The appearance of trigeminal ectopic ganglia within the surface ectoderm in the chick embryo

Aberrant ganglion cell morphogenesis associated with the ophthalmic placode was observed within the epidermis of the chick embryo at stages 20 through 26. This process was studied using immunohistochemistry and scanning and transmission electron microscopy. Clusters of ganglion cells were covered by epidermis and seen as epidermal nodules distributed diffusely in the supraorbital region. The ganglion cell clusters found within these nodules were not delineated by basement membrane from the epidermis, and were provisionally termed ectopic ganglia. The cell bodies within the ganglia possessed neurites containing microtubules and neurofilament protein. The cell bodies were also immunoreactive to monoclonal antibodies, E/C8 and HNK-1, indicating neuronal phenotypes. The neurites were often associated with the ophthalmic nerve or its branches as well as being distributed in the ectoderm. The ectopic ganglia seemed to represent the delayed gangliogenesis of a part of the ophthalmic placode which failed to detach from the ectoderm during ophthalmic ganglion formation.