Axonal growth and target selection during development: retinal projections to the ventrobasal complex and other "nonvisual" structures in neonatal Syrian hamsters.

In newborn hamsters, there is a direct retinal projection to the ventrobasal complex, the principal thalamic somatosensory nucleus. The projection decreases precipitously between the second and third postnatal days. A few retinofugal axons remain dorsally along the lateral border of the nucleus on day 4, and none are present thereafter. In neonatal hamsters, retinofugal axons project to additional "nonvisual" nuclei including the periventricular and anterior nuclei of the hypothalamus, zona incerta, substantia nigra, inferior colliculus, pons, and mesencephalic tegmentum. Some of these connections remain in adult hamsters, although in apparently reduced density or relative volume, while others disappear. The contribution of transient connections to the normal morphological or functional development of the brain remains to be clarified. The combined results of this and other studies show that the normally transient retino-ventrobasal projection is a substrate for abnormal connections in that it can be permanently stabilized by appropriate neurosurgery on the day of birth.