High magnification images of the photosensitive outer segments of a chicken retina. Image E shows the cytoplasm surrounding the stacks of membrane-enclosed disks using a technique that causes the cytoplasm to fluoresce. Image D is a Bright field image (scale bar: 20 μm). PE, pigment epithelium; PL, layer of outer and inner segments of photoreceptor cells; MLE, Membrana limitans externa; ONL, outer nuclear layer. Credit: Blank et al. (2011) "Oxygen supply from the bird's eye perspective: Globin E is a respiratory protein in the chicken retina", Journal of Biological Chemistry