THE JOURNAL OF COMPARATIVE NEUROLOGY 360:599-611 (1995) Fibroblasts at the Transection Site ofthe Injured Goldfish Optic Nerve and Their Potential Role During Retinal Axonal Regeneration

The region at and around the site of optic nerve transeetion (ONS) in goldfish, topologically the equivalent of the glial seal' in mammals, is reported to remain free of astrocytes over weeks, but its cellular constituents are unknown. To leam what type of cell occupies the site of injury and thus provides support for the rapidly regenerating retinal growth cones, immunostaining experiments at the light micro­ scopic level and electron microscopic examinations were undertaken. Between 2 and 30 days after ONS, an area up to 150 f1m wide at the transection site exhibits intense anti-fibronectin immunoreactivity. This site contained cells and processes with ultrastructural characteristics of fibroblasts and abundant collagen fibrils. Moreover, on fibroblast cultures derived from regenerating optic nerves, retinal axons grew to considerable density in vitro. Since fibroblasts are constituents of the interfascicular spaces and outer nerve sheath of the normal goldfish optic nerve, the present data imply that fibroblasts of either source migrate into the.lesion. Judging from fibronectin immunostaining they remain there during the passage of regenerat­ ing axons, and thus may provide physical and perhaps molecular support for axon growth. The fibroblasts are again restricted to interfascicular spaces after restoration of the astrocytic glia limitans around regenerated fascicles. . © 1995 Wiley·Liss, Inc. Indexing terms: eNS injury, retinal ganglion cells, glial scar, mesenchymal cells, axon growth support Whereas fish regenerate axons after transection of their optic nerve or spinal cord to full recovery of function, this capability for spontaneous and lengthy regrowth of CNS fiber tracts has been lost in mammals (Skene, 1989). Specific proteins found on oligodendrocytes and CNS myelin in mammals and birds inhibit axon regrowth (Schwab et a1., 1993). Such inhibition is not exerted by fish oligodendrocytes and fish CNS myelin (Vanselow et al., 1990; Bastmeyer et al., 1991). Moreover, astrocytes and mesenchymal cells (i.e., fibroblastic cells of the meninges) form a so-called glial scar at the lesion sites of injured CNS fiber tracts in mammals (Reier, 1986). This glial scar appears to represent a moIeculaI' and perhaps physical barrier to axon growth. In fish, however, sites of injury in the optic nerve (Gaze, 1970) and spinal cord (Sharma et al., 1993) are freely crossed by regenerating growth cones. A number of studies have analyzed the glia components after crush injury or transection of the fish optic nerve (Nona et a1., 1989; Levine, 1991, 1993; Blaugrund et a1. , 1993). These studies agree that the site of injury remains devoid of astrocytes for at least 2 weeks after surgery. None of the proteins characteristic of normal or reactive fish astrocytes © 1995 WILEY-LISS, INC. such as glial fibrillary acidic protein (GFAP) (Stafford et al., 1990), specific cytokeratins (Giordano et a1., 1989), or vimentin (Maggs and Scholes, 1986) were detected in cells in and around the region where the nerve had been severed. However, regenerating growth cones form within 2-4 days after surgery at the eye-side (distal) portion of the optic nerve stump (Lanners and Grafstein, 1980) and have arrived in the brain-side (proximal) nerve stump by 8-12 days (Murray, 1982; Lowenger and Levine, 1988; Paschke et al., 1992; Strobel and Stuermer, 1994). Thus, some sort of cellular or extracellular structural support is suspected to be available to the regrowing axons in this region to allow axons to cross the lesion site. Having grown across the lesion, leading growth cones contact a variety of cellular profiles (Strobel and Stuermer, 1994), whereas growth cones which follow appear to fasciculate with the leading Aceepted March 27, 1995. Address reprint requests to C.A.O. Stuermer, Faculty of Biology, Univ. of Konstanz, D·78434 Konstanz, Germany. Sabine Hirsch is now at Max·Planck·Institut für Entwicklungsbiologie, D-75076 Tübingen, Germany.