Schematic drawings of the first-instar larval CNS

From Ito et al. (1995) Roux's Archiv Dev. Biol. 204: 284-307

The lateral (A), horizontal (B), and perspective (C) views of the larval CNS just after hatching. C shows the neuropile of the T1 neuromere and the abdominal ventral nerve cord. One larval abdominal neuromere (A1-A6) just after hatching is 11-13 µm thick, 70-80 µm wide, and 35-40 um high. Caudal neuromeres (A7 and A8/9) are narrower and distorted. The thoracic neuromeres are thicker, narrower, and higher than the abdominal ones.
The metameric Cartesian coordinate system is shown in A and B. "%NAP" (Percent Neuromere Antero-Posterior) indicates positions along the longitudinal axis; "0 %NAP" corresponds to the anterior segment border, "100 %NAP" to the posterior segmental border. The mediolateral axis is represented by "%NML" (Percent Neuromere Medio-Lateral); 0 %NML corresponds to the midline and 100 %NML to the lateralmost surface of the CNS. The vertical axis is indicated as "%NVD" (Percent Neuromere Ventro-Dorsal); 0 %NVD is the ventral surface and 100 %NVD dorsal.
The segment borders are marked by the "dorsoventral channel" (DV channel), a duct-like structure penetrating the nervous system vertically on the midline. The inner surface of the channel is contiguous with the outer VNC surface (Fig. 3A and H). In early embryos the channels first appear as the space between a pair of longitudinal connectives and the neighboring neuromeres (Fig. 6K and 14E).
With numerous intrinsic axon fibers associated with the commissures and longitudinal tracts, the larval neuromere is much larger and more complex than in the early embryos. The midline region between the commissure neuropile is almost devoid of cell bodies (see also Fig. 3). Each neuromere carries three nerves: a pair of peripheral nerves and a neurohemal organ. The neurohemal organ connects to the dorsal VNC at the segment border. The abdominal neurohemal organ forms a V-shaped bifurcation to send a pair of dorsal nerves (Hertweck, 1931; "transverse nerves" in Gorczyca et al., 1994) to both sides of the body wall, where they are called segment boundary nerves (Bodmer and Jan, 1987). The bifurcation occurs just above the VNC in the embryo. In larvae, especially in late stages, the stalk between the VNC and the bifurcation point becomes elongated, running above the dorsal midline (see Fig. 1 of White and Kankel, 1978 for late larvae). The thoracic segments lack the lateral projections from the neurohemal organ (A and B, see also Fig. 3 of Naessel et al., 1988 for Calliphora).
The peripheral nerve consists of two separate fiber bundles until embryonic stage 15-16 (see Fig 11B and C); they fuse during stage 16-17. The level where the nerves leave the VNC is relatively ventral in thoracic segments and more dorsal in the abdomen (A). The peripheral nerve has two nerve roots. The intersegmental nerve root (ISNR) crosses the segment border and forms two branches. The anterior branch enters the neuropile of the anterior segment in the dorsalmost region (90 %NVD) at about 50-70 %NAP (see also Fig. 3D). The posterior branch enters the neuropile slightly more ventrally (75 %NVD) and posteriorly (65-80 %NML; see also Fig. 3E). Both branches are associated with the mediolateral fiber tracts in the dorsalmost neuropile. The segmental nerve root (SNR) forms many small branches that enter the ventralmost region of the neuropile (50-60% NVD) at various anteroŠposterior levels and invade the ventralmost neuropile. In midŠstage embryos, all the nerve roots run perpendicularly to the body axis. As the nervous system contracts, most of them are skewed to pass through the cortex obliquely. This distortion of the nerve roots does not affect the cortex structure.

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