Image from Wikimedia Commons
As we approach Halloween, one part of the human anatomy will begin to get more attention than any other: the skeleton! Though often thought of as a passive system in the body, the skeleton is far from a non-living scaffold of mineral deposits. Osteocytes, the cells encased in bone, actively regulate bone structure and density through complex signaling pathways that affect their extracellular matrix. Though containing organic fibers like collagen, the bone matrix is made up largely of inorganic deposits of hydroxyapatite, a mineral consisting primarily of calcium and phosphate. This mixture of organic and inorganic components allows our skeleton to be strong and also grow and change with the rest of the body. When an organism with an endoskeleton dies, these inorganic mineral deposits don’t decompose, meaning skeletons are often used to symbolize death.
Skeletons, however, do a lot more than just offer a convenient choice of decoration as we approach October 31. Because bone structure is regulated by metabolically-active cells, diet and exercise can make a large impact on bone health. Osteocytes in bone are connected by “canals” of fluid in a structure called the lacunocanalicular matrix. The lacunocanalicular matrix functions primarily for the diffusion of nutrients and signaling molecules between osteocytes, and it has recently been suggested that fluid flow in the lacunocanalicular matrix may have effects as diverse as promoting the production of adenosine triphosphate, or ATP; releasing prostaglandins, a type of signaling molecule; extending dendrites, the connections between bone cells; and preventing apoptosis, or programmed cell death. This kind of fluid flow is promoted by weight-bearing exercise, and this may explain why exercise leads to a healthier skeleton. Diet can also play a role in bone health. Making sure we have enough calcium and vitamin D3 in our diets is imperative to keeping a healthy skeleton, and there is currently a lot of debate over whether the type and amount of simple sugars consumed can affect the functioning of osteocytes. Though our body naturally synthesizes vitamin D3 when exposed to sunlight, it may be a good idea to take vitamin D3 supplements during the winter months when it is difficult to spend time outside, or if you happen to be a vampire. Bone health has been negatively impacted in poor urban environments where people often spend little time outside and have diets low in calcium.
Just like any undead minion worth its salt, our skeletons are also good at dealing with damage. Damage to bone tissue causes the expression of proteins that cause apoptosis, or programmed cell death, while osteocytes near the apoptotic cells express antiapoptotic molecules to protect them from unnecessary death. While it may seem counterintuitive that the cells effectively commit suicide to deal with damage, the death of these cells allows for the resorption of the raw materials in the damaged area. These apoptotic cells also express elevated levels of RANK ligand, a molecule responsible for the movement of osteoclasts to the damaged area. Osteoclasts help clear the mineral deposits from the damaged area so the bone can be repaired. After the area of injury is clear, osteoblasts, the cells that form new bone, will begin to form new matrix in the area, and, after becoming encased in new bone, mature into osteocytes. The scientifically valid lesson to be learned from this information: if you are given the choice between fighting ghosts or skeletons, choose ghosts.