Axonal regrowth in the amyelinated optic nerve of the myelin‐deficient rat: Ultrastructural observations and effects of ganglioside administration

Abstract

It has been postulated that myelin degradation products may inhibit regrowth of mammalian central axons and that central nervous system (CNS) myelin and oligodendrocytes may constitute a "nonpermissive substrate" for axonal growth. To address these issues, we utilized an X-linked rat mutant, myelin-deficient or md. In the optic nerve of this mutant, 40 days and more postnatally, normal myelin is absent and oligodendrocytes are few (Dentinger et al. Brain Res. 344:255-266, 1985). Twenty-eight days before sacrifice, we operated on four groups of 50-day-old md rats and age-matched normal littermates according to the following protocols: 1) unilateral intraorbital optic nerve crush; 2) beginning within 1 hour of nerve crush, daily intraperitoneal injection of GM1 ganglioside (20 mg/kg) dissolved in phosphate-buffered saline (PBS); 3) daily intraperitoneal injection of PBS alone, also begun within 1 hour of nerve crush; 4) severance of the optic nerve immediately behind the papilla 16 or 21 days after the primary crush lesions. Additionally, normal and md rats were killed 4 and 14 days after unilateral optic nerve injury. Nerves of unoperated md rats and their normal littermates were also processed. In the operated animals that did not receive GM1, ultrastructural analysis 4, 14, and 28 days after lesioning revealed that md optic nerves contained significantly greater numbers of regenerating axons, including growth cones and varicosities, than nerves of normal rats. Notably, 28 days postoperatively, (group 1), regenerating axons were still abundant in md nerve, whereas, in nerves of normally myelinated littermates, axonal numbers were diminished markedly. Regenerating optic axons of both md and normally myelinated rats were oriented by linear astrocytic arrays and often were enclosed by astrocytic cytoplasm. In normal littermates, GM1 administration (group 2) induced a significant increase in the number of axons within the operative lesion. Paradoxically, GM1 inhibited the ordinarily robust regeneration of md axons. PBS-injected md and normal rats (group 3) showed no significant differences from noninjected, operated animals. Severance of the nerve at the papilla (group 4) 7-12 days before sacrifice confirmed the origination of axonal regrowth by retinal ganglion cells. The data provide in vivo support for a role of myelin breakdown products or the secretory products of oligodendroglia in the inhibition of regenerative axonal sprouting within mammalian CNS.