@bluedolphin-- I'm not an expert on this topic but I think that dendrites are found in the spine as well.
It's not surprising that dendrites would be found outside of the brain though. We have nerve cells all over our body. They are responsible for the communication that takes place between the various organs and the cells. For example, when we touch something hot and burn ourselves, the reason we feel pain and pull away is also due to the work of these nerve cells. They are the ones communicating the pain to our brain and then the brain takes action by pulling our hand away.
Of course, dendrites don't grow in every area where there are nerve cells. There are different factors that cause their growth. I don't know enough about it to explain though. Is there anyone here who can answer this more accurately?