Bones come in a variety of shapes and sizes and have a complex internal and external structure.
They are lightweight yet strong and hard, and serve multiple functions.
Bone tissue is made up of different types of bone cells.
Modified (flattened) osteoblasts become the lining cells that form a protective layer on the bone surface.
In the human body at birth, there are approximately 270 bones 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 Greek word for bone is ὀστέον ("osteon"), hence the many terms that use it as a prefix—such as osteopathy.
Bone is not uniformly solid, but consists of a flexible matrix (about 30%) and bound minerals (about 70%) which are intricately woven and endlessly remodeled by a group of specialized bone cells.
Their unique composition and design allows bones to be relatively hard and strong, while remaining lightweight.
The elasticity of collagen improves fracture resistance.
It is the bone mineralization that give bones rigidity.
Bone is actively constructed and remodeled throughout life by special bone cells known as osteoblasts and osteoclasts.
Within any single bone, the tissue is woven into two main patterns, known as cortical and cancellous bone, and each with different appearance and characteristics.
The hard outer layer of bones is composed of cortical bone, which is also called compact bone as it is much denser than cancellous 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 or Haversian system.
Volkmann's canals at right angles connect the osteons together.
The columns are metabolically active, and as bone is reabsorbed and created the nature and location of the cells within the osteon will change.
The endosteum is the boundary between the cortical bone and the cancellous bone.
The primary anatomical and functional unit of cortical bone is the osteon.
Cancellous bone, also called trabecular or spongy bone, is the internal tissue of the skeletal bone and is an open cell porous network.
This makes it weaker and more flexible.
The greater surface area also makes it suitable for metabolic activities such as the exchange of calcium ions.
Cancellous bone is typically found at the ends of long bones, near joints and in the interior of vertebrae.
The primary anatomical and functional unit of cancellous bone is the trabecula.
The trabeculae are aligned towards the mechanical load distribution that a bone experiences within long bones such as the femur.
Thin formations of osteoblasts covered in endosteum create an irregular network of spaces, known as trabeculae.
Trabecular marrow is composed of a network of rod- and plate-like elements that make the overall organ lighter and allow room for blood vessels and marrow.
Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone.
The words cancellous and trabecular refer to the tiny lattice-shaped units (trabeculae) that form the tissue.
It was first illustrated accurately in the engravings of Crisóstomo Martinez.
In newborns, all such bones are filled exclusively with red marrow or hematopoietic marrow, but as the child ages the hematopoietic fraction decreases in quantity and the fatty/ yellow fraction called marrow adipose tissue (MAT) increases in quantity.
In adults, red marrow is mostly found in the bone marrow of the femur, the ribs, the vertebrae and pelvic bones.
Bone is a metabolically active tissue composed of several types of cells.
Within the marrow of the bone there are also hematopoietic stem cells.
Osteoblasts are mononucleate bone-forming cells.
The osteoid seam is a narrow region of newly formed organic matrix, not yet mineralized, located on the surface of a bone.
Osteoid is primarily composed of Type I collagen.
The osteoblast creates and repairs new bone by actually building around itself.
First, the osteoblast puts up collagen fibers.
These collagen fibers are used as a framework for the osteoblasts' work.
The brand-new bone created by the osteoblast is called osteoid.
Once the osteoblast is finished working it is actually trapped inside the bone once it hardens.
When the osteoblast becomes trapped, it becomes known as an osteocyte.
Other osteoblasts remain on the top of the new bone and are used to protect the underlying bone, these become known as lining cells.
Osteocytes are cells of mesenchymal origin and originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produced.
The spaces the cell body of osteocytes occupy within the mineralized collagen type I matrix are known as lacunae, while the osteocyte cell processes occupy channels called canaliculi.
The many processes of osteocytes reach out to meet osteoblasts, osteoclasts, bone lining cells, and other osteocytes probably for the purposes of communication.
Osteocytes remain in contact with other osteocytes in the bone through gap junctions—coupled cell processes which pass through the canalicular channels.
New bone is then formed by the osteoblasts.
Bone is constantly remodeled by the resorption of osteoclasts and created by osteoblasts.
Osteoclasts are large cells with multiple nuclei located on bone surfaces in what are called Howship's lacunae (or resorption pits).
These lacunae are the result of surrounding bone tissue that has been reabsorbed.
