From bone remodeling to function and anatomy, we shine a light upon the complex phenomenon that is bone.
Bone is the complex tissue that makes up the human skeleton and is primarily made up of collagen. Collagen is a protein with a soft framework which binds to calcium phosphate for further strength and solidity.
Bone is made up of two different types of living, growing tissue. The outer and inner layers of bone differ in terms of their density. The inside of bone is almost hollow and made up of marrow. This ensures the skeleton is both strong and light.
Bones also contain other proteins, vitamins, minerals, nerves, and blood vessels.
Fun Fact – 99% of the body’s calcium is in the bones and teeth!
Cortical (compact) bone is the dense, hard tissue forming the outer shell. This makes up 80% of total bone in the human body. The cortical bone determines the shape of the bone, alongside providing strength and allowing firm attachments of tendons.¹
Trabecular (cancellous) bone is the spongy interior in which the cortical bone surrounds. This softer tissue usually contains bone marrow, which produces red and white blood cells, and platelets. This makes up just 20% of total bone, however it’s surface area is much greater than cortical bone and is made up of a honeycomb-like structure.
While all bones are made up of this structure, they are not all identical. Bones can differ in shape, size and function. There are 5 main types of bones in the human body:
Long bones are longer in length than width and are primarily made from compact bone. They usually have epiphysis (growth plates) at either end of the bone, protected by cartilage. Most bones in the arms and legs are long bones. The longest bone in the human body is the femur (thighbone)!
Short bones are cube-shaped, roughly as wide and thick as they are long. They are often found in joints such as the wrists or ankles, providing stability. These bones contain relatively large quantities of bone marrow.
Flat bones are thin and often curved, providing protection to vital organs. They are usually very strong and a base for muscular attachment. The shoulder blades, hips, breastbone, and skull are all examples of flat bones.
Sesamoid bones are embedded in tendons, protecting them from wear. One example is the kneecap.
Irregular bones have more complex shapes and structures, such as the spine or pelvis. They consist mainly of trabecular bone, with only a thin layer of cortical bone. These bones do not fit into any of the other categories.
As strange as it may seem, our bones are constantly regenerating to replace old bone tissue with new bone tissue. This process is called bone remodeling.²
Bone remodeling is the outcome of two opposite processes:
Osteoclasts break down and digest old, mineralized bone through resorption. They emit hydrochloric acid to strip minerals such as calcium from the bone.
Osteoblasts produce new bone by creating a protein (osteoid) which becomes bone. Bone cells and blood vessels deposit minerals such as calcium in the osteoid.
These two cells work together to help maintain your bone density.
Bone remodeling is crucial in preventing the build-up of old bone and creating new bone to fit your changing mechanical needs.³ This process can also help fix broken bones and renew damaged tissue. This 4–8 month cycle repeats continuously throughout our lives, with around a fifth of all your bone tissue replaced each year. Infact, depending on your age, every 5-10 years all your bodies bones are completely replaced!
From around the age of 30, the osteoblasts slow down and are unable to create new bone as quickly as osteoclasts digest old bone. This causes the bone remodeling process to become imbalanced and bone mass begins to decline.
Osteoporosis is a prevalent bone disease partially caused by an imbalance in the bone remodeling process. When new bone is not created as quickly as old is broken down, bone density and mass start to decline. Overtime, this can lead to osteopenia and osteoporosis.
Osteopenia is often seen as the stage before osteoporosis where bone mass and bone mineral density are below average, but not yet osteoporotic.
Humans are born with around 270 bones. However, as we age many of our bones fuse together. This forms the human adult skeleton which only has 206 bones.
Bones stop growing in length during adolescence, but bone mass and mineral density keep changing throughout your lifetime.
Bone mass and mineral density continue to increase until they reach their peak, usually between the ages of 25-30. After this point, bone mass and mineral density will slowly start to decrease.
Alongside age, there are many other risk factors which can also impact bone health, such as hormone changes during menopause, or a family history of osteoporosis. If you have any concerns about your bone health, make sure you talk to your GP.
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