TARGET-DERIVED AS WELL AS SOURCE-DERIVED FACTORS DIRECT THE MORPHOGENESIS OF ANOMALOUS RETINO-THALAMIC PROJECTIONS

Neonatal tectal lesions in hamsters result in the elimination of a maj or central target of retinal axons, massively denervate the lateral po sterior nucleus of the thalamus (LP), and lead to a marked increase of the retino-LP projection. In such animals, retino-LP axons show all o f the normally-occurring terminal types. In addition, large clusters o f varicosities, whose tubular configuration resembles the major type o f tecto-LP terminals observed in normal animals, are also noted if the tectal lesion is made on the day after birth (Pi). If, however, the n eonatal lesion occurs on P5 rather than on P1, terminals resembling no rmal tecto-LP endings are rarely observed; rather, the distribution an d morphology of retino-LP terminals bear a greater resemblance to thos e seen in normal hamsters, but the size and complexity of the terminal s, particularly those that form string-like arrangements, is significa ntly increased. Our findings suggest that the altered morphology of so me abnormally induced retino-LP terminals may be orchestrated by targe t-associated signals. However, there are age-related limitations on th e degree to which afferent systems can vary their terminal morphology; these restrictions may derive from the target, or may be a function o f intrinsic changes within the cells of origin of the afferent fibers. (C) 1997 Wiley-Liss, Inc.