Bone
Gopla Grove, Chinese Psycology Online, http://www.zgxl.org

Bone Development and Growth

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.

Intramembranous

Intramembranous ossification involves the replacement of sheet-like connective tissue membranes with bony tissue. Bones formed in this manner are called intramembranous bones. They include certain flat bones of the skull and some of the irregular bones. The future bones are first formed as connective tissue membranes. Osteoblasts migrate to the membranes and deposit bony matrix around themselves. When the osteoblasts are surrounded by matrix they are called osteocytes.

Endochondral Ossification

Endochondral ossification involves the replacement of hyaline cartilage with bony tissue. Most of the bones of the skeleton are formed in this manner. These bones are called endochondral bones. In this process, the future bones are first formed as hyaline cartilage models. During the third month after conception, the perichondrium that surrounds the hyaline cartilage "models" becomes infiltrated with blood vessels and osteoblasts and changes into a periosteum. The osteoblasts form a collar of compact bone around the diaphysis. At the same time, the cartilage in the center of the diaphysis begins to disintegrate. Osteoblasts penetrate the disintegrating cartilage and replace it with spongy bone. This forms a primary ossification center. Ossification continues from this center toward the ends of the bones. After spongy bone is formed in the diaphysis, osteoclasts break down the newly formed bone to open up the medullary cavity.

The cartilage in the epiphyses continues to grow so the developing bone increases in length. Later, usually after birth, secondary ossification centers form in the epiphyses. Ossification in the epiphyses is similar to that in the diaphysis except that the spongy bone is retained instead of being broken down to form a medullary cavity. When secondary ossification is complete, the hyaline cartilage is totally replaced by bone except in two areas. A region of hyaline cartilage remains over the surface of the epiphysis as the articular cartilage and another area of cartilage remains between the epiphysis and diaphysis. This is the epiphyseal plate or growth region.

Bone 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.

Classification of Bones

Long Bones 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. 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.

Divisions of the Skeleton

The adult human skeleton usually consists of 206 named bones. These bones can be grouped in two divisions: axial skeleton and appendicular skeleton. The 80 bones of the axial skeleton form the vertical axis of the body. They include the bones of the head, vertebral column, ribs and breastbone or sternum. The appendicular skeleton consists of 126 bones and includes the free appendages and their attachments to the axial skeleton. The free appendages are the upper and lower extremities, or limbs, and their attachments which are called girdles. The named bones of the body are listed below by category.

Axial Skeleton (80 bones)

Skull (28)

Cranial Bones

Facial Bones

Auditory Ossicles

Hyoid (1)

Vetebral Column

Thoracic Cage

Appendicular Skeleton (126 bones)

Pectoral girdles

Upper Extremity

Pelvic Girdle

Lower Extremity

Referrence:Bone(Chinese Version)

Bones, Muscles, and Joints (KidsHealth-Teens-Your Body-Body Basics Library http://kidshealth.org/index.html)

Without our bones, we couldn't stand, walk, run, or even sit. Our bones provide support for our bodies and help form our shape. They protect our organs against injury and enable us to move. They're strong enough to support our entire weight, yet are very lightweight themselves. Some bones protect other parts of the body, as the skull protects the brain, and others work together to make body parts move, such as the arm and leg bones.

Joints occur where two bones meet. They make the skeleton flexible - without them, movement would be impossible. Muscles are also crucial for movement: they're the tough, elastic tissues that pull our bones when we move. Working together, bones, joints, and muscles enable us to do all the physical activities we do every day.

How Are Bones, Muscles, and Joints Important for Living?

From our head to our toes, our skeleton provides support and protection. The skull protects the brain and forms the shape of our face. The spinal cord, which is a crucial pathway for the flow of messages between the brain and the body, is protected by the backbone, or spinal column. The ribs form a cage that shelters the heart, lungs, liver, and spleen, and the pelvis helps protect the bladder, intestines, and in girls and women, the reproductive organs.

