Horseradish peroxidase (HRP) has been injected into periocular, upper beak, or lower jaw tissues of chick embryos aged 6–8 days of incubation. Subsequent mapping of the distribution of labeled neurons in the trigeminal ganglion indicates that the somatotopic organization of neurons is essentially identical to that found after hatching.
At these stages most labeled cells are in the distal parts of the bilobed ganglion. There is no indication that any of these neurons, most but not all of which are derived from epidermal placodes, establish any erroneus projections. Most of the proximo-central or core neurons, which are of neural crest origin, are immature at these stages. On the eighth day of development, by which time the embryo is responsive to tactile stimulation of the beak, few of these core cells have established projections to more distal (rostral) periocular or upper beak areas or to the lower jaw. In contrast, injection of HRP above the eye results in labeling of many of the core neurons. This suggests that there is an asymmetry in the time of neurite outgrowth from core neurons that is correlated with their peripheral projections but not with their time of terminal mitosis.
Following injection of HRP into temporal periocular tissues, most of the core cells in the maxillo-mandibular lobe contain peroxidase. This is probably due to uptake by growth cones emerging along the maxillary ramus.
Examination of contralateral trigeminal ganglia indicates that the transmedian projections of these sensory fibers form later in development. Branches of the mandibular motor nerve extend throughout the lower jaw and presumptive jaw-closing muscle anlagen, as evidenced by incorporation of HRP into motoneurons of the trigeminal nucleus.