Bone

Bones form the skeletal system of vertebrates, wherein they serve several important functions including being the points of attachment for muscle movement, protection of important organs, a source of blood cell formation, and calcium storage.

The living bones in our bodies use oxygen and give off waste products in metabolism. They contain active tissues that consume nutrients, require a blood supply and change shape or remodel in response to variations in mechanical stress.

Functions

Bones work together with muscles as simple mechanical lever systems to produce body movement. The skeleton supports the body against the pull of gravity. The large bones of the lower limbs support the trunk when standing.

Bones provide a rigid framework that support and protect the soft organs of the body. The fused bones of the cranium surround the brain to make it less vulnerable to injury. Vertebrae surround and protect the spinal cord and bones of the rib cage help protect the heart and lungs of the thorax.

Bones contain more calcium than any other organ. The intercellular matrix of bone contains large amounts of calcium salts, the most important being calcium phosphate. When blood calcium levels decrease below normal, calcium is released from the bones so that there will be an adequate supply for metabolic needs. When blood calcium levels are increased, the excess calcium is stored in the bone matrix. The dynamic process of releasing and storing calcium goes on almost continuously.

Hematopoiesis, the formation of blood cells, mostly takes place in the red marrow of the bones. In infants, red marrow is found in the bone cavities. With age, it is largely replaced by yellow marrow for fat storage. In adults, red marrow is limited to the spongy bone in the skull, ribs, sternum, clavicles, vertebrae and pelvis. Red marrow functions in the formation of red blood cells, white blood cells and blood platelets.

Structure

Diagram of the spongy and compact layers of the bone

There are two types of bone tissue: compact and spongy. The names imply that the two types differ in density, or how tightly the tissue is packed together. There are three types of cells that contribute to bone homeostasis. Osteoblasts are bone-forming cell, osteoclasts resorb or break down bone, and osteocytes are mature bone cells. An equilibrium between osteoblasts and osteoclasts maintains bone tissue. Bone is made of different types tissues including the outer layer of the bone, the compact middle layer, and the spongy third layer.

Periosteum The periosteum is hard and smooth. It provides protection to the rest of the layers of the bone. It is almost like the "skin" of the bones. The periosteum provides a site for the attachment of muscles, and also contains nerves, blood vessels, and lymphatic vessels.[1]

Compact The compact bone is extremely dense and surrounds the spongy bone. The compact bone also contains many Haversian canals, which also contain nerves and blood vessels. This layer of bone gives the skeleton its strength and allows it to support your bodies mass.[2]

Spongy The spongy bone is where all of the blood cells are created from the bone marrow. Spongy bone contains two different types of bone marrow: red and yellow marrow. The red bone marrow produces red and white blood cells and platelets. The yellow bone marrow, however, is used to store fats.[3]

Types

The bones of the body come in a variety of sizes and shapes. The four principal types of bones are long, short, flat and irregular.

Long Bones
Bones that are longer than they are wide are called long bones. They consist of a long shaft with two bulky ends or extremities. They are primarily compact bone but may have a large amount of spongy bone at the ends or extremities. Long bones include bones of the thigh, leg, arm, and forearm.

Short Bones
Short bones are roughly cube shaped with vertical and horizontal dimensions approximately equal. They consist primarily of spongy bone, which is covered by a thin layer of compact bone. Short bones include the bones of the wrist and ankle.

Flat Bones
Flat bones are thin, flattened, and usually curved. Most of the bones of the cranium are flat bones.

Irregular Bones
Bones that are not in any of the above three categories are classified as irregular bones. They are primarily spongy bone that is covered with a thin layer of compact bone. The vertebrae and some of the bones in the skull are irregular bones.

All bones have surface markings and characteristics that make a specific bone unique. There are holes, depressions, smooth facets, lines, projections and other markings. These usually represent passageways for vessels and nerves, points of articulation with other bones or points of attachment for tendons and ligaments.

Growth

Bones grow in length at the epiphyseal plate by a process that is similar to endochondral ossification. The cartilage in the region of the epiphyseal plate next to the epiphysis continues to grow by mitosis. The chondrocytes, in the region next to the diaphysis, age and degenerate. Osteoblasts move in and ossify the matrix to form bone. This process continues throughout childhood and the adolescent years until the cartilage growth slows and finally stops. When cartilage growth ceases, usually in the early twenties, the epiphyseal plate completely ossifies so that only a thin epiphyseal line remains and the bones can no longer grow in length. Bone growth is under the influence of growth hormone from the anterior pituitary gland and sex hormones from the ovaries and testes.

Even though bones stop growing in length in early adulthood, they can continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or to weight. The increase in diameter is called appositional growth. Osteoblasts in the periosteum form compact bone around the external bone surface. At the same time, osteoclasts in the endosteum break down bone on the internal bone surface, around the medullary cavity. These two processes together increase the diameter of the bone and, at the same time, keep the bone from becoming excessively heavy and bulky.

Development

The terms osteogenesis and ossification are often used synonymously to indicate the process of bone formation. Parts of the skeleton form during the first few weeks after conception. By the end of the eighth week after conception, the skeletal pattern is formed in cartilage and connective tissue membranes and ossification begins.

Bone development continues throughout adulthood. Even after adult stature is attained, bone development continues for repair of fractures and for remodeling to meet changing lifestyles. Osteoblasts, osteocytes and osteoclasts are the three cell types involved in the development, growth and remodeling of bones. Osteoblasts are bone-forming cells, osteocytes are mature bone cells and osteoclasts break down and reabsorb bone.

There are two types of ossification: intramembranous and endochondral.

Repair

Main Article: Bone fracture

Your bones are tough stuff - but even tough stuff can break. Like a wooden pencil, bones will bend under strain. Most fractures occur in the upper extremities: the wrist, the forearm, and above the elbow. Also if the pressure is too much, or too sudden, bones can snap.

Bones can fracture in a number of different ways. A fracture may be a straight break across the bone (transverse fracture), slanting (oblique fracture) or winding (spiral fracture). The break may run along the shaft of the bone (longitudinal fracture), or the bone may be shattered into pieces (comminuted fracture). Young bone is softer and more able to bend than adult bone, so children's bones often fracture on one side but bend on the other - known as a greenstick fracture.[4]

There are many different types of fracture, the most common of which is a simple fracture, when a bone breaks cleanly. If you put out your arms when you fall, you could end up with an impacted fracture, where the ends of two bones are forced into one another. A sharp sudden twist of a bone in a game of football could result in a jagged spiral fracture. Road accidents often cause comminuted fractures, where a bone breaks into fragments, or compression fractures, where a bone is crushed.[5]