Distribution of trace elements in the mammalian retina and cornea by use of particle-induced X-ray emission (PIXE): localisation of zinc does not correlate with that of metallothioneins

Abstract

Proton-induced X-ray emission (PIXE) in combination with 3D depth profiling with Rutherford backscattering spectrometry (RBS) was used to establish the distribution and concentration of trace elements within individual corneal and retinal areas in frozen sections from adult male Wistar rats (n = 6). The distribution of endogenous trace elements in the cornea and retina is non-homogenous. The most abundant metal in the cornea is calcium followed by zinc. Iron and copper are present in small amounts localised particularly to the epithelium. Iron is also identified in keratocytes. Relatively high levels of calcium occur in the corneal epithelial cell bodies. Zinc has a wide intense distribution across the corneal epithelium (with greater levels in the basal part) and posterior stroma. In the retina, zinc is the most common metal followed by iron and copper. Relatively high levels of zinc exist in the retinal pigment epithelium (RPE), photoreceptor inner segments (RIS) and inner nuclear layer (INL). Chelatable zinc was localised with fluorescent TSQ in the RPE, RIS and plexiform layers. It is interesting to note that the highest levels of total zinc and the greatest intensity of chelatable zinc staining do not coincide. In the RPE and corneal epithelium, zinc co-localised with the zinc-containing metallothioneins (MT). However, there was a clear mismatch between the localisation of the most intense levels of zinc in the neuroretina (i.e. INL) and corneal posterior stroma with that reported for MT. For example, the presence of zinc is not particularly associated with the retinal ganglion cells, retinal area that contains MTs in significant amounts. While high amounts of zinc are present in the INL and corneal posterior stroma, which are largely devoid of MTs. This probably represents pools of static, catalytic and structural zinc associated with substances other than the MTs.