Median Neuroblast:

The MNB is an unpaired NB that forms at S4 from the mesectodermal cells along the ventral midline of the CNS.

Klambt et al. (1991) analyzed enhancer trap lines expressed at the midline and concluded that the MNB and VUM precursors generate distinct lineages. Bossing and Technau (1994) reported that the MNB generates a clone of motoneurons with bifurcating bilateral projections and local interneurons; they also scored very similar unpaired midline clones which they assigned to "VUM precursors."

All insect model systems have intensively studied Dorsal Unpaired Median (DUM) neurons that derive from the MNB (Plotnikova, 1969; Crossman et al., 1971; Hoyle, 1974; Hoyle, 1978; Hoyle and Dagan, 1978; Goodman and Spitzer, 1979; Thompson and Siegler, 1991, 1993; Condron et al., 1994; Campbell et al, 1995; Condron and Zinn, 1995). The DUM neurons include octopamine-positive neurosecretory cells that extend bifurcating axons to various (unidentified) muscle targets and GABA-positive interneurons (Goodman and Spitzer, 1979; Goodman et al., 1979; Condron et al., 1994; Condron and Zinn, 1995). In each of these insects, ventrally positioned cells with similar properties (VUMs) exist, but whether they arise from the MNB is unclear (Pflueger et al., 1993; K. Thompson, personal communication).

We generated 12 MNB clones, 2 at stage 15, 4 at stage 16 and 5 at stage 17; all were at the anterior edge of the engrailed stripe, included engrailed positive cells, and produced similar axonal projections. Our stage 15 clones contained 2 cells with projections similar to those observed by Bossing et al (1994), however, by stage 17 the clones were more complex and always contained more than two cells. We suggest that the "VUM neurons" are early born progeny of the MNB, rather than a distinct cell lineage.

A. Neurosecretory cells/motoneurons

This clone contains both large and small cells, but because their axons are tightly fasciculated as they enter the neuropil it is impossible to assign projections to individual cells. Presumably the larger cells (7.2 x 6.8 mm; n=3) are the motoneurons/neurosecretory cells which extend bilateral projections to body wall muscle targets. At stage 15, there is only one bifurcating projection to the posterior root of the ISN; by stage 17, we observed a robust projection to SNb (n=5) as well (Fig. MNB). The ISN projection always extends to the dorsal muscle group, but target selection appears to be random (consistent with a neurosecretory function). We never saw branches to SNa, SNc or SNd muscle targets (Fig. MNB).

B. Interneurons

We observed 2-6 smaller cells (average diamter 4.2 mm, n=6) that we presume to generate the interneuronal projection (Fig. MNB). The interneurons bifurcate dorsally and remain at an extremely dorsal position, extending to segment boundaries in both connectives.

References:

Bossing, T., and Technau, G. M. (1994). The fate of the CNS midline progenitors in Drosophila as revealed by a new method for single cell labeling. Development 120: 1895-1906

Campbell, H. R., Thompson, K. J., and Siegler, M. V. S. (1995). Neurons of the median neuroblast lineage of the grasshopper: a population study of the efferent DUM neurons. J Comp Neurol 358: 541-551.

Condron, B. G., Patel, N. H., and Zinn, K. (1994). engrailed controls glial/neuronal decisions at the midline of the central nervous system. Neuron 13: 541-54. Condron, B. G., and Zinn, K. (1994). The grasshopper median neuroblast is a multipotent progenitor cell that generates glia and neurons in distinct temporal phases. J Neurosci 9: 2687-96.

Condron, B. G., and Zinn, K. (1995). Activation of cAMP-dependent protein kinase triggers a glial to neuronal cell fate switch in an insect neuroblast lineage. Curr Biol 5(1): 51-61.

Crossman, A. R., Kerkut, G. A., Pitman, R. M., and Walker, R. J. (1971). Electrically excitable nerve cell bodies in the central ganglia of two insect species Periplaneta americana and Schistocerca gregaria. Investigation of cell geometry and morphology by intracellular dye injection. Comp Biochem Physiol 40A: 579-594.

Goodman, C. S., and Spitzer, N. C. (1979). Embryonic development of identified neurones: differentiation from neuroblast to neuron. Nature 280: 208-14.

Goodman, C. S., O'Shea, M., McCaman, R., and Spitzer, N.C. (1979). Embryonic development of identified neurons: temporal pattern of morphological and biochemical differentiation. Science 204: 1219-22.

Hoyle, G. (1974).Dorsal unpaired median insect neurons make neurosecretory endings on skeletal muscle. J Exp Zoology 187: 159-165. Hoyle, G. (1978). The dorsal, unpaired, median neurons of the Locust metathoracic ganglion. J Neurobiol 9(1): 43-57.

Hoyle, G., and Dagan, D. (1978). Physiological characteristics and reflex activation of DUM (Octopaminergic) neurons of Locust metathoracic ganglion. J Neurobiol 9(1): 59-79.

Klambt, C., Jacobs, J. R., and Goodman, C. S. (1991). The midline of the Drosophila central nervous syetm: a model for the genetic analysis of cell fate, cell migration and growth cone guidance. Cell 64: 801-15.

Pflueger, H.-J., Witten, J. L., and Levine, R. B. (1993). Fate of abdominal ventral unpaired median cells during metamorphosis of the Hawkmoth, Manduca sexta. J Comp Neurol 335: 508-522.

Plotnikova, S. N. (1969). Effector neurons with several axons in the ventral nerve cord of the Asian grasshopper Locusta migratoria. J Evol Biochem Physiol 5: 276-8.

Thompson, K. J., and Siegler, M. V. S. (1991). "Anatomy and physiology of spiking local and intersegmental interneurons in the median neuoblast lineage of the grasshopper". J Comp Neurol 305: 659-675.

Thompson, K. J., and Siegler, M. V. S. (1993). "Development of segment specificity in identified lineages of the Grasshopper CNS". J Neurosci 13(8):3309-18.