Ultrasound probing of embryonic mouse brains shows some disruption of normal developmental activity, say researchers

Credit: Jim Gathany/CDC

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SYDNEY, 9 August 2006 - The impact of ultrasounds on embryonic brains may be more damaging that was previously thought, according to U.S. researchers.

A study conducted by Pasko Rakic and colleagues at Yale University in New Haven, Conneticutt, has found that a significant number of nerve cells in the brain of embryonic mice do not migrate to the appropriate location following exposure to ultrasound.

"Proper migration of neurons during development is essential for normal development of the brain's cerebral cortex, and its function can be impaired if neuronal migration is disrupted," said Rakic, chairman of the Department of Neurobiology at the university.

His team analysed how effectively neurons in the brains of one 146 mouse embryos migrated to the brain's cerebral cortex once exposed to ultrasound waves.

They found that after several prolonged exposures, a small number of neurons did not migrate to their necessary position in the upper layers of the cerebral cortex, and instead moved to the lower layers or became embedded in supporting white matter in the brain.

In an accompanying commentary, Verne Caviness of Boston's Massachusetts General Hospital and Ellen Grant of Harvard Medical School, explain how the implications of this research for the developing brain is unknown.

They argued that since the number of misplaced cells is so small, their effect may be little more than minimal background noise. The cells also appear to retain their intended cell characteristics, despite migrating to the wrong position.

Caviness and Grant describe how after the neurons have migrated, a large proportion of them are naturally eliminated as cells die from the development of new tissues in the brain as the embryo grows. Essentially, all of the misplaced cells in the mouse's brain may be eliminated and will be of no consequence for the organisation of the cortex.

This study does not necessarily suggest that ultrasound will have the same affect on human foetuses. Rakic and his colleagues said there were clear diferences between their research and the average pre-natal ultrasound women would undergo during pregnancy.

The brains of the embryonic mice were exposed with a larger volume of ultrasound waves and more frequently than a human foetus would be, they said. The authors also highlighted the differences in the biology of mice and humans, making it difficult to predict what affect the ultrasounds may have on humans.

"It is not known whether or to what extent ultrasound waves affect migrating neurons in developing humans," said Rakic.

He highlighted the difficulties in identifying the position of neurons, since it requires researchers to label the DNA replication, a procedure that cannot be used in humans. This means that the misplaced cells in the embryonic brain could be missed totally when tests are taking place.

However, they said the study serves as a reminder that we can't always take routine medical procedures for granted. While the research does not suggest ultrasounds are anything to be concverned about, it does highlight an area of embryonic development that calls for more study.

Rakic and his colleagues intend to extend the study to non-human primates to see if similar effects occur.