Bone

A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the organs of the body, produce red and white blood cells, store minerals, help regulate acid-base homeostasis, provide structure and support for the body, and enable mobility and hearing. Bones come in a variety of shapes and sizes and have complex internal and external structures.

Bone tissue is a form of hard tissue, specialised connective tissue that is mineralized and has an intercellular honeycomb-like matrix. This matrix helps give the bone rigidity. Bone tissue is made up of different types of bone cells: osteoblasts and osteocytes (which form and mineralise bone), and osteoclasts (which resorb bone).

In the human body at birth, approximately 300 bones are present. Many of these fuse together during development, leaving a total of 206 separate bones in the adult, not counting numerous small sesamoid bones. The largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear.

Gross anatomy

The human body contains five types of bones: long, short, flat, irregular, and sesamoid. Long bones, like those in the limbs, fingers, and toes, are longer than they are wide and include areas called the diaphysis and epiphysis. Short bones, such as those in the wrist and ankle, are cube-shaped and provide stability. Flat bones, like those in the skull and sternum, are thin and curved. Sesamoid bones, found in tendons, include the patella and pisiform. Irregular bones, such as those in the spine and pelvis, don’t fit into the other categories and have complex shapes.

Bones serve many important functions. They form the skeleton, giving the body structure and support, and protect important organs like the brain and heart. Bones also help produce blood cells and store minerals like calcium and phosphorus. Additionally, bones play a role in maintaining the body’s balance of acids and bases and help regulate blood sugar levels.

Tissue

Bone is made up of a flexible material called matrix and minerals that give it strength. The matrix is mostly collagen, which helps bones bend without breaking. Minerals like calcium phosphate make bones hard and strong.

Bones have two main types of tissue: cortical bone on the outside, which is very dense and gives bones their solid appearance, and cancellous bone inside, which is more like a sponge. Cancellous bone helps with storing and sharing important minerals, especially calcium, and is where new blood cells are made.

Histology and physiology

Bone is made of living cells and minerals. Important cells include osteoblasts, which build bone, osteocytes, which help maintain bone, and osteoclasts, which break down old bone. These cells work together to keep bones strong and healthy.

Bones are made of a mix of organic materials, mostly type I collagen, and minerals like hydroxyapatite. Collagen gives bone flexibility, while minerals make it hard and strong. This combination helps bones support and protect the body.

Development

The formation of bone is called ossification. During the fetal stage of development this happens in two ways: intramembranous ossification and endochondral ossification. Intramembranous ossification makes flat bones like those in the skull from connective tissue. Endochondral ossification makes most other bones from cartilage.

Endochondral ossification starts with cartilage that turns into bone. It makes long bones and many other bones in the body. This process includes forming a cartilage model and then replacing it with bone. Bone development in childhood is very important. Regular physical activity, like running and jumping, helps bones grow strong and stay healthy.

Remodeling

Bone is always being made and replaced in a process called remodeling. This helps keep bones strong and repair small damages. Special cells called osteoblasts build bone, while osteoclasts break down old bone. This process is controlled by various signals and chemicals in the body.

Volume

Bone volume depends on how fast bone is formed and broken down. Growth factors, which are special substances, can affect bone formation. These factors help control the activity of cells that build bone, influencing how much bone is made in different areas.

Clinical significance

See also: Bone disease

Many diseases can affect bones, such as arthritis, fractures, infections, and osteoporosis. These conditions are treated by different kinds of doctors, including rheumatologists who care for joints, and orthopedic surgeons who fix broken bones. Other doctors like rehabilitation specialists, radiologists, and family doctors also help with bone health.

When someone visits a doctor for a bone problem, the doctor will first ask questions and examine the patient. Then, they often take pictures of the bones using radiography, which can include X-rays, CT scans, MRI scan, or a Bone scan. Sometimes, they might also take a small sample of fluid from around the joints.

Fractures

Fractures happen when a lot of force hits a bone, or when a bone is weakened by conditions like osteoporosis. Common fractures include wrist fractures and hip fractures. Doctors use X-rays, CT scans, and MRIs to see where the bone is broken and decide how to treat it. Treatment often involves pain relief and keeping the broken area still so it can heal. Sometimes surgery is needed to fix the bone in place.

Tumors

Tumors can grow in bones. Some tumors are harmless and do not spread, while others can be more serious. Examples of harmless tumors include osteoma and osteochondroma.

Cancer

Cancer can start in the bones or spread there from other parts of the body. When cancer spreads to bones, it can make the bones weaker and more likely to break. Treating bone cancer depends on the type and stage of the cancer and may include radiotherapy, chemotherapy, or other treatments.

Diabetes

Type 1 diabetes is when the body cannot make enough insulin, while type 2 diabetes is when the body does not use insulin well. Both types can affect bone health, making bones weaker and more likely to break. Research is ongoing to find better ways to protect bones in people with diabetes.

Osteoporosis

Main article: Osteoporosis

Osteoporosis is a condition where bones become weak and break more easily. It is common in older women, especially after menopause, but can affect anyone. Doctors can find osteoporosis using special X-ray machines. Treatment includes healthy eating, exercise, and sometimes medicine to strengthen the bones.

Bone health

Good bone health is important for staying strong and active. Eating well and exercising when you’re young can help build strong bones for life. Children and teens can improve their bone health by getting enough calcium and vitamin D and by being active.

Osteology

The study of bones and teeth is called osteology. Scientists use this knowledge in fields like anthropology, archeology, and forensic science to learn about people from the past. They can figure out things like a person's age, health, or ancestry by looking at their bones.

Anthropologists and archeologists also examine special bone tools created by ancient humans, including both Homo sapiens and Homo neanderthalensis.

Other animals

Main articles: Bird anatomy and Exoskeleton

Bird skeletons are very lightweight because their bones are smaller and thinner, which helps them fly. Many bird bones are hollow and have little marrow. A bird's beak is made of bone covered in a material called keratin. Some animals, like deer, have special bone structures such as antlers that sit outside their skin.

Certain marine animals, such as marine mammals and marine turtles, have different bone structures compared to humans. Some animals, especially herbivores, eat bones to get important minerals like phosphate. Many bone diseases that affect humans can also affect other animals.

Society and culture

Bones from animals have been used for many purposes throughout history. In prehistoric times, people made tools and art from bones, and today bones are still used to make items like buttons, beads, and handles. Bones can also be boiled to create a strong glue once important for many things but now mainly used in restoring antiques. Broth, a tasty soup, is often made by simmering bones with other ingredients.

Bones have also played special roles in different cultures. In ancient China, people wrote questions on bones and then burned them to find answers in the cracks. The wishbones of birds are sometimes used in traditions to decide who gets to make a wish. Some cultures believe pointing a bone at someone brings bad luck.

Additional images

Here are some detailed images that show different aspects of bones:

• Cells in bone marrow
• Scanning electron microscope of bone at 100× magnification
• Structure detail of an animal bone

These pictures help us see how bones are built and what they look like up close.