Bone facts for kids
Bones are parts of the human or animal skeleton of vertebrates. They are stiff organs which make up the endoskeleton. Our body is made of flesh and blood built on a framework of bones. Without our bones, we could not stand up or move about. They also protect organs inside our body. The skull protects the brain and the ribs protect the heart and lungs. The jaw and cheekbones support the facial muscles, which help us smile. The number of bones in a human's neck is the same as a giraffe's.
Contents
What they do
The bones are the framework of the body. Without them we would be a pile of organs on the ground and would not be able to move. The bones are also very important for our protection, that is, the rib cage protects the heart and lungs, cranium (skull) protects the brain, pelvis protects the reproductive organs, vertebrae protect the spinal cord. Bones need to be maintained by taking regular exercise and by having lots of calcium from foods like milk, and dark leafy greens such as spinach. The red bone marrow in the middle of the bigger bones is what makes the red blood cells that we need.
Structure
Quick facts for kids Osteon |
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| Diagram of compact bone from a transverse section of a long bone's cortex. | |
| Latin | Osteon |
| Gray's | subject #18 89 |
Bone is not uniformly solid. It consists of a flexible matrix (about 30%) and bound minerals (about 70%). Bone matrix is 90 to 95% composed of elastic collagen fibers, also known as ossein, and the remainder is ground substance.
The hard outer layer of bones is composed of cortical bone, which is also called compact bone. It forms the hard exterior (cortex) of bones. The cortical bone gives bone its smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult human skeleton. It facilitates bone's main functions—to support the whole body, to protect organs, to provide levers for movement, and to store and release chemical elements, mainly calcium. It consists of multiple microscopic columns, each called an osteon. Each column is multiple layers of osteoblasts and osteocytes, which are specialized bone cells. Osteoblasts are involved in the creation and mineralization of bone tissue while osteocytes and osteoclasts are involved in the reabsorption of bone tissue.
Cancellous bone or spongy bone, also known as trabecular bone, is the internal tissue of the skeletal bone and is an open cell porous network. Cancellous bone is typically found at the ends of long bones, near joints, and in the interior of vertebrae. Cancellous bone often contains red bone marrow. Bone marrow produces blood cells for the blood system, and lymphocytes for the immune system. Within the marrow of the bone there are also hematopoietic stem cells. These cells give rise to other cells, including white blood cells, red blood cells, and platelets.
Types
Five types of bones are found in the human body: long, short, flat, irregular, and sesamoid.
- Long bones are characterized by a shaft, the diaphysis, that is much longer than its width; and by an epiphysis, a rounded head at each end of the shaft. They are made up mostly of compact bone, with lesser amounts of marrow, located within the medullary cavity, and areas of spongy, cancellous bone at the ends of the bones. Most bones of the limbs, including those of the fingers and toes, are long bones. The exceptions are the eight carpal bones of the wrist, the seven articulating tarsal bones of the ankle and the sesamoid bone of the kneecap. Long bones such as the clavicle, that have a differently shaped shaft or ends are also called modified long bones.
- Short bones are roughly cube-shaped, and have only a thin layer of compact bone surrounding a spongy interior. Short bones provide stability and support as well as some limited motion. The bones of the wrist and ankle are short bones.
- Flat bones are thin and generally curved, with two parallel layers of compact bone sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones, as is the sternum.
- Sesamoid bones are bones embedded in tendons. Since they act to hold the tendon further away from the joint, the angle of the tendon is increased and thus the leverage of the muscle is increased. Examples of sesamoid bones are the patella and the pisiform.
- Irregular bones do not fit into the above categories. They consist of thin layers of compact bone surrounding a spongy interior. As implied by the name, their shapes are irregular and complicated. Often this irregular shape is due to their many centers of ossification or because they contain bony sinuses. The bones of the spine, pelvis, and some bones of the skull are irregular bones. Examples include the ethmoid and sphenoid bones.
Development
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The formation of bone is called ossification. During the prenatal development this occurs by two processes: intramembranous ossification and endochondral ossification. Intramembranous ossification involves the formation of bone from connective tissue whereas endochondral ossification involves the formation of bone from cartilage.
Intramembranous ossification mainly occurs during formation of the flat bones of the skull but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as mesenchyme tissue rather than from cartilage. The process includes: the development of the ossification center, calcification, trabeculae formation and the development of the periosteum.
Endochondral ossification occurs in long bones and most other bones in the body; it involves the development of bone from cartilage. This process includes the development of a cartilage model, its growth and development, development of the primary and secondary ossification centers, and the formation of articular cartilage and the epiphyseal plates.
Fractures
In normal bone, fractures occur when there is significant force applied, or repetitive trauma over a long time. Fractures can also occur when a bone is weakened, such as with osteoporosis, or when there is a structural problem, such as when the bone remodels excessively (such as Paget's disease) or is the site of the growth of cancer. Common fractures include wrist fractures and hip fractures, associated with osteoporosis, vertebral fractures associated with high-energy trauma and cancer, and fractures of long-bones. Not all fractures are painful. When serious, depending on the fractures type and location, complications may include flail chest, compartment syndromes or fat embolism. Compound fractures involve the bone's penetration through the skin. Some complex fractures can be treated by the use of bone grafting procedures that replace missing bone portions.
Fractures and their underlying causes can be investigated by X-rays, CT scans and MRIs.
Interesting facts about bones
- 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.
- The Greek word for bone is ὀστέον ("osteon"), hence the many terms that use it as a prefix—such as osteopathy.
- The largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear.
- Many animals, particularly herbivores, practice osteophagy—the eating of bones. This is presumably carried out in order to replenish lacking phosphate.
Images for kids
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Micrograph of cancellous bone
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Bone cells
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Skeletal System of Human Body
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Transmission electron micrograph of decalcified woven bone matrix displaying characteristic irregular orientation of collagen fibers
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Structure of a long bone
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Reduced bone mineral density in Osteoporosis (R), increasing the likelihood of fractures
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Human femurs and humerus from Roman period, with evidence of healed fractures
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Skeletal fluorosis in a cow's leg, due to industrial contamination
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Leg and pelvic girdle bones of bird
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Structure detail of an animal bone
See also
In Spanish: Hueso para niños