Osteoclasts mature and/or migrate to discrete bone surfaces.
Main article: Extracellular matrix
Bones consist of living cells embedded in a mineralized organic matrix.
This matrix consists of organic components, mainly type I collagen—"organic" referring to materials produced as a result of the human body—and inorganic components, primarily hydroxyapatite and other salts of calcium and phosphate.
Above 30% of the acellular part of bone consists of the organic components, and 70% of salts.
These effects are synergistic.
The exact composition of the matrix may be subject to change over time due to nutrition and biomineralization, with the ratio of calcium to phosphate varying between 1.3 and 2.0 (per weight), and trace minerals such as magnesium, sodium, potassium and carbonate also being found.
Type I collagen composes 90–95% of the organic matrix, with remainder of the matrix being a homogenous liquid called ground substance consisting of proteoglycans such as hyaluronic acid and chondroitin sulfate, as well as non-collagenous proteins such as osteocalcin, osteopontin or bone sialoprotein.
Collagen consists of strands of repeating units, which give bone tensile strength, and are arranged in an overlapping fashion that prevents shear stress.
The function of ground substance is not fully known.
Two types of bone can be identified microscopically according to the arrangement of collagen: woven and lamellar.
- Woven bone (also known as fibrous bone), which is characterized by a haphazard organization of collagen fibers and is mechanically weak.
- Lamellar bone, which has a regular parallel alignment of collagen into sheets ("lamellae") and is mechanically strong.
Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all fetal bones, but is later replaced by more resilient lamellar bone.
Woven bone is weaker, with a smaller number of randomly oriented collagen fibers, but forms quickly; it is for this appearance of the fibrous matrix that the bone is termed woven.
It is soon replaced by lamellar bone, which is highly organized in concentric sheets with a much lower proportion of osteocytes to surrounding tissue.
Lamellar bone, which makes its first appearance in humans in the fetus during the third trimester, is stronger and filled with many collagen fibers parallel to other fibers in the same layer (these parallel columns are called osteons).
After a fracture, woven bone forms initially and is gradually replaced by lamellar bone during a process known as "bony substitution."
Compared to woven bone, lamellar bone formation takes place more slowly.
The orderly deposition of collagen fibers restricts the formation of osteoid to about 1 to 2 µm per day.
Lamellar bone also requires a relatively flat surface to lay the collagen fibers in parallel or concentric layers.
The extracellular matrix of bone is laid down by osteoblasts, which secrete both collagen and ground substance.
These synthesise collagen within the cell, and then secrete collagen fibrils.
The collagen fibers rapidly polymerise to form collagen strands.
At this stage they are not yet mineralised, and are called "osteoid".
Around the strands calcium and phosphate precipitate on the surface of these strands, within days to weeks becoming crystals of hydroxyapatite.
This cleaves the phosphate groups and acts as the foci for calcium and phosphate deposition.
The vesicles then rupture and act as a centre for crystals to grow on.
More particularly, bone mineral is formed from globular and plate structures.
There are five types of bones 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. The bones of the wrist and ankle are short bones.
- Flat bones are thin and generally curved, with two parallel layers of compact bones 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.
Main article: Anatomical terms of bone
Other anatomical terms are also used to describe the location of bones.
Some anatomists still use Latin to refer to bones.
The term "osseous", and the prefix "osteo-", referring to things related to bone, are still used commonly today.
Some examples of terms used to describe bones include the term "foramen" to describe a hole through which something passes, and a "canal" or "meatus" to describe a tunnel-like structure.
A protrusion from a bone can be called a number of terms, including a "condyle", "crest", "spine", "eminence", "tubercle" or "tuberosity", depending on the protrusion's shape and location.
In general, long bones are said to have a "head", "neck", and "body".
When two bones join together, they are said to "articulate".
If the two bones have a fibrous connection and are relatively immobile, then the joint is called a "suture".
The formation of bone is called ossification.
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.
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.
Endochondral ossification begins with points in the cartilage called "primary ossification centers."
They mostly appear during fetal development, though a few short bones begin their primary ossification after birth.
They are responsible for the formation of the diaphyses of long bones, short bones and certain parts of irregular bones.
Secondary ossification occurs after birth, and forms the epiphyses of long bones and the extremities of irregular and flat bones.
The diaphysis and both epiphyses of a long bone are separated by a growing zone of cartilage (the epiphyseal plate).
