This unit, we dragged the skeleton out of the (supply) closet and learned all about bones! The skeleton is actually an organ system called the skeletal system, and while at first, it may seem like bones are relatively inanimate, bones are actually constantly undergoing change and vital processes.
The different types of bone cells. Image courtesy of Wikipedia Commons.
Let us first examine the anatomy of a bone on a microscopic level. Within the bone, there are osteogenic cells, and conveniently, "Osteo" means "Bone" in Greek. Obviously, our bones don't stay the same size throughout our entire life, otherwise, we would remain baby-sized forever. We actually undergo a process called ossification, or osteogenesis, in order to remodel and grow our bones. This process takes place from fetal development to around 25 years old, when our bones are completely ossified. This utilizes two types of cells: the osteoblasts and osteoclasts. Osteoblasts are the bone-building cells, and osteoclasts are the bone-destroying cells. Together, they work in harmony to remodel the bone in a constant state of flux. As old bone is destroyed, new bone is formed, keeping our bones nice and fresh. In fact, we create an entirely new skeleton approximately every 7 years! Mature bone cells are called osteocytes, and they conduct the daily processes of bones to maintain homeostasis. Bones have a compact bone exterior that is hard and dense, and a spongy bone interior that is more porous and filled with bone marrow/blood vessels. Through the Haversian Systems, osteons, or cylindrical canals, contain the bone's blood supplies. They are surrounded by lamellae which run perpendicularly to the osteons in order to provide more sturdiness. The lacunae are small spaces in the bone that contain the previously mentioned osteocytes.
Now we will look at bones on a macroscopic level, as well as discuss the different types of classification of bones. Bones can be classified into 4 different groups: long, short, flat, or irregular. Long bones are longer than they are wide, like the femur. Short bones are cuboidal shaped, like the carpals in your hand. Flat bones are, well, flat, like the ribs or the sternum. Lastly, irregular bones are the bones that don't fit into any other category, like the odd-shaped pelvis. In the skeleton, there are also joints that allow for movement in our bodies. Fibrous joints are surrounded by dense connective tissue and are synarthrosis, or immovable. An example would be the sutures of your skull. Cartilaginous joints are surrounded by cartilage and are amphiarthrosis, or allow for little movement. Synovial joints are the joints we typically think of, and they are surrounded by a joint capsule containing synovial fluid. They are diarthrosis, or fully moveable, and include joints like your elbows. To see a fun activity where we examined and classified bones on our own of rodents, refer to the Owl Pellet Lab.
In this lab, Michelle and I analyzed an owl pellet, which is the regurgitated remains of a barn owl's meal (a yummy rodent or bird) that could not be digested. When we first unwrapped the pellet, it looked like a lump of fur. It felt like I was petting my old pet hamster. After we began examining the inside and carefully crumbling away the dark fur parts to expose the bone, it was evident that we had multiple rodents in our pellet, as shown by the 11 humerus and femur bones we found. We believed we had vole bones in our pellet. Our reasoning for this was: the shape of the humerus bones, which were long but thin and had the defining triangle spike on the side; the shape of the tibias and fibulas, which were fused in our case; and most notably the shape of the skull pieces, which had the large sharp front teeth and the tiny back teeth, as well as the deeper eye socket. These characteristics clearly point to a vole, as the other rodents ehad differently shaped humerus bones (the shrew's lacked the spike on the side and the mole's was vaguely circular), fibulas and tibias, or skulls (the shrew had an indiscernible eye socket, and the mole's was also more shallow).
The bones we found had some similarities and differences to a human skeleton. The human humerus is also in the shape of a long line. Both the vole and the human humerus has two wider epiphyses at each end of it. The skull also has the same basic features, such as an eye socket, the sharp teeth like the canine teeth in humans, and the flat molars near the back of the jaw. However, the vole has the distinct triangle on the side of the humerus, which humans lack. The radius and ulna are also very different in the vole, with the ulna being remarkably thin in comparison to the radius. (Maybe vice versa.) The tibia and fibula were also fused in the vole and not in a human.
