The mammalian eye is in many respects similar to the eyes of most other vertebrates, consisting of a focal apparatus, the pupil and lens, that direct light onto the photosensitive retina to produce an image. The retina is a discretely layered structure containing two primary classes of photoreceptive cells, the rods and cones, which appose the retinal pigment epithelium. The additional proximal layers of the retina consist of secondary neurons and their processes, and are involved in the first steps of visual processing. The output from the retina is via the axons of the retinal ganglion cells which form the optic nerve, projecting to the visual centers of the brain. In addition to the primary image-forming role in vision, recent studies have highlighted an additional photoreceptive pathway within the eye, based upon directly light-sensitive retinal ganglion cells expressing the photopigment melanopsin (Provencio et al., 2000; Berson et al., 2002; Sekaran et al., 2003; Foster and Hankins, 2002). This pathway is involved in detecting gross changes in light levels (irradiance) and is involved in the regulation of circa-dian rhythms as well as regulating other non-image forming tasks such as the pupillary light response and acute suppression of pineal melatonin (Lucas et al., 2001; Lucas et al., 1999).
The eye, and more specifically the retina, forms an ideal substrate for the study of neural physiology, as it forms a discrete outpost of the central nervous system, containing a wide range of neurons specialized to a variety of different tasks (Dowling, 1987). Investigating patterns of gene expression within the eye is therefore an essential aspect of the analysis of retinal physiology. qPCR provides an ideal tool for the investigation of temporal changes in gene expression in response to stimuli, as well as enabling characterization of pathways involved in retinal development and retinal degeneration.
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