Macular pigment — its role in health and disease

Abstract

Interest in macular pigment has increased over the last two decades with the realisation that it plays a major role in maintaining retinal homeostasis and that it may contribute to retinal pathologies [2, 15]. Macular pigment consists predominantly of two hydroxy carotenoids namely zeaxanthin and lutein (Fig. 1). However, a number of oxidised forms of lutein and zeaxanthin can also be identified in the retina of which meso-zeaxanthin (a product of oxidative modification of zeaxanthin) is the most common. Visually, macular pigment presents as a yellow colouration concentrated in the fovea. However, more objective methodologies such as spectrophotometry and HPLC have mapped the topographical distribution of macular pigment both across and within the layers of the retina. The absorption spectrum of macular pigment peaks in the blue light region of the visible spectrum (460 nm) but does not absorb in the green. By comparing the absorption of green and blue light in tissue sections through the fovea of adult primates it was demonstrated that macular pigment is concentrated in the foveola and avascular fovea regions [21]. Further analysis showed the macular pigment to be predominately located in the photoreceptor axons with significant but lesser amounts in the inner retinal neurons. Significantly lower levels are observed throughout the retina and macular pigment is often found in association with photoreceptor outer segments [18]. Biochemical analyses confirms that the macular carotenoids are assymetrically distributed throughout the retina but are concentrated in the macular and account for 36% of the total retinal carotenoids [4].