Abnormal recrossing retinotectal projections after early lesions in Syrian hamsters: age-related effects.

If the superficial layers of the right superior colliculus (SC) of a newborn hamster are removed, fibers from the left eye not only terminate in the surviving deeper layers of the right SC, but also cross the tectal midline and terminate in the medial third of the left SC. If the right eye is also removed at birth, the abnormally recrossing fibers from the left eye will spread over the entire surface of the left SC31. In this series of experiments, we ablated the right SC of hamsters at birth, but enucleated the right eye on different days postnatally in order to examine the spreading of the axon terminal pattern as a function of age. When the animals were young adults, the pattern of retinofugal projection of the left eye was traced using the Fink-Heimer technique or with autoradiography. It was found that the projections from the left eye continue to spread over the entire left SC when the right eye was removed up until day 10, though their distribution was more sparse when the eye was removed on days 7-10 than when it was removed on days 0-6. When the removal of the right eye was delayed until day 12, the lateral spreading of recrossing axons was markedly reduced. When the right eye was removed on day 14, the distribution of the projections from the left eye was restricted to the medial third of the left SC, just as if no early eye removal had been performed. It appears that after a critical age is reached, even when terminal space is available the axons and axon terminals will not move, at least not over any appreciable distance.