system
Given the following post, respond as if you're commenting on Reddit:
human
What does a surgeon do to reform bone after a surgery?

I’m thinking of brain surgery where a surgeon will have to actually cut through the skull to work with the brain. After the surgery is complete how to they put the skull back together?
gpt
The surgeon supplies metal plates and screws, the body does the rest. 

Bone growth depends on 3 main things: osteoinduction, osteoconduction, and a source of nutrition and cells. The latter is the easiest to understand: you need a reasonable blood supply or you won't heal. That is true of any wound, and holds true for bone healing. Typically new osteocytes, or bone cells, are derived from mesenchymal stem cells. These cells are a type of pluripotent stem cells and exist in adults. MSCs live in the marrow, which is richly vascularized. MSCs will migrate from the marrow to a site of bone injury by following chemical trails left by chemokines and other proteins. These proteins are actually embedded within the matrix of the bone itself, so the act of breaking a bone automatically releases a chemical signal flare to the MSCs. Once the MSCs get to the area of need, they need to differentiate into osteocytes. This is osteoinduction, or inducing the stem cells to make bone. This is done with specific growth factors, the prime example being bone morphogenetic protein 2, or BMP-2. These proteins change how the stem cell expresses its DNA, causing the cell to terminally differentiate and start laying down bone material. This deposition of the extracellular matrix (ECM) is coordinated by osteoconduction. Osteoconduction requires a suitable surface on which bone can grow. Certain metals and surface finishes have been studied in terms of osteoconduction. Titanium is highly osteoconductive, porous Titanium even more so. However, porosity decreases mechanical strength, placing design constraints on many implants. Some surgeons will also use ground up bone as an osteoconductive surface, some use collagen scaffolds. 

Most importantly though, the pieces of bone you want to fuse have to be held together, or else the ECM laid down will end up cartilaginous and not bony. Bone naturally grows stronger when under more load in accordance with Wolff's Law, so surgeons will usually force repaired bone edges together with their plates and screws as well to encourage healing.