One of the constants of our present age of change and progress is that biology is always more complex than first thought. It's a given. Evolution is under no constraint to produce designs that fall within the bounds of complexity we feel comfortable taking in at one sitting.
The scientific community will still master our biology - reverse-engineer it, replicate it, and ultimately greatly improve on it - but we shouldn't be surprised when hoped-for simplicities in biological mechanisms fail to materialize. That's all the more reason for greater support and funding for efficient, goal-driven medical research.
MIT scientists report that adult stem cells produced in the brain are pre-programmed to make only certain kinds of connections - making it impossible for a neural stem cell originating in the brain to be transplanted to the spinal cord, for instance, to take over functions for damaged cells.
"A stem cell that produces neurons that could be useful to replace neurons in the cerebral cortex (the type of neurons lost in Alzheimer's disease) will be most likely useless to replace neurons lost in the spinal cord. [Moreover], because there are many different types of neurons in the cerebral cortex, it is likely that we will have to figure out how to program stem cells to become many different types of neurons, each of them with a different set of pre-specified connections."
"In the stem cell field, it is generally thought that the main limitation to achieve brain repair is simply for the new neurons to reach a given brain region and to ensure their survival. Once there, it has been assumed that stem cells will ‘know what to do’ and will become the type of neuron that is missing. It seems that is not the case at all. Our experiments indicate that things are much more complicated."