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Toxicity of nanoparticles: Zinc oxide on the brain

Nanotechnology has revealed that tiny particles have unique properties for use in medicine and technology. However, the large surface areas per unit mass of nanoparticles make them highly reactive, which raises concerns about toxicity to humans or damaging to the environment.

Now, Minghong Wu and co-workers¹ at Shanghai University have discovered that zinc oxide nanoparticles can damage or kill stem cells in the brains of mice. Their findings suggest that further precautions should be taken to protect people working with nanomaterials.

Zinc oxide nanoparticles have been used in food additives, paints, electronic devices and sunscreens. They have even been proposed for medical applications, since they can inhibit harmful bacteria.
 

Unfortunately, many scientists believe that these same properties of zinc oxide could pose a risk to humans. Research has shown that the smallest particles could reach mouse brains. In particular they could reach neural stem cells (NSCs), important cells that can differentiate into any type of brain cell. NSCs are vital for replacing lost neurons, and damage to them could result in neurodegenerative diseases.

To investigate this possibility, the researchers prepared cultures of mouse NSCs, and added zinc oxide nanoparticles ranging from 10 to 200 nanometres in size. After 24 hours, many of the NSCs showed clear signs of apoptosis (controlled cell death), for example changes in shape, broken nuclei or ruptured cell membranes. The amount of apoptosis in the cultures depended strongly on the concentration of zinc oxide nanoparticles (Fig.1).

Surprisingly, the size of the zinc oxide nanoparticles did not seem to make any difference to the number of cells dying by apoptosis, even though one would expect smaller particles to be more harmful.
 

This implies that the main toxic effects come from dissolved zinc ions in the culture medium. In fact, the researchers found that the nanoparticles had a similar toxic effect to direct zinc ions from zinc chloride.

The zinc ions probably induce apoptosis by targeting mitochondria in the NSCs, thus inhibiting cell respiration. Wu and co-workers recommend further research into these effects, so that the scientific community can review the safe working guidelines for people developing or using zinc oxide nanomaterials.

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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