At skeletal maturity (18 to 25 years of age), all of the cartilage is replaced by bone, fusing the diaphysis and both epiphyses together (epiphyseal closure).
In the upper limbs, only the diaphyses of the long bones and scapula are ossified.
The epiphyses, carpal bones, coracoid process, medial border of the scapula, and acromion are still cartilaginous.
The following steps are followed in the conversion of cartilage to bone:
- Zone of reserve cartilage. This region, farthest from the marrow cavity, consists of typical hyaline cartilage that as yet shows no sign of transforming into bone.
- Zone of cell proliferation. A little closer to the marrow cavity, chondrocytes multiply and arrange themselves into longitudinal columns of flattened lacunae.
- Zone of cell hypertrophy. Next, the chondrocytes cease to divide and begin to hypertrophy (enlarge), much like they do in the primary ossification center of the fetus. The walls of the matrix between lacunae become very thin.
- Zone of calcification. Minerals are deposited in the matrix between the columns of lacunae and calcify the cartilage. These are not the permanent mineral deposits of bone, but only a temporary support for the cartilage that would otherwise soon be weakened by the breakdown of the enlarged lacunae.
- Zone of bone deposition. Within each column, the walls between the lacunae break down and the chondrocytes die. This converts each column into a longitudinal channel, which is immediately invaded by blood vessels and marrow from the marrow cavity. Osteoblasts line up along the walls of these channels and begin depositing concentric lamellae of matrix, while osteoclasts dissolve the temporarily calcified cartilage.
|Functions of Bone|
Bones have a variety of functions:
Bones serve a variety of mechanical functions.
Together the bones in the body form the skeleton.
They provide a frame to keep the body supported, and an attachment point for skeletal muscles, tendons, ligaments and joints, which function together to generate and transfer forces so that individual body parts or the whole body can be manipulated in three-dimensional space (the interaction between bone and muscle is studied in biomechanics).
This means that bone resists pushing (compressional) stress well, resist pulling (tensional) stress less well, but only poorly resists shear stress (such as due to torsional loads).
Mechanically, bones also have a special role in hearing.
The cancellous part of bones contain bone marrow.
Bone marrow produces blood cells in a process called hematopoiesis.
Unlike red and white blood cells, created by mitosis, platelets are shed from very large cells called megakaryocytes.
This process of progressive differentiation occurs within the bone marrow.
After the cells are matured, they enter the circulation.
Every day, over 2.5 billion red blood cells and platelets, and 50–100 billion granulocytes are produced in this way.
As well as creating cells, bone marrow is also one of the major sites where defective or aged red blood cells are destroyed.
- Mineral storage – bones act as reserves of minerals important for the body, most notably calcium and phosphorus.
Determined by the species, age, and the type of bone, bone cells make up to 15 percent of the bone.
- Fat storage – marrow adipose tissue (MAT) acts as a storage reserve of fatty acids.
- Acid-base balance – bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts.
- Detoxification – bone tissues can also store heavy metals and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for excretion.
- Endocrine organ – bone controls phosphate metabolism by releasing fibroblast growth factor 23 (FGF-23), which acts on kidneys to reduce phosphate reabsorption. Bone cells also release a hormone called osteocalcin, which contributes to the regulation of blood sugar (glucose) and fat deposition. Osteocalcin increases both the insulin secretion and sensitivity, in addition to boosting the number of insulin-producing cells and reducing stores of fat.
- Calcium balance – the process of bone resorption by the osteoclasts releases stored calcium into the systemic circulation and is an important process in regulating calcium balance. As bone formation actively fixes circulating calcium in its mineral form, removing it from the bloodstream, resorption actively unfixes it thereby increasing circulating calcium levels. These processes occur in tandem at site-specific locations.
Main article: Bone remodeling
Bone is constantly being created and replaced in a process known as remodeling.
This ongoing turnover of bone is a process of resorption followed by replacement of bone with little change in shape.
This is accomplished through osteoblasts and osteoclasts.
Cells are stimulated by a variety of signals, and together referred to as a remodeling unit.
Approximately 10% of the skeletal mass of an adult is remodelled each year.
It has been hypothesized that this is a result of bone's piezoelectric properties, which cause bone to generate small electrical potentials under stress.
