EmbryoNRDevFIG 4: Development of the amphid commissure and the lateral route to the nerve ring (NR)

A-C. TEM images (cross sections) of a 350-360 min embryo (after first cell division, approximately between bean and comma stages) showing the nascent commissure fibers traveling between the head muscle (Head mu) and the SIBD neuron that has lodged itself between the muscle and the hypodermis (Hyp). Anterior and posterior to these nascent commissure fibers, head muscles are attached to the hypodermis. (Image source: [Hall archive] N611_sections386, 431, 522.)
Inset. Schematic representation of the amphid commissure development. At this stage muscle cells (Mu) that surround the pharynx primordium may be preventing a more direct, i.e. lateral, route to the NR (indicated with crosses) for the processes of the lateral ganglia neurons that are trying to reach the NR. Commissural axons must therefore travel under the muscle to reach the ventral ganglion. SIBD neurons that are positioned between the muscle and the hypodermis at this stage may be providing a bridge between the lateral and ventral ganglia for the early commissural axons. EC, excretory cell; LG, lateral ganglion; VG, ventral ganglion.
D&E. Cross sections of TEM images of comma stage when the NR pioneer bundles are seen on each side (marked by orange lines), and 1.5-fold embryos when the NR is visible around the pharynx primordium, are shown. (Image source: Shaham Lab.)
F. Schematic representation of the NR development in 430 min embryo (after first cell division). Muscle cells have migrated to their final positions next to hypodermis, leaving a trailing process behind that remains attached close to the pharynx. Processes from the remaining lateral ganglia neurons can now reach the NR directly from a lateral route rather than having to travel ventrally first. The pioneering neuron to grow a lateral process into the NR is thought to be AVA. Ventrally, RIH is positioned to navigate processes growing into the NR (C. Norris, D. H. Hall, E. Hedgecock, unpublished observations).

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