Growth and restriction of the ipsilateral retinocollicular projection in the opossum.

The distribution of optic nerve fibers and terminals in the superior colliculus (SC) was followed throughout its development in pouch young opossums in order to establish the normal sequence of events leading to the formation of mature patterns. Up to 7 days of life in the pouch, labeled fibers can be followed only as far as the rostral aspect of the optic tract. The earliest evidence for crossed retinal projections in the SC is found at 10 days of age. In parasagittal sections, the label extends along the rostrocaudal tectal axis from the rostral border to the presumptive caudal pole of the SC. Unequivocal evidence for ipsilateral retinocollicular projection is found at 15 days extending to all but the caudal 5th of the rostrocaudal extent of the SC. The projections from both eyes overlap extensively in the SC at 22 days and after this age significant changes occur, mostly at the ipsilateral side: a sub-pial tier of fine label develops excluding both rostral and caudal collicular poles; a deeper tier of coarse label extends from the rostral to the caudal pole and a third, patchy tier of label is found at the prospective strata griseum superficiale and griseum intermediate. By 47 and 60 days the tangential distribution of the projections is virtually indistinguishable from the adult pattern although laminar segregation does not seem as sharp as in the adult. Comparisons of the changeable patterns of ipsilateral retinocollicular projections from 22 to 34 days with the invariant, aberrant pattern in adult animals submitted to uniocular enucleation at either age suggests that the preservation of a juvenile pattern does not provide a comprehensive explanation for the formation of aberrant projections.