Despite first impressions, bones are living, active tissues that are constantly being remodeled.
Bones have many functions. They support the body structurally, protect our vital organs, and allow us to move. Also, they provide an environment for bone marrow, where the blood cells are created, and they act as a storage area for minerals, particularly calcium.
At birth, we have around 270 soft bones. As we grow, some of these fuse. Once we reach adulthood, we have 206 bones.
The largest bone in the human body is the thighbone or femur, and the smallest is the stapes in the middle ear, which are just 3 millimeters (mm) long.
Bones are mostly made of the protein collagen, which forms a soft framework. The mineral calcium phosphate hardens this framework, giving it strength. More than 99 percent of our body's calcium is held in our bones and teeth.
Bones have an internal structure similar to a honeycomb, which makes them rigid yet relatively light.
The structure of bones
Bones are composed of two types of tissue:
1. Compact (cortical) bone: A hard outer layer that is dense, strong, and durable. It makes up around 80 percent of adult bone mass.
2. Cancellous (trabecular or spongy) bone: This consists of a network of trabeculae or rod-like structures. It is lighter, less dense, and more flexible than compact bone.
Also found in bones:
- osteoblasts and osteocytes, responsible for creating bone
- osteoclasts or bone resorbing cells
- osteoid, a mix of collagen and other proteins
- inorganic mineral salts within the matrix
- nerves and blood vessels
- bone marrow
- membranes, including the endosteum and periosteum
Bones are not a static tissue but need to be constantly maintained and remodeled. There are three main cell types involved in this process.
Osteoblasts: These are responsible for making new bone and repairing older bone. Osteoblasts produce a protein mixture called osteoid, which is mineralized and becomes bone. They also manufacture hormones, including prostaglandins.
Osteocytes: These are inactive osteoblasts that have become trapped in the bone that they have created. They maintain connections to other osteocytes and osteoblasts. They are important for communication within bone tissue.
Osteoclasts: These are large cells with more than one nucleus. Their job is to break down bone. They release enzymes and acids to dissolve minerals in bone and digest them. This process is called resorption. Osteoclasts help remodel injured bones and create pathways for nerves and blood vessels to travel through.
Bone marrow is found in almost all bones where cancellous bone is present.
The marrow is responsible for making around 2 million red blood cells every second. It also produces lymphocytes or the white blood cells involved in the immune response.
Bones are essentially living cells embedded in a mineral-based organic matrix. This extracellular matrix is made of:
Organic components, being mostly type 1 collagen.
Inorganic components, including hydroxyapatite and other salts, such as calcium and phosphate.
Collagen gives bone its tensile strength, namely the resistance to being pulled apart. Hydroxyapatite gives the bones compressive strength or resistance to being compressed.
What do bones do?
Bones serve several vital functions:
Bones provide a frame to support the body. Muscles, tendons, and ligaments attach to bones. Without anchoring to bones, muscles could not move the body.
Some bones protect the body's internal organs. For instance, the skull protects the brain, and the ribs protect the heart and lungs.
Cancellous bone produces red blood cells, platelets, and white blood cells. Also, defective and old red blood cells are destroyed in bone marrow.
Electron microscope image of trabecular bone (x100 magnification).
Image credit: Sbertazzo
Storing minerals: Bones act as a reserve for minerals, particularly calcium and phosphorous.
They also store some growth factors, such as insulin-like growth factor.
Fat storage: Fatty acids can be stored in the bone marrow adipose tissue.
pH balance: Bones can release or absorb alkaline salts, helping blood to stay at the right pH level.
Detoxification: Bones can absorb heavy metals and other toxic elements from the blood.
Endocrine function: Bones release hormones that act on the kidneys and influence blood sugar regulation and fat deposition.
Calcium balance: Bones can raise or reduce calcium in the blood by forming bone, or breaking it down in a process called resorption.
Types of bone
There are five types of bones in the human body:
Long bones: These are mostly compacted bone with little marrow and include most of the bones in the limbs. These bones tend to support weight and help movement.
Short bones: Only a thin layer of compact bone, these include bones of the wrist and ankle.
Flat bones: Usually bones that are thin and curved. They consist of two outer layers of compact bone and an inner layer of spongy bone. Flat bones include most of the bones of the skull and the sternum or breastbone. They tend to have a protective role.
Sesamoid bones: These are embedded in tendons, such as the patella or kneecap. They protect tendons from wear and stress.
Irregular bones: As their name implies, these are bones that do not fit into the first four categories and are an unusual shape. They include the bones of the spine and pelvis. They are often protecting organs or tissues.
Bones of the skeleton are split into two groups:
Appendicular skeleton — bones of the limbs, shoulders, and pelvic girdle.
Axial skeleton — bones of the skull, vertebral column, thoracic cage.
Bone is constantly under construction.
Bone is always being remodeled. This is a two-part process:
1. Resorption when osteoclasts break down and remove bone.
2. Formation when new bone tissue is laid down.
An estimated 10 percent of an adult's skeleton is replaced each year.
Remodeling allows the body to fix damaged sections, reshape the skeleton during growth, and regulate calcium levels.
If one part of the skeleton is put under increased stress over time, for instance, during sport or exercise, the sections of bone under most pressure will become thicker in response.
What is osteoporosis?
Osteoporosis is a bone disease where there is a reduction in bone mineral density. This increases the risk of fractures occurring. Osteoporosis is most common in women after the menopause. However, it can happen in premenopausal women and men.
Osteoporosis occurs either when removal or resorption of bone happens too quickly, new bone is formed too slowly, or for both reasons. It can be caused by having inadequate calcium, a vitamin D deficiency, consuming excessive alcohol, or smoking tobacco.
In a nutshell
Although they get less attention than other body parts, bones are more than just a protective scaffold on which the human body is built.
Bones also maintain appropriate levels of many compounds and regulate hormonal pathways. Bones are the unsung heroes of anatomy.