Fig. 2. Glia in the larval visual system.

The larval visual system becomes populated by glia from at least two different sources, the eye disc and in the developing optic lobe, the glial precursor centre (GPC). A. The glia of the eye disc (brown nuclei) all originate in the optic stalk (osg) and migrate into the eye disc. There are four types of differentiated glia in the eye disc: two large basally-located carpet glia (cg, outline in dashed lines), the eye disc marginal glia (edmg; not to be confused with the lamina marginal glia), wrapping glia (wg) and surface glia (surg). Undifferentiated glia (udg, outline in dashed lines) migrate along the basal surface of the eye disc, below the carpet glia, until they come into contact with newly differentiated photoreceptors (R) just posterior to the morphogenetic furrow (mf). Differentiating glia then migrate apically and develop extensions to surround photoreceptor axons, becoming wrapping glia. A grey dashed line indicates a rotation of the brain relative to the optic lobe for illustration purposes, but both wrapping and carpet glia extend into the optic lobes. In the optic lobe, glia are derived from the GPC which lies proximal to the lamina furrow (LF) and the Outer Optic Anlage (OOA), from whence neuronal precursors arise. Three types of lamina glia (dark blue) derive from the GPC; these include at least some of the satellite glia (sg), as well as the epithelial (eg) and marginal glia (mg). The GPC also gives rise to the glia lining the medulla neuropile (light blue) including the outer chiasmal glia (xg) and the medulla neuropile glia (mng). Cells underlying the larval marginal glia are usually labelled medulla glia in the literature, failing to acknowledge that in the adult an additional layer of glia, those of the outer chiasm ‘small’ and ‘giant’ glia (Tix et al., 1997), lies between the marginal glia and medulla glia. Lamina precursor cells (LPC) are displaced to the lamina where, as lamina neurons (ln), their cell bodies come to lie between the satellite and the epithelial glia. Likewise, medulla precusor cells (MPC), possibly ganglion mother cells, ultimately give rise to medulla neurons (mn). Subperineurial glia (supng; pink nuclei) derived from the epithelium and mesodermally derived perineurial glia (png; grey nuclei) surround the entire optic lobe as a sheath to form distinct components of the blood brain barrier. The inner glia of the optic stalk and the medulla cortex glia are not illustrated. B. A cross section of the eye disc shows the relative apical/basal locations of the glia and their locations in relation to the photoreceptors. Figures modified from originals in Chotard et al. (2005) and Silies et al. (2007).