Development of the crossed retinocollicular projection in the mouse.

Changes in the distribution of axons of the crossed retinal projection within the superior colliculus of the developing mouse were studied by means of normal fiber and Golgi impregnations and by anterograde horseradish peroxidase labelling. Retinal axons advance along the optic tract from gestational days E12 to E14 and first invade the superior colliculus on E15. Over the subsequent days until birth (E19), the retinal axons extend within rostrocaudally oriented fascicles that distribute through the full thickness of the uppermost collicular layer, the stratum superficiale (SS). A dramatic transformation of this fiber stratification pattern into the mature pattern occurs over the first postnatal week. The fiber bundles are progressively cleared from the upper half of SS, identified as the future stratum griseum superficiale (SGS). Concurrently, the fiber bundles in the deep SS, identified as the stratum opticum (SO), give rise to individual, nonfasciculated fibers, which arborize within SGS. The contralateral retinal origin of the transient population of axons in SGS as well as the majority of axons that persist in SO is evident from the observation that they degenerate following neonatal enucleation. The number of fiber bundles lost is estimated to be 40-50% of the total population present in the superficial layers at birth. The combined set of observations indicates that axon elimination plays a major role in shaping the laminar pattern of retinal innervation of the colliculus. Retinal ganglion cell death, and not axon pruning, is proposed as the most probable mechanism by which axon fascicles are eliminated from SGS.