X-ray microanalysis of ocular melanin in pigs maintained on normal and low zinc diets

Retinal-choroidal tissues were collected from older sows that were fed either control (100% Zn), intermediate zinc (59% Zn) or low zinc (21% Zn) diets for a 6-month period. The tissues were prepared for elemental energy dispersive X-ray analysis and examined by scanning transmission electron microscopy. Melanosomes of the retinal pigment epithelium and choroidal melanocytes were specifically analysed. The elemental spectra of a specific type of melanosome within a sample region were consistently similar. The elemental spectra of choroidal melanosomes differed slightly from that of RPE melanosomes, having greater levels of calcium, iron and zinc. Changes of the elemental content of both types of melanosomes were observed in the animals that were maintained on low zinc nutrition. Iron and zinc decreased in level, while copper increased. Calcium decreased in the RPE melanosomes of intermediate and low zinc diet animals. However, calcium only decreased in choroidal melanosomes of the low zinc-fed sows, having increased substantially in the intermediate zinc-fed group. Abnormal melanosomes were concomitantly seen in the melanocytes of both the intermediate and low zinc groups.