Avian corneal innervation: Inhibition of nerve ring formation by 6-diazo-5-oxo-l-norleucine

Normal innervation of embryonic avian cornea is achieved in two distinct phases. During phase I, nerves extend from the ventrotemporal region both dorsally and ventrally around the cornea, but not into it, ultimately encircling the 10th-day cornea. Phase II commences as nerves extend radially from the ring into the corneal stroma and from there into the epithelium. The effect of the glutamine analog, 6-diazo-5-oxo-l-norleucine (DON), on this normal sequence of events has been examined. In ovo administration of 5 μg DON on the 5th day of development inhibits the incorporation of ³⁵S]sulfate in sulfated glycosaminoglycans in both the cornea and control tissues and inhibits the completion of phase I. Phase II of corneal innervation appears to be affected only indirectly and extension of nerves into the cornea does occur. However, the number of nerves entering the DON-treated cornea is dramatically reduced. Administration of DON on the 7th or 9th days of development does not affect corneal innervation, but does demonstrate a clear effect on ³⁵S]sulfate incorporation in sulfated glycosaminoglycans by the cornea and control tissues. These data suggest that nerve ring completion is not a prerequisite for extension of nerves into the cornea and suggest an integral role for glycosaminoglycans in facilitating phase I, but not phase II, of corneal innervation.