Research results

Histological distribution of mercury and selenium in bottlenose dolphin (Tursiops truncates) brain

○ Masumi Marumoto, , Kohji Marumoto, Masaaki Nakamura, Mineshi Sakamoto (National Institute for Minamata Disease) and Shozo Tsuruta (Aichi Gakuin University)

Introduction

Cetaceans of the family Mylidae accumulate high levels of mercury in their organs through ingestion of fish and shellfish. Although these cetaceans are exposed to methylmercury throughout their lives, no deaths from methylmercury poisoning have ever been reported. Some methylmercury may pass the blood brain barrier and reach the brain, but there are no reports of it causing neuronal cell injury. Therefore, the present study examined the distribution of mercury and selenium in the brain of the bottlenose dolphin Tursiops truncatus, a cetacean of the family Mammalia, using an electron probe micro-analyser (EPMA) to search for their localisation and to determine why exposure to methylmercury does not cause cell injury. why exposure to methylmercury does not cause cellular injury.

Materials & Methods

Samples were taken from the cerebrum and cerebellum of three adult female bottlenose dolphins, and pathology specimens were prepared according to routine methods for pathological retrieval, autometallography, and mercury and selenium localisation analysis using EPMA. In addition, total mercury and methylmercury-selenium concentrations were measured.

Results & Discussion

Units are microgram per gram of wet weight.

Total mercury concentrations in the brain exceeded 10 ppm, but the proportion of methylmercury was small and mostly inorganic mercury.

The molar ratio of selenium and inorganic mercury in the brain was almost one.

No lesions were found in all organs searched, including the cerebrum, cerebellum, skeletal muscles and lungs of the bottlenose dolphin.

Autometallography searches to visualise inorganic mercury revealed mercury deposition in the cytoplasm of a wide range of neuronal cells and in microglia in the cerebrum. In the cerebellum, mercury was found predominantly in the Bergmann glia, but not in the neurons. These findings were similar to those found in Minamata disease cases.

The EPMA analysis showed maps of the three elements superimposed on each other. Phosphorus, which is abundant in the cell nucleus, and nitrogen, which is abundant in the parenchyma, were mapped simultaneously with the element of interest, making the location of the element easier to understand.

In the left-hand diagram, red shows phosphorus and light blue shows where mercury and selenium are co-occurring. The nucleus is clearly shown, indicating that mercury and selenium are present in the cytoplasm. In the right-hand diagram, dark blue shows the localisation of nitrogen, pink shows phosphorus and light blue shows the localisation of mercury. The presence of the parenchyma and the nucleus is shown, so that the location of mercury is more clearly indicated.

Although many mercury and selenium deposits were found in the neurons of the cerebrum of hand dolphins, no findings suggestive of neuronal injury were observed. It is unlikely that mercury crosses the cerebrovascular barrier as selenated mercury, and it was speculated that methylmercury reaching the brain is mineralised in the cytoplasm of neurons and microglia and combined with selenium to exist as selenated mercury.

Acknowledgments

The authors would like to thank the late Dr Yoshifumi Takahashi and Satsuki Nagase (Aichi Gakuin University), Mao Uchikado and Miwa Chijiiwa (Department of Basic Medical Science, National Institute for Minamata Disease), Shigemi Onizuka, Fumika Hashimoto, Akane Morimoto and Ai Motoyama (Department of Environment and Public Health) for their support in presenting this study.