Bones contain large amounts of calcium, phosphorus, and sodium, and smaller amounts of other minerals. Calcium makes your bones hard, so they can protect you and support your weight. Extra calcium is stored in your bones, and when your body needs calcium, the bones release it. The amount of vitamins and minerals, especially calcium itself, that you eat directly affects how much calcium is stored in your bones.

Bones also have another critical function: they contain red bone marrow that makes most of the blood cells that flow through our bodies. This marrow contains special cells called stem cells, which produce the body's red blood cells - about 200 billion every day. When you lose blood, the red blood cells, which carry oxygen throughout your body, are depleted. But chemical messages in the blood signal your bone marrow to increase its production of red blood cells to replace those that were lost. Many white blood cells (which help fight infection) and platelets (which help with clotting) are also formed in the marrow.

That's why bone marrow can be used to treat certain diseases of the blood or immune system, such as leukemia and immunodeficiency disorders. When a person's white blood cells are not working properly or have been destroyed as part of treatment to cure cancer, doctors can remove bone marrow from a healthy person who is a "genetic match" and transplant it into the person who is sick. If the process works, the white blood cells in the healthy bone marrow produce normal cells to replace those that are defective or have been destroyed.

When it comes to moving, bones need a little help from our joints and muscles. Joints allow bones to move as muscles pull them. Some open and close like a hinge (such as fingers and elbows), whereas others allow for more complicated movement. A shoulder or hip joint, for example, allows for backward, forward, sideways, and rotating movement.

Muscles also help to support many functions that are necessary for growth and maintaining a strong body. They move the jaw so we can chew food, and they move food through the digestive system. Muscles in the heart drive circulation, and those in the chest make it possible for us to breathe. When we smile and talk, muscles help us communicate, and when we head to the gym, they help us exercise to stay physically fit and healthy.

Basic Anatomy

The human skeleton has 206 bones. About 70% of an adult's bones are composed of minerals; the remaining 30% is organic matter, mainly the protein collagen.

There are two forms of bone - compact and cancellous. Compact bone is the solid, hard outside part of bone. It looks like ivory and is very strong - six times stronger than a steel bar of the same weight. Holes and channels run through it, carrying blood vessels and nerves to its inner parts.

Cancellous bone, which looks spongy, is inside the compact bone. It is made up of a meshlike network of tiny pieces of bone called trabeculae (pronounced: tru-beh-cue-lay). The spaces in this network are filled with red marrow, found mainly at the ends of bones, and yellow marrow, which is mostly fat.

The human body has more than 650 muscles, which make up half of a person's body weight. They are connected to bones by tough, cordlike tissues called tendons, which allow the muscles to pull on bones. The narrow bands on the top of your hands are tendons; you can see them lengthen and shorten as they pull on your fingers to make them move. Bones are fastened to other bones by long, fibrous straps called ligaments, which wrap around the joints. Cartilage is a flexible, rubbery substance that supports bones and protects them when they rub against each other. Together, bones, muscles, tendons, and ligaments form the musculoskeletal (pronounced: mus-cue-low-skel-uh-tull) system.

Humans have three different kinds of muscle - skeletal, smooth, and cardiac. Skeletal muscles are attached to bone, mostly in the legs, arms, abdomen, chest, neck, and face. These muscles are called striated because they are made up of fibers that have horizontal stripes. They hold the skeleton together, give the body shape, and help it with everyday movements. Skeletal muscles can contract (shorten or tighten) quickly and powerfully, but they get tired easily and have to rest between workouts. Their size varies greatly depending on the job they do, and they are known as voluntary muscles because you can control their movement.

Smooth, or involuntary, muscles are also made of fibers, but they look smooth. Generally, we can't consciously control these muscles; rather, they're controlled by the nervous system automatically. The walls of the stomach and intestines, which help break up food, contain smooth muscle. Smooth muscle is also in the walls of blood vessels, which squeezes blood through the vessels and helps maintain blood pressure. Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don't get tired easily.

