DEVELOPMENT OF THE RETINOFUGAL PROJECTIONS IN THE EMBRYONIC AND LARVAL ZEBRAFISH (BRACHYDANIO-RERIO)

Studies of the projection from the vertebrate retina have contributed significantly to current concepts of neural development. The zebrafish has recently become a favored system for the study of development in general and neural development in particular. Although the development of both the optic nerve and the retinotectal projection of the zebraf ish has been described, the retinofugal projection in its entirety has not. This paper describes it and also addresses the issue of projecti onal exuberance: i.e., transient projections to targets that are not i nnervated in the adult. The retinofugal projection of embryonic and la rval zebrafish (32 hours to 7 days post-fertilization) was labeled by intraocular injection of DiI -dioctadecyl-3,3,3',3',tetramethylindocar bocyanine perchlorate) and then studied in wholemounts and sections. T he first optic axons crossed the chiasm at 32 hours post-fertilization and projected in a straight line to reach the tectum at about 44 hour s. At 48 hours, a few optic axons deviated along either the tract of t he posterior commissure or the tract of the postoptic commissure. By 7 2 hours (about the time of hatching) optic axons arborized in ten dist inct regions, termed arborization fields. At 6-7 days post-fertilizati on, the same ten arborization fields (nine contralateral, one bilatera l) were evident. Most of the arborization fields were located in the s uperficial neuropil and were not associated with morphologically ident ifiable clusters of somata. On the basis of various landmarks, the ten arborization fields are identified as precursors of retinorecipient n uclei previously described in other adult cypriniform fishes. The deve lopment was characterized by the nearly complete absence of any transi ent projections. Thus, the idea that axonal outgrowth is initially exu berant and trimmed back later is not supported by these results. (C) 1 994 Wiley-Liss, Inc.