Neurones situated outside the isthmo-optic nucleus and projecting to the eye in adult birds.

The centrifugal projection to the eye has been studied by retrograde horseradish peroxidase (HRP) transport in adult pigeons and chickens. About 1500 large neurones outside the contralateral isthmo-optic nucleus and 20 or so ipsilateral ectopic cells contain HRP 0.5-3.5 days after intravitreal HRP injections. The number of ectopic neurones which project to the contralateral eye is more than 20% of the number of labelled cells within the isthmo-optic nucleus. In contrast to the monopolar isthmo-optic neurones, the ectopic cells are a distinct population of large multipolar cells. Previously it has been suggested that many of these cells degenerate during the development of the chick. This study shows that they persist in the adult avian brain where they can be detected in large numbers by the presence of retrogradely transported HRP, provided that an HRP chromogen reaction of sufficient sensitivity is used. The exact target for their axons is unknown.