Cardiac muscle is found in the heart. The walls of the heart's chambers, the ventricles and atria, are composed almost entirely of muscle fibers. Cardiac muscle is also an involuntary type of muscle, because, for the most part, we can't consciously control it. Its quick, powerful contractions force blood out of the heart, producing the heartbeat.

Joints are classified by how much the bones they connect can move against one another. Immovable, or fibrous, joints don't move. The dome of the skull, for example, is made of bony plates, which must be immovable to protect the brain. Between the edges of these plates are links, or joints, of fibrous tissue. Fibrous joints also hold the teeth in the jawbone.

Partially moveable joints move a little. Also called cartilaginous joints, they are linked by cartilage, as in the spine. Each vertebrae in the spine moves in relation to the one above and below it, and together these movements give the spine its flexibility.

Freely moveable, or synovial, joints move in many directions. The main joints of the body - found at the hip, shoulders, elbows, knees, wrists, and ankles - are freely moveable. They are filled with synovial fluid, which acts as a lubricant to help the joints move easily.

Normal Physiology

Bones begin to develop long before birth. When the skeleton first forms, it is made of flexible cartilage, but within a few weeks it begins the process of ossification, where the cartilage is replaced by hard calcium phosphate and stretchy collagen, the two main components of bone. It takes about 20 years for this process to be completed.

The bones in a kid are smaller than in an adult and contain "growing zones" called growth plates. These plates consist of columns of multiplying cartilage cells that grow in length, and then change into bone. On average, girls' arms and legs grow until about age 14, whereas guys' grow for a year or 2 longer.

Bone building continues throughout your life, as your body constantly renews the bones' living tissue. Bone contains three types of cells: osteoblasts, which make new bone and help repair damage; osteocytes, which carry nutrients and waste products to and from blood vessels in the bone; and osteoclasts, which break down bone and help to "sculpt" and shape it.

Osteoclasts are very active in kids and teens, working on bone as it is remodeled during growth and as fractures heal. Approximately 3% of children's bone is broken down and rebuilt each year, but in adults, this turnover is much slower.

Even when you sit perfectly still, your body is still moving because your muscles are constantly at work: your heart beats, your intestines push food along its way, your breathing makes your chest rise and fall, and your blood vessels pulsate. The movements your muscles make are coordinated and controlled by the brain. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex (above your ear) and the cerebellum (behind your ear).

When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles involved, causing them to contract. Interestingly, the motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa. The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it's in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain sends a message to the muscles in your arm and you move it. When you run, the messages to the brain are more involved, because many muscles have to work in rhythm.

Muscles move body parts by contracting and then relaxing. Your muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint and bring it closer to your body. Then, when you've completed the movement, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the biceps contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow.

There are three kinds of freely moveable joints that play a big part in voluntary movement: hinge, pivot, and ball-and-socket. Hinge joints allow movement in one direction, as seen in the knees and elbows. Pivot joints allow a rotating motion, like that of the head moving from side to side. Ball-and-socket joints allow the greatest freedom of movement. The hips and shoulders have this type of joint, in which the spherical end of a long bone fits into the hollow of another bone.

Diseases, Conditions, Disorders, and Dysfunctions

As strong as bones are, they can break. Muscles can weaken, and joints can be damaged by injury or disease. The following are examples of problems that can affect our bones, muscles, and joints:

Arthritis - In arthritis, joint inflammation, pain, and loss of movement occur, which can be caused by a variety of underlying problems and conditions. It affects mostly older people, although people of all ages can have arthritis. In osteoarthritis, the cartilage in the joints loses some of its ability to stretch and wears away, allowing the bones of the joint to grate against each other. It commonly affects the knuckle joints and can also involve the hips, knees, and spine. In rheumatoid arthritis, for unknown reasons the body's immune system attacks the cartilage in the joints, making them stiff, painful, and sometimes deformed.

