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Glial Cells in the Optic Lobe of Drosophila melanogaster

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E. Eule, S. Tix1, 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

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Summary

Genetically diverse populations of glial cells in the optic lobe of wild type adult Drosophila melanogaster were disclosed by the enhancer trap technique. Our genetic classification agrees with previous typologies based on morphological criteria. Due to the expression of the lacZ reporter gene, the population size and position of glial cell types could be recorded. Some classes of glial cells show variability with regard to number and arrangement.

Key words: Visual system - glial cells - pattern formation - enhancer trap technique.

Overview of the optic lobe

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Fig.1: Horizontal semithin plastic section of an adult wild type head. Overview of the left adult optic lobe. From left to right the lamina, medulla and lobula complex (lobula and lobula plate) can be distinguished. The outer optic chiasm (Xo) connects lamina and medulla, while the inner optic chiasm (Xi) houses fibres of medulla and lobula complex. Bar denotes 25 Ám.

Overview of the outer optic chiasm

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Fig. 2: Horizontal semithin plastic section of a wild type optic lobe, focussing on the outer optic chiasm. The proximal satellite glial cells (pSGl) can be found at the border between the cell body cortex and the neuropile of the lamina (Fig 3; small arrowheads). They comprise flat cell bodies and are strongly labelled. The cell bodies of the medulla neuropile glial cells (MNGl) can be seen at the interface of medulla cortex and the neuropil (large arrowheads) (see also fig. 6). Processes of these glial cells deeply penetrate the medulla neuropile. Bar denotes 10 Ám.

Glial cells of the lamina

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Fig. 3: Reporter gene expression in glial cell types of the lamina. [a] and [b]: Sagittal semithin sections, anterior is left. [a]: line 3-93, [b]: line 3-74. [c]: line 3-74, horizontal section through the lamina (distal is left, anterior is up). Bars denote 10 Ám.

[a]: The large nuclei of the pseudocartridge glial cells (PGl) are found between the fenestrated layer and the lamina cortex. [a] - [c]: The nuclei of the epithelial glial cells (EGl) form a regular row in the lamina neuropile. The cell bodies of the lamina monopolar cells can be identified distally. [b]: The small cell bodies of the distal satellite glial cells (dSGl) lie in the lamina cortex. They show convex depressions (arrows), reflecting their close association with neuronal cell bodies. [b], [c]: Arrowheads point to the flat and strongly stained nuclei of the fenestrated glial cells (FGl) just underneath the basement membrane of the compound eye. The marginal glial cells (MGl) close off the lamina neuropile proximally [c].

Glial cells of optic chiasms

outer chiasm small glial cells (oCsGl)

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Fig. 4: High magnification of the outer optic chiasm of an adult individual of line T1184 with cytoplasmatic lacZ expression. Enhancer trap expression is restricted to the outer chiasm small glial cells (oCsGl), which enwrap fibre bundles inside the outer optic chiasm exclusively. Outside the optic chiasm, these axons do not have a blue envelope. bar = 10 Ám.

outer and inner chiasm giant glial cells (oCgGl iCgGl)

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Fig. 5: Giant glial cells of the optic chiasms. Huge glial nuclei are stuck inbetween the fibre bundles at the level of crossings in both optic chiasms. The cells of the inner chiasm and the outer chiasm seem to be homologous with regard to position. They form a vertical row spanning the entire crossing region of. [a]: Horizontal view of the optic lobe of line 3-93. [b]: Frontal section through the outer optic chiasm (Xo). [c]: Frontal section through the inner optic chiasm (Xi). Outer chiasm giant glial cells, (oCgGl); inner chiasm giant glial cells, (iCgGl). Anterior is up, bar denotes 25 Ám.

Glial cells of the medulla

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Fig. 6: Glial cells of the medulla. [a]: Horizontal section through the optic lobe of a 3-159 fly with selective label of the medulla neuropile glial cells (MNGl). These cells occur not only at the distal interface between the medulla cortex and neuropile, but they are also found laterally and proximally (arrowheads). Processes of the MNGl extend into the neuropile (fig 2). Anterior is up, bar = 25 Ám. MNGl are not stereotypically arranged (Fig.6c).

[b]: Medulla satellite glial cells (MSaGl) are uncovered in the medulla cortex of 2-25 flies. In this example, the nucleus of one such cell is surrounded by diffuse blue label entirely restricted to the medulla cortex. The diffuse label is due to small processes of this glial cell, extending into its neighborhood. Anterior is up, bar = 10 Ám.

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Fig. 6c: Frontal view on the surface ot the medulla neuropile. The medulla neuropile glial cells (MNGl) are not arranged in a stereotypic pattern.

Glial cells of the lobula

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Fig.7: Glial cells of the lobula complex. [a]: Horizontal, semithin section of the lobula. Lobula neuropile glial cells (LoNGl) occur at the anterior interface between lobula cortex and neuropile (arrow heads). Processes of the LoNGl extend into both cortex and neuropile.

[b]: Horizontal section of the lobula plate. Like the LoNGL, the lobula plate neuropile glia (LPNGl) is situated between cortex and neuropile (arrow heads). In the cortex, a lobula plate satellite glial cell (LPSaGl) can be detected (little arrow).

[c]: Horizontal section of the lobula plate of enhancer trap line 3-74. Almost all cells of the lobula plate are stained. The little arrow points to a densely blue LPSaGl . The marker is accumulated at the surface of its cell body. [a]-[c]: Bar denotes 10 Ám; distal is left, anterior is up.

General glial cells of the cns

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Fig. 8: Glial cells typical for the entire central nervous system, illustrated by horizontal sections through adult heads of line 3-93. [a]: High magnification of the right optic lobe, showing parts of medulla and lobula plate. A thin blue layer of perineurial cells (PN) surrounds the complete CNS. The trachea-associated cells are labelled, too. Inside the CNS, these are called tracheal glia (TrGl) (arrow); outside the CNS these cells are by standard definition non-glial (arrow head). [b]: The large discoid cell bodies of the subperineurial glial cells (SuPnGl) are located proximally to the layer of perineurial cells. Bar denotes 10 Ám; distal is right, anterior is up.

Gradients of Gene Expression in the Visual System

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Fig. 9: Spatial gradients of gene expression in the visual system. [a]: line 3-74, and [b]: line T601; two lines show a blue marker gradient in the eye and optic lobe. Enhancer trap activity increases from dorsal to ventral (in both lines. The gradients affect neurons and glial cells: [a] cone cells in the eye (not shown) and epithelial glial cells in line 3-74 (arrows), [b] nucleoli of the retinula cells (arrows), the entire lamina neuropile (arrows) and the perineurial cells surrounding the medulla in line T601. Dorsal is up, frontal is left (a) and right (b) respectively. Bars denote 50 Ám.

elav staining

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Fig. 10: Double staining of cryostate sections of wild type heads with the elav antiserum (brown) and X-Gal (blue). . [a] line 3-66, [b] line 3-159. The elav antiserum recognizes only neurons. The inner chiasm giant glial cells (iCgGl) [a] and the medulla neuropile glial cells (MNGl) [b] are not elav- positive. This shows clearly that both cell types are non-neuronal elements of the optic lobe. bar denotes 25 Ám, anterior is up.

Table 1: Summary of glial cells in the optic lobe.

Table 2: Enhancer trap lines used in this study

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