Oriented Axon Outgrowth from Avian Embryonic Retinae in Culture

The neural retina of avian embryos was spread on a membrane filter and cut in any desired orientation. Strips cut across the retina of 4- to 7-day chick or 3- to 6-day quail embryos were explanted onto collagen gels. Vigorous neurite outgrowth was seen for about 3 days, by which time many neurites were 3 mm long. Horseradish peroxidase (HRP) labeling showed that the cells producing the neurites were large and formed a layer near the inner limiting membrane, indicating that the neurites in vitro were axons of retinal ganglion cells. The size of the neurite population and the regions from which neurites emerged varied with the donor age, while most neurites sprouted from the side of the explant formerly closest to the optic fissure. This pattern closely resembled that of axon growth in the normal retina, as revealed by SEM, silver staining, and HRP labeling. Mitotic inhibitors (Ara-C and FUdR) did not alter the neurite outgrowth. Pretreatment of retinae with trypsin or collagenase did not disorganize axons at the time of explanation, but tended to equalize neurite emergence on each side of the retinal strips. We suggest that microenvironmental factors, especially the enzyme-labile inner limiting membrane, are important for axon guidance in the retina.