Fracture - A fracture occurs when a bone breaks; it may be cracked, snapped through, or shattered. After a bone fractures, new cells fill the gap and repair the break. Applying a strong plaster cast, which keeps the bone in the correct position until it heals, is the usual treatment. If the fracture is complicated, metal plates can be screwed into the bone to better stabilize the fracture and aid healing.

Muscular dystrophy - This inherited disease affects the voluntary muscles, causing them to weaken and deteriorate. The most common form in childhood is Duchenne muscular dystrophy. Because the abnormal gene causing this form of the disorder is carried on the X chromosome, the condition almost exclusively affects boys.

Osteoporosis - In osteoporosis, bone tissue becomes brittle, thin, and spongy. Bones break easily, and the spine sometimes begins to crumble and collapse. Older people, especially older women, can develop osteoporosis as a result of insufficient exercise and calcium intake, in combination with hormonal changes and genetic factors. Building up adequate stores of calcium in the bones as a child, teen, and young adult is a key factor in preventing or delaying the development of osteoporosis at a later age.

Rickets - Kids who don't get enough sunlight exposure (to help the body produce its own vitamin D) or enough vitamin D in their diets can develop rickets. Vitamin D helps the body absorb and use the important bone-building minerals calcium and phosphorus. In some cases, kids may develop rickets as the result of a genetically inherited disorder, or in association with liver or kidney diseases. The bones of kids who have untreated rickets do not grow normally and may become deformed. These children may become bowlegged because their bones are too soft and weak to bear the weight of their bodies.

Tendonitis - This condition usually arises from overexercising a muscle. The tendon and tendon sheath become inflamed, which can be painful. Resting the muscles helps relieve the condition.

Torn tendon - A common injury is a torn Achilles tendon, which connects the calf muscles to the heel. This tendon can snap, but it usually can be repaired by surgery.

Torn muscle - Sudden movement can tear the fibers inside muscle. This can cause pain, bleeding, and soreness.

Glossary

ball-and-socket joint: a type of joint that permits great freedom and range of movement (for example, shoulder, hip)
cancellous bone: spongy bone
cardiac muscle: heart muscle
cartilage: rubbery material that cushions and protects bones
compact bone: hard tissue that makes up one of the outer layers of a bone
extensor: a muscle that straightens a joint or extends a limb
flexor: a muscle that brings two body parts closer together by bending a joint and flexing a limb
hinge joint: a joint that allows movement in one direction (for example, knee)
immovable joint: a joint that does not permit movement, like those in the skull; also called a fibrous joint
joint: any point where two or more bones meet
ligament: a thick, fibrous cord that joins one bone to another, providing stability while allowing movement of a joint
muscles: body tissues made up of specialized muscle cells and fibers involved in body movement and function of the heart and other internal organs
musculoskeletal system: the body system made up of bones, joints, ligaments, muscles, and tendons
ossification: bone formation; the replacement of cartilage by bone
osteoblasts: bone cells that make new bone for growth and repair damage to a bone
osteoclasts: bone cells that break down bone tissue and aid in the shaping and remodeling of bones
osteocytes: bone cells that carry nutrients and waste products to and from blood vessels in the bone
partially movable joint: a joint that moves a little, like those in the spinal column; also called a cartilaginous joint
pivot joint: a joint that allows a rotating motion
skeletal muscles: muscles that work with the bones of the skeleton; also called voluntary muscles
skeleton: the body's bony supporting structure
smooth muscles: specialized involuntary muscles found in such places as the walls of intestines and blood vessels
stem cells: cells in the bone marrow that produce blood cells
synovial fluid: fluid found in a synovial joint that lubricates the joint and protects it from wear
tendon: a strong cord, composed of fibers of collagen, that attaches a muscle to a bone
trabeculae: tiny pieces of bone found in cancellous bone