition in pathfinding.
Udolph et al, 1993 and Bossing et al, 1996, are able to identify a more ventrally positioned cortical glial cell, as well; we do not observe that cell consistently (see Fig 1-1 and movies). Of the 5 stage 17 clones we generated, we observed that cell once.
References:
Broadus, J., Skeath, J.B., Spana, E. P., Bossing, T., Technau, G.M., and Doe, C.Q. (1995). New neuroblast markers and the origin of the aCC/pCC neurons in the Drosophila central nervous system. Mech Dev 53: 393-402.
Bossing, T., Technau, G. M., and Doe, C.Q. (1995). Huckebein is required for glial development and axon pathfinding in the NB 1-1 and NB 2-2 lineages in the Drosophila central nervous system. Mech Dev 55: 53-64.
Carlson, S.D., and Saint Marie, R.L. (1990). Structure and Function of Insect Glia. Ann Rev Entomol 35:597-421. Cui, X., and Doe, C.Q. (1992). ming is expressed in neuroblast sublineages and regulates gene expression in the Drosophila central nervous system. Development 116(4): 943-52.
Doe, C. Q., Smouse, D., and Goodman, C. S. (1988). Control of neuronal fate by the Drosophila segmentation gene even-skipped. Nature 333:376-8. Doe, C. Q. (1992). Molecular markers for identified neuroblasts and ganglion mother cells in the Drosophila central nervous system. Development 116: 855-863.
Fortini, M.E., Lai, Z.C., and Rubin, G.M (1991). Drosophila zfh-1 and zfh-2 genes encode novel proteins containing both zinc-finger and homeodomain motifs. Mech Dev 34(2-3):113-22.
Goodman, C. S., Raper, J., Ho, R. K., and Chang, S. (1982). "Pathfinding by neuronal growth cones in grasshopper embryos." in Developmental Order: Its Origin and Regulation. (eds Subtelny,S., and Green, P.B.) pp. 275-316 (Liss, New York, 1982).
Hoyle, G. (1986). Glial cells of an insect ganglion. J Comp Neurol 246(1):85- 103. Ito, K., Urban, J., and Technau G. M. (1995). Distribution, Classification and Development of Drosophila glial cells in the late emrbyonic and early larval ventral nerve cord. Roux's Arch Dev Biol 204: 284-307.
Jacobs, J. R., and Goodman, C. S. (1989a). Embryonic development of axon pathways in the Drosophila Central Nervous System. I. A glial scaffold appears before the first growth cones. J. Neurosci 9(7): 2402-11.
Jacobs, J. R., and Goodman, C. S. (1989b). Embryonic development of axon pathways in the Drosophila Central Nervous System. II. Behaviour of pioneer growth cones. J. Neurosci 9(7): 2412-22.
Landgraf, M., Bossing, T., Technau, G. M., and Bate, M. (1997). The origin, location and projections of the embryonic abdominal motoneurons of Drosophila melanogaster. J. Neurosci 17(24): 9642-55.
McDonald, J.A., Holbrook, S., Isshiki, T., Weiss, J., Doe, C.Q., and Mellerick, D.M. (1998). Dorsoventral patterning in the Droosphila central nervous system: the vnd homeobox gene specifies ventral column identity. Genes Dev 12: 3603- 12.
McNeill, H., Yang, C.H., Brodsky, M., Ungos, J., and Simon, M.A. (1997). Mirror encodes a novel PBX-class ofhomeoprotein that functions in the definition of the dorsal-ventral border in the Drosophila eye. Genes Dev 11(8): 1073-82.
Skeath, J. B. (1998). The Drosophila EGF-receptro controls the formation and specification of NBs along the dorso-ventral axis of the Drosophila embryo. Development 125: 3301-12. Sink, H., and Whitington, P. (1991a). Location and connectivity of abdominal motoneurons in the embryo and larvae of Drosophila melanogaster. J. Neurobiol 22: 298-311.
Sink, H., and Whitington, P. (1991b). Pathfinding in the central nervous system and periphery by identified embryonic Drosophila motor axons. Development 112(1): 307-16.
Thomas, J. B., Bastiani, M. J., Bate, M., and Goodman, C. S. (1984). From grasshopper to Drosophila: a common plan for neuronal development. Nature 310: 203-207.
Udolph, G., Prokop, A., Bossing, T., and Technau, G.M. (1993). A common precursor for glia and neurons in the embryonic CNS of Drosophila gives rise to segment specific lineage variants. Development 118: 765-775.
White, K., DeCelles, N.L., and Enlow, T.C. (1983). Genetic and developmental analysis of the locus vnd in Drosophila melanogaster. Genetics 104(3): 433-48.
Yang, X., Bahri, S., Klein, T., and Chia, W. (1997). Klumpfuss, a putative Drosophila zinc finger transcription factor, acts to differentiate between the identities of two secondary precursor cells within one neuroblast lineage. Genes Dev 11(11):1396-1408.