MP3:

MP3 (Midline Precursor 3) forms as an unpaired precursor from the mesectodermal cells along the ventral midline of the CNS.

In grasshopper, MP3 generates the H-cell and H-sib (Goodman et al., 1979; Bate et al., 1981; Goodman et al., 1981; Bate and Grunewald, 1981; Goodman et al., 1984). The H-cell is an interneuron that generates complex arborizations on both sides of the midline, with a single robust projection connecting the two in a letter-H arrangement (in T3 and A1 segments); other segments have similar but simpler projection patterns.

In Drosophila, Bossing and Technau (1994) describe a 2 cell midline clone (UMI; unpaired median interneurons) that contains an neuron resembling the H cell -- the neuron projects into the posterior commissure, bifurcates, and extends in both anterior and posterior directions on both sides of the midline -- but because they saw no segmental variation they were reluctant to call it an MP3 homolog. We generated 6 MP3 clones: 2 at stage 15, 1 at stage 16 and 3 at stage 17 (Fig. MP3). Our stage 15 clones are similar to those observed by Bossing and Technau (1996); but by stage 17 the clones were more complex and clearly matched the segment-specific axon projections reported for the grasshopper MP3 neurons. In T3, the axon projections form an "H" with bilateral projections in a medial fascicle of the longitudinal connectives for at least three segments, and additional bilateral projections in a lateral fascicle of the connective; the lateral projections are always asymmetric and the shorter projection branches dramatically (Fig. MP3). In A1, the clone has similar projections. A lower abdominal segment contained projections that extended bilaterally but lacked A/P extensions in the connectives. These segment-specific patterns are similar to the MP3 clone in grasshopper.

References:

Bate, C. M., and Grunewald, E. B. (1981). Embryogenesis of an insect nervous system II: a second class of neuron precursor cells and the origin of the intersegmental connectives. J Embryol exp Morph 61: 317-30.

Bate, C. M., Goodman, C. S., and Spitzer, N. C. (1981). Embryonic development of identified neurons: segment specific differences in the H cell homologues. J Neurosci 1(1): 103-6.

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.

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.

Goodman, C. S., Bastiani, M., Doe, C. Q., du Lac, S., Helfand, S. L., Kuwada, J. Y., and Thomas, J. B. (1984). Cell recognition during neuronal development. Science 225: 1271-79.