Glial Cells in the Optic Lobe - Introduction

E. Eule, S. Tix (1), and K.-F. Fischbach
Institut für Biologie III, Schänzlestrasse 1, 79104 Freiburg i. Br., Germany. (1) New address: California Institute of Technology, Pasadena, California 91125, USA

Recently, considerable attention has been paid to the role of glial cells in development, maintenaince, and regeneration of the nervous system in both, vertebrates and invertebrates. Apart from their role in ensheathing axons and operating as mechanical support and insulation devices, glial cells are involved in glycogen and neurotransmitter metabolism (Coles and Tsacopoulos 1981, Pentreath and Kai-Kai 1982, Pentreath et al. 1986). Glial cells regulate ionic homeostasis in the environment of neurons and are important for neuromodulation (Treherne and Schofield 1981). Moreover, glial cells are major constituents of the bloodbrain barrier (Butt 1991), and they participate in wound healing of the vertebrate brain (Bevan 1990, Snow et al. 1990, Laywell and Steindler 1991). A similar function has also been demonstrated for the glial cells in invertebrates (Wigglesworth 1959, see review of Lane 1981).

Glial cells provide inductive cues for outgrowing axons in development (Rakic 1971, Silver et al. 1982, Bastiani and Goodman 1986, Jacobs and Goodman 1989, Jacobs et al. 1989) as well as for regenerating neurons (David and Aguayo 1981, Blanco and Lane 1990, Bunge and Hopkins 1990, Smith et al. 1987, 1990, Lefkowitz et al. 1991) and for neurons growing in tissue cultures (Fredieu and Mahowald 1989, Hatten 1990, Pixley 1992). On the other hand, glial differentiation and function is strongly dependent on neurons (Tolbert and Oland 1989, 1990, Winberg et al. 1992, Georgiou et al. 1994). Furthermore, glial cells have the potential to migrate (Büssow 1980, Stone and Dreher 1987, Watanabe and Raff 1987, Giangrande et al. 1993, Choi and Benzer 1994). Genetic analysis of glial functions is especially advanced in Drosophila. In the embryonic nervous system glial cells separate the anterior and posterior commissures (Klämbt and Goodman 1991, Klämbt et al.1991). Some genes essential for the correct differentiation of these glial cells have been characterized (Nambu et al. 1980, Crews et al. 1988, Rothberg et al. 1990, Klämbt et al. 1991, Rutledge 1992).

Recently, Winberg et al. (1992) have described a set of glial cells in the differentiating optic lobes of third instar Drosophila larval brains in the direct vicinity of the lamina precursors. The authors suggest that these cells act as "stop signals" and guide posts for Rn axons after growing through the optic stalk into the hemisphere. Choi and Benzer (1994) have demonstrated that RBG cells of the fly's retina originate in the optic stalk and migrate into the eye imaginal disc during larval development. Furthermore, glial cells appear to regulate neuroblast proliferation from the optic anlagen (Ebens et al. 1993). In the adult nervous system, disrupted glial cell function is suspected to contribute to a reversal in electrophysiological response to light (Xiong et al. 1994). In the drop-dead mutant showing progressive brain degeneration, glial cells fail to mature (Buchanan and Benzer 1993). Inspite of their importance for both optic lobe development and adult vision, little is known about the adult complement of glial cells, whereas the neuronal components of the adult optic lobe have been described in detail (Fischbach and Dittrich 1989).

With the invention of the enhancer trap technique (O'Kane and Gehring 1987, Bellen et al. 1989, Wilson et al. 1989) it is now possible to identify glial cells by selective expression pattern of the bacterial lacZ reporter gene. Since there was no rigorous description of glial cells in the Drosophila adult optic lobe, we established a catalogue of such cell types, using enhancer trap P-element lines. It will also be shown that the reporter gene expression can be used to trace the fate of glial cell types in structural brain mutants. The characterisation of the different glial cells in the adult optic lobe is a necessary prerequisite for a better understanding of visual system function and will allow developmental tracing of the identified glial cell populations.

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