As we grow, various bones fuse together and as adults our body’s skeleton consists of 206 bones, various ligaments and cartilage.
Osteology, derived from the Greek word osteon, meaning bone, is the branch of anatomy that describes the structure of the skeleton. Our bones are what give the body its’ shape and support. Bones act as a shield to help protect delicate and vital internal parts like the heart, lungs, kidneys and brain from violent outside forces. Bones, joints and ligaments, working in cooperation with the muscles and tendons, make it possible for the body to move. Bones are the repositories for nearly all the body’s calcium. The red bone marrow manufactures blood cells.
There are four different types of bones where shape is the basis for categorization, flat, long, short and irregular. Flat bones are like those found in the skull, ribs, sternum and scapula. Long bones are the type found in the extremities, like the femur or thighbone in the leg or ulna in the arm. Short bones are found in the wrists and ankles and some of the irregular bones are found in the vertebrae and at the sutures in the skull.
When using origin as the category there are two types, membrane and cartilage. As the fetus develops, membrane bones develop directly from sheets of connective tissue membrane. These include some bones of the face and skull. Cartilage bones develop from what’s termed a temporary cartilage model, which is eventually replaced by bone tissue. Sesamoid bones, named as such because they resemble a sesame seed, develop in tendons or in capsules of joints.
When structure is the basis, there are two types of bone tissue. Compact bone is like its’ name implies, hard dense and composes the outer layer of all developed bones. Cancellous, or spongy, bone is contained inside the compact bone and allows the bone to not only be hard but somewhat flexible. Without the cancellous bone our bones would be very brittle. When steel is tempered the same practice of a softer inner part and a harder outer surface is employed to give the finished product, like a knife, the ability to bend without breaking. Some steel is made too brittle, like cheap drill bits, and even though they may be sharp, they break quite easily. Bone of the cancellous type is found in the long bones, sternum, vertebrae and between layers of the cranium bones.
A typical long bone has a diaphysis or shaft and two epiphyses or ends. The shaft is covered by a membrane called the periosteum and is what the muscles attach to. The shaft contains osteoblast, bone forming cells of the red bone marrow, and blood vessels that carry nutrient to the bone. The inside of the shaft, or medullary cavity, is lined with a membrane and contains a yellow bone marrow that’s made up primarily of fat cells. The interior of the epiphyses is where red blood cells are produced, called homopoiesis, in the red bone marrow.
If we were able to watch a bone develop and grow, we’d see that in the embryo the bone is completely cartilaginous and for each bone there is a temporary cartilage model. Red blood cells and osteoblasts, an osteoblast is a cell that’s responsible for the formation, or ossification, of bone, gradually change the model to permanent bone and periosteum, a layer of dense connective tissue that covers the bone surface, except at the joints also known as the articular surfaces. The outer layer of the periosteum is very dense and contains large numbers of blood vessels. The inner layer is more cellular, contains osteoblasts and fewer blood vessels. In the long bones the ends remain cartilaginous for years. Bone production begins first in the centers of the bone and then in the ends. Even after the ends are completely formed a thin layer of cartilage remains between the end and the shaft. As the growth in length continues, thickness is achieved as the osteoblasts deposit new bone tissue in the periosteum. Sometime between puberty and the 25th year, final bony fusion of all separate parts occurs ending the growth process. A large part of the bone formed during the growth period is destroyed in the growth process. The destruction is performed by specialized cells called osteoclasts