BRAIN RESEARCH ELSEVIER Brain Research 705 (1995) 315-324 Research report Microglia in normal and regenerating visual pathways of the tench ( Tinca tinca L., 1758; Teleost)" a study with tomato lectin Almudena Velasco, Elena Caminos, Elena Vecino, Juan M. Lara, Jose Aij6n * Departamento de Biologia Cellulary Patologia, Facultadde Medicina, Universidad de Salamanca, Avda. del Campo, Charro, s / n, 37007 Salamanca, Spain Accepted 12 September 1995 Abstract We have studied the microglial cells in the normal and regenerating visual pathways of Tinca tinca (Cyprinid, Teleost) by using the lectin from Lycopersicum esculentum (tomato), which, in our case, has been demonstrated as a specific marker for teleost microglia. In the normal fish, there are tomato lectin positive microglial cells in the retina, optic nerve, and optic rectum. Following optic nerve crush, we observed a more extensive labeling of the microglia in the crushed optic nerve and in the contralateral optic tectum affecting the stratum opticum and stratum fibrosum et griseum superficiale. In both cases, there was an increase of rounded and i,~ss ramified microglial cells, and granular cells. This response of a more extensive labeling of microglial cells increases to a maximum at 2-3 weeks after the crush; the density of labeled microglial cells decreases after 3 months after crushing. However, in the retina no changes were observed after optic nerve crush. These results suggest that the microglial cells could play an important role in regeneration of fish optic pathway, as other neuroglial cells do. Keywords: Fish; Microglia; Regeneration; Tomato lectin; Visual pathway I. Introduction Fish and amphibian retinal ganglion cells, in contrast to those of mammals, have the capacity for regenerating their axons after optic nerve lesion [2,12]. The axons of the visual system re-establish synaptic contacts with their pre- vious targets in the tectum, restoring vision [38,39]. The principal obstacles to axonal regeneration in the CNS of mammals appear to be the reactive astrocytes, oligodendrocytes and microglial cells which possess differ- ent factors that inhibit neurite growth. Reactive astrocytes form a glial scar in rat optic nerve that impedes axonal elongation [28], and oligodendrocytes possess myelin asso- ciated proteins that inhibit the axonal growth [6,10]. More- over, there are great differences between mammalian and lower vertebrates in the time-scale of removal of the degeneration products by means of microgliai cells and macrophages. In mammals, axonal debris disappeared in 5 months [3], whereas in lower vertebrates the removal of degenerating axons occurred rapidly within one week after optic nerve crush [9,46]. * Corresponding author. Fax: (34) (23) 294549. 0006-8993/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0006-8993(95)01204-4 Macrophages and microglial cells appear to play an important role in gliogenesis and neuronal repair. In mam- mals, they interact with the proliferation of astroglial cells by releasing several factors such as cytokines [17,18], and there are factors secreted by microglia involved in the response to lesion in the fish visual system [13]. The descriptions of microglial cells in the brain of the lower vertebrates have been scarce and these cells show a slightly different morphology to that of mammals [9,40]. Different techniques have been used to study microglial cells in vertebrates including silver impregnation techniques [35], enzyme histochemistry [7,20,29] and immunohistochem- istry [32,33]. In the last years, lectins have been utilized as histochemical markers for the study of microglial cells since they bind to specific sugar groups on most mem- branes [26,43]. Some examples are wheat germ agglutinin (WGA), Ricinus communis agglutinin (RCA120) , and Grif- fonia simplicifolia B 4 isolectin (GSA-I B4) [8,26,44]. We have utilized a lectin obtained from Lycopersicum esculentum (tomato), with affinity for poly-N-acetyl lac- tosamine sugar residues [47], since this has permitted the identification of ameboid and ramified microglial cells in postnatal and adult rat brain [1]. In this paper, we describe for the first time the morphology and distribution of