Every time we hurt ourselves, our body goes through the same biological process required to repair our wounds. Although this process makes us healthy again, it never completely restores our bodies back to how they were and often leaves a scar. You might wonder why healing isn’t perfect or why we can’t regenerate perfectly like sea stars. This article is an attempt at answering those questions.
A quick explanation of the healing process explains why we get scars. When we have a cut, for example, our body goes through the four processes of hemostasis, inflammation, proliferation, and maturation. Hemostasis and inflammation simply stop the bleeding and protect the body from infection, while proliferation and maturation actually heal the wound. During proliferation, cells in our blood like fibroblasts and collagen make up something called granulation tissue, which fills the gap that has been made in our skin. The skin is ready to heal after proliferation, transitioning into the maturation phase, which is responsible for leaving the scar. During maturation, Structural fibers called collagen fibers are reorganized into parallel lines and are reinforced to prevent tearing. The scar tissue made by this is weaker and looks different than normal tissue because of its parallel structure. In contrast, normal tissue is more connected and organized almost randomly.
Surprisingly, our inability to regenerate limbs like sea stars has nothing to do with the healing process, but actually has a lot to do with the growing process. The reason humans can’t regenerate is because all the cell types that cause regrowth in humans go through a process called embryogenesis. Embryogenesis causes regenerative cells to differentiate permanently, making them more mature for their role, but in the process, making it so they can’t become any other cell type. Later in life mostly all of these regenerative cells, or stem cells, differentiate and mark the end of growth. Since most are past the point of no return, they won’t be replaced if they leave. This means bone cells, muscle cells, neurons, and many other cells can’t regenerate, all being necessary to regenerate a limb.
As it turns out these two questions are very much unrelated. To sum it all up in one sentence, we have scar tissue because our bodies only reorganize collagen fibers in parallel lines, and we can’t fully regenerate because our stem cells mature to the point that they can’t be regrown.