Growth cones of regenerating retinal axons contact a variety of cellular profiles in the transected goldfish optic nerve.

Following optic nerve transection in goldfish, retinal axons regenerate. To determine what the growth cones use as a substrate for their growth, regenerating growth cones were labeled by horseradish peroxidase (HRP) application to the retina 5-6 days after intraorbital optic nerve section (ONS) and identified at 10-11 days after ONS in the brain sided (distal) portion of the optic nerve in thick and serial ultrathin sections. Leading growth cones (n = 5) were found in intimate contact with a variety of elements: with myelin fragments alone, with myelin fragments and glial cells, and with the basal lamina of the glia limitans and the surface of a fibroblast outside the boundary of previous fascicles. In ultrathin sections of conventionally treated regenerating optic nerves, (unlabeled) axon profiles--in addition to myelin fragments--were seen to be in contact with an astrocyte and an oligodendrocyte, suggesting that the growth cones of these axons may have been associated with those cells. The data suggest that leading growth cones of regenerating axons may be capable of growing along myelin fragments and on a wide variety of cellular surfaces in the goldfish optic nerve.