Postnatal development of the occipito-tectal pathway in the rat.

The postnatal development of the occipito-tectal pathway was studied by making single injections of 3H-leucine into the striate cortex of rats ranging in age from newborn to postnatal day 50 (P50). After these injections, the earliest age at which autoradiographic labeling was found in the ipsilateral superior colliculus (SC) was P4. Two main stages were recognized in the development of the occipito-tectal pathway. In the first stage, from P4 to P9, the silver grain pattern over the SC was suggestive of axonal labeling. The label was tangentially and radially exuberant involving the prospective stratum opticum, the adjacent part of the stratum griseum superficiale and also the strata intermediale. A rough topographic order in the projection existed at least from P6. The second stage, from P9 to P17, was characterized by the ingrowth of axonal arbors into the collicular strata superficiale and by the disappearance of the tangentially exuberant projections. Quantitative estimations of the degree of tangential exuberancy of the projection showed that it underwent a reduction of almost 50% from P7 to P17. By P17, the radial and tangential patterns of termination of the occipito-tectal pathway appeared virtually mature. No projections to the contralateral SC were observed at any age. The results of the present study indicate that the mature topographic pattern of the occipito-tectal projection is attained through two separate steps which may involve a number of different mechanisms. In the first step, occipital axons grow orderly -although in an exuberant manner- towards their roughly appropriate tectal locations, remaining to a large extent confined to the collicular white matter. In the second step, further refinement of the topographic map is achieved both by selective growing of terminal arbors into tangentially restricted regions of the tectal surface, and, by retraction of tangentially exuberant projections.