Extraction, detection, and imaging of the macular carotenoids

The term "macular carotenoids" refers to the lutein, zeaxanthin, and meso-zeaxanthin that are highly concentrated at the center of the human retina. Intraretinal levels of these carotenoids are inversely associated with the risk of age-related macular degeneration (AMD), and oral supplementation with these carotenoids can significantly reduce AMD risk. To make macular carotenoid analysis more accessible, we systematically review the current methods for extraction, detection, and imaging of macular carotenoids in both basic and clinical research. We first introduce carotenoid extraction methods from the retina, retinal pigment epithelium (RPE)/choroid, serum, and liver of the human and animal models, such as mice and Japanese quails, as well as from algae, bacteria, and chicken egg yolks and cultured cells. We then review macular carotenoid detection by spectroscopy and HPLC, while particularly introducing carotenoid separation via cyano columns, chiral columns, and C30 columns. In the end, we summarize the common methods used to image carotenoids in living human eyes: resonance Raman spectroscopy, autofluorescence attenuation spectroscopy, and reflection spectroscopy, and we then review the utility of confocal resonance Raman microscopy to image the macular carotenoids in tissue sections of human and mouse retinas.