Arguably, the greatest frontier in biomedical research remains the mystery of the human brain. Much of what we know comes from animal models, but that leaves major gaps since nothing is like the sophistication and complexity of the human brain.

One method scientists have used to further their understanding is through stem cells. They've coaxed certain stem cells to produce pea-sized collections of brain tissue in culture. They stimulate the stem cells with chemicals called growth factors inside a chamber known as a bioreactor. Over weeks, clumps of brain tissue called organoids develop and contain nerve cells with a structure that mimics the typical six layers of the human cortex. That's the outer layer of the brain where higher thinking happens.

But recently researchers have begun stirring the ethics pot. They've implanted these organioids inside mice and the pea-sized human brain tissue have connected to the animals' blood supply which will eventually allow them to grow larger and integrate further with the animal's brain function.

TEven though this will yield new insights into the human brain, helping us understand autism or Alzheimer's, what about the question of whether we're humanizing the mouse? No one knows yet whether these mice have new capabilities, higher intelligence, or greater consciousness.

It's unlikely because the amount of human brain tissue is small, however, are we crossing a line especially for future studies? When will the threshold be crossed as an animal becomes too human? Here is where our progress begins to pass our ethical scaffolding. This work will become even more divisive as more of this type of research is done.