From the Eye to the Brain: Development of the Drosophila Visual System (original) (raw)

2016, Current Topics in Developmental Biology

of the eye will not be further described here as it has been reviewed extensively in the recent past (Paulk et al. 2011, Kumar 2012, Lamb 2013). 2.2 Optic lobe Fly neurons are organized into approximately 50 areas composed mainly of neuronal processes, called neuropils, with their cell bodies localized at the periphery. Four of these neuropils form the optic lobe: lamina, medulla, and the lobula complex which is further subdivided into the lobula and the lobula plate neuropils (Morante and Desplan 2004). Two major types of neurons can be identified within the optic lobes: "Interneurons" whose cell bodies and projections remain within the optic lobe, and "projection" neurons, which connect the optic lobe to the CB (Fig. 1 D, E) (Hofbauer and Campos-Ortega 1990) 2.2.1 Lamina-Photoreceptors from each ommatidium involved in motion vision ("outer photoreceptors") first innervate the lamina neuropil, which manifests a columnar organization in which each pixel of the visual field corresponds to one cartridge (Meinertzhagen and Sorra 2001). The lamina is mostly composed of interneurons, whose projections do not leave the optic lobe with their cell bodies located in the lamina cortex region. Lamina neurons can be divided in two populations: Five types of monopolar neurons that contact a single cartridge and project retinotopically into the medulla, and amacrine cells that contact several cartridges within the lamina (Fischbach and Dittrich 1989, Hofbauer and Campos-Ortega 1990, Tuthill et al. 2013). 2.2.2 Medulla-The medulla neuropil receives direct innervation from color ("Inner") photoreceptors, as well as from lamina monopolar neurons. The medulla neuropil is stratified in ten layers (M1-M10) with the region between layers M1 and M6 referred to as 'distal medulla' that receives these external inputs (Fischbach and Dittrich 1989, Morante and Desplan 2008, Takemura et al. 2008). The 'proximal medulla' (layers M7 to M10) receives information from the distal medulla and further computes visual information. The medulla neuropil is organized in repetitive columnar units, oriented perpendicular to the ten layers that are reminiscent of lamina cartridges (Fischbach and Dittrich 1989). Retinotopic connections between medulla columns, photoreceptors and lamina cartridges ensure an accurate representation of the visual world (