Embryonic development of the chick primary trigeminal sensory‐motor complex

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Abstract
The objective of this study is to define the development of all components in the chick embryonic trigeminal primary sensory‐motor complex, from their first appearance through the formation of central and peripheral axonal projections up to stage 34 (8 days of incubation). This was accomplished by two labeling procedures: application of the monoclonal antibody HNK‐1, which binds to the precursors of all these components except the placode‐derived neurons, and application of HRP to axons cut immediately distal to the trigeminal ganglion.Single immunopositive motor neuron precursors are present at stage 12. These accumulate in the transient medial motor column, whose neurons initiate axon outgrowth by stage 13–14, concomitant with the onset of translocation of their somata to form the definitive trigeminal lateral motor column (LMC). Intiially these translocating somata accumulate on the medial margin of the LMC. Beginning on incubation day 5, axons growing from newly formed motor neurons pass beside the lateral margin of the LMC, and the nuclei of these cells subsequently follow this pathway. These events follow a rostral‐to‐caudal sequence, and this phase of motor nucleus formation is complete by day 8. The lateral translocation of some caudally located nuclei is arrested beginning on day 5. This cessation, which proceeds rostrally, demarcates neurons that form the dorsal motor nucleus of the trigeminal complex.Sensory neurite formation is intiated in ophthalmic placode‐derived cells at stage 14.5, one stage later by maxillomandibular neurons, and from mesencephalic V cells at stage 15. Neural crest cells do not initiate axon formation until at least day 4 to 5. Following application of HRP distal to the condensing ganglion at stage 16, labeled ophthalmic nerve projections appear in contact with the wall of the hindbrain centrally and overlying the optic vesicle peripherally. Fibers forming the descending tract elongate rapidly, reaching the level of the VIIth nerve root (200 m̈m caudal to the trigeminal root) by stage 18 and the cervical cord by stage 22. Labeled terminal arborizations of descending trigeminal afferents are first visible at stage 22 and are evident along the entire descending and proximal ascending tracts by stage 27. Later‐developing descending axons grow in close association with existing trigeminal fibers, though a few growth cones are consistently evident superficial to the other fibers. No projections different from those reported in adult birds are seen, nor are there any contralateral afferent projections.Peripheral axons from neurons in the mesencephalic trigeminal nucleus emerge from the trigeminal ganglion beginning at stage 21. These cells are labeled only when tracer is applied to the mandibular nerve.