PRECOCIOUS INVASION OF THE OPTIC STALK BY TRANSIENT RETINOPETAL AXONS

This study demonstrates that the fetal optic nerve contains a conspicu ous population of transient retinopetal axons. Implants of the carbocy anine dye, DiI, were made into the retina or diencephalon of fetal fer rets to label the retinopetal axons retrogradely or anterogradely, res pectively, and sections were immunostained for P-tubulin to label the early differentiating axons in the optic nerve. Dye implants into the optic nerve head, but not the retinal periphery, retrogradely labeled somata in the ventrolateral diencephalon, provided the implants were m ade before embryonic day (E) 30. When dye implants were made into the ventrolateral diencephalon, these same retinopetal axons were anterogr adely labeled, coursing through the optic nerve but never invading the retina. The axons course as 2-5 fascicles from their cells of origin and turn laterally to enter the optic nerve where it joins the future hypothalamus. The retinopetal cells can be retrogradely labeled as ear ly as E20, before optic axons have left the retina. The optic nerve an d fiber layer are immunoreactive for beta-tubulin on E24 and thereafte r, whereas on E20 and E22, they are immunonegative. Yet at these early embryonic ages, immunopositive fascicles of axons course from the die ncephalon into the optic stalk, confirming the precocious nature of th e retinopetal projection. Implants of dye made into the future optic n erve head at these very early stages also retrogradely label retinopet al cells in the future chiasmatic region. These cells are distributed primarily on the side ipsilateral to the midline, but a few can be fou nd contralateral to it. Both these, as well as the retinopetal axons a rising from the ventrolateral diencephalon, may serve a transient guid ance function for later developing optic axons. (C) 1995 Wiley-Liss, I nc.