In a world first, surgeons in the Chinese city of Zhengzhou are planning to inject stem cells derived from human embryos into the brains of patients with Parkinson's disease with the aim of treating their debilitating symptoms.

Meanwhile, another medical team in the same city is aiming to target vision loss using embryonic stem cells (ESC) to replace lost cells in the retina, marking a new direction in China in the wake of major changes in how the country regulates stem cell treatments.

While similar treatments on Parkinson's patients have already been tested in Australia, those trials relied on cells taken from eggs that were forced to divide without first being fertilised in an effort to circumvent any ethical concerns.

Stem cells are a little like blank slates that are yet to take on a specific task. If you rewind the clock on any of your body's tissues, its cells will become less specialised, until you're left with a cell with a lot of potential to become nearly anything.

In the case of both kinds of embryonic stem cells, divided egg cells are subjected to various treatments to encourage them to develop into replacement cells that could treat a condition in a recipient.

The symptoms of Parkinson's disease are largely caused by a loss of nervous tissue deep inside the brain in an area called the basal ganglia.

Losing those cells means a loss of a neurotransmitter called dopamine, and with it a lower ability to control nervous impulses that would prevent muscles in the extremities from activating.

In the case of a condition called macular degeneration, damage to a layer of tissue called the retinal pigment epithelium at the back of the eye causes the light-catching cells above it to die.

By turning ESC into cells that can naturally develop into the tissues that have deteriorated such as the precursors to neurons that can produce dopamine, or into retinal tissue and then injecting it into the target site, the researchers hope to improve the lost functions.

Not everybody is convinced of the success of trials such as those being done in China and last year in Australia.

A stem cell biologist from the Scripps Research Institute in California, Jeanne Loring, believes the choice of cell used in both Parkinson's disease trials won't be specialised enough to match expected results.

"Not knowing what the cells will become is troubling," Loring told David Cyranoski at Nature.

But the research team in China remains confident in its decision.

Qi Zhou from the Chinese Academy of Sciences Institute of Zoology in Beijing is the stem cell specialist leading both sets of ESC trials, and says four years of animal trials conducted on monkeys have so far showed promising results.

"We have all the imaging data, behavioural data, and molecular data to support efficacy," Zhou told Nature.

He also claims the team conducting the Parkinson's trial have been selective with their potential candidates, choosing patients who will have the least chance of rejecting the ESCs from the cell bank.

In 2015, China introduced tough new regulations to deal with the growing problem of 'rogue clinics' offering stem cell treatments without due record keeping or process, making it hard to evaluate safety, or even the types of cells used in the treatments.

The changes are set to improve the ethics and safety of stem cell treatments by enforcing the use of cells through a regulatory body, ensuring informed patient consent, and permitting treatments only through authorised hospitals.

Time will tell if the regulations can be enforced, but for stem cell researchers, the changes are positive.

"It will be a major new direction for China," stem cell scientist Pei Xuetaotold Nature.

If the results are as good as the teams in Australia and China predict, it could also set new standards for the world.

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World-first trials have been launched to treat Parkinson's and blindness with embryonic stem cells - ScienceAlert