The action of osteoblasts and osteoclasts are controlled by a number of chemical enzymes that either promote or inhibit the activity of the bone remodeling cells, controlling the rate at which bone is made, destroyed, or changed in shape.
The cells also use paracrine signalling to control the activity of each other.
Osteoprotegerin is secreted by osteoblasts and is able to bind RANK-L, inhibiting osteoclast stimulation.
These hormones also promote increased secretion of osteoprotegerin.
Osteoblasts can also be induced to secrete a number of cytokines that promote reabsorption of bone by stimulating osteoclast activity and differentiation from progenitor cells.
Vitamin D, parathyroid hormone and stimulation from osteocytes induce osteoblasts to increase secretion of RANK-ligand and interleukin 6, which cytokines then stimulate increased reabsorption of bone by osteoclasts.
These same compounds also increase secretion of macrophage colony-stimulating factor by osteoblasts, which promotes the differentiation of progenitor cells into osteoclasts, and decrease secretion of osteoprotegerin.
Bone volume is determined by the rates of bone formation and bone resorption.
Recent research has suggested that certain growth factors may work to locally alter bone formation by increasing osteoblast activity.
Numerous bone-derived growth factors have been isolated and classified via bone cultures.
These factors include insulin-like growth factors I and II, transforming growth factor-beta, fibroblast growth factor, platelet-derived growth factor, and bone morphogenetic proteins.
Evidence suggests that bone cells produce growth factors for extracellular storage in the bone matrix.
The release of these growth factors from the bone matrix could cause the proliferation of osteoblast precursors.
Essentially, bone growth factors may act as potential determinants of local bone formation.
Research has suggested that cancellous bone volume in postmenopausal osteoporosis may be determined by the relationship between the total bone forming surface and the percent of surface resorption.
See also: Bone disease
A number of diseases can affect bone, including arthritis, fractures, infections, osteoporosis and tumours.
Other doctors, such as rehabilitation specialists may be involved in recovery, radiologists in interpreting the findings on imaging, and pathologists in investigating the cause of the disease, and family doctors may play a role in preventing complications of bone disease such as osteoporosis.
When a doctor sees a patient, a history and exam will be taken.
Bones are then often imaged, called radiography.
Other tests such as a blood test for autoimmune markers may be taken, or a synovial fluid aspirate may be taken.
Main article: Bone fracture
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.
Not all fractures are painful.
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 are described by their location and shape, and several classification systems exist, depending on the location of the fracture.
A common long bone fracture in children is a Salter–Harris fracture.
When fractures are managed, pain relief is often given, and the fractured area is often immobilised.
This is to promote bone healing.
In addition, surgical measures such as internal fixation may be used.
Because of the immobilisation, people with fractures are often advised to undergo rehabilitation.
Main article: Bone tumour
There are several types of tumour that can affect bone; examples of benign bone tumours include osteoma, osteoid osteoma, osteochondroma, osteoblastoma, enchondroma, giant cell tumour of bone, and aneurysmal bone cyst.
Main article: Bone metastases
Cancers that arise in bone are called "primary" cancers, although such cancers are rare.
Bone may also be affected by cancers in other parts of the body.
This increases bone reabsorption, and can lead to bone fractures.
Bone tissue that is destroyed or altered as a result of cancers is distorted, weakened, and more prone to fracture.
If the cancer is metastatic, then there might be other symptoms depending on the site of the original cancer.
Some bone cancers can also be felt.
Cancers of the bone are managed according to their type, their stage, prognosis, and what symptoms they cause.
Many primary cancers of bone are treated with radiotherapy.
- Osteomyelitis is inflammation of the bone or bone marrow due to bacterial infection.
- Osteomalacia is a painful softening of adult bone caused by severe vitamin D deficiency.
- Osteogenesis imperfecta
- Osteochondritis dissecans
- Ankylosing spondylitis
- Skeletal fluorosis is a bone disease caused by an excessive accumulation of fluoride in the bones. In advanced cases, skeletal fluorosis damages bones and joints and is painful.
Main article: Osteoporosis
Osteoporosis is most common in women after menopause, when it is called "postmenopausal osteoporosis", but may develop in men and premenopausal women in the presence of particular hormonal disorders and other chronic diseases or as a result of smoking and medications, specifically glucocorticoids.
Osteoporosis usually has no symptoms until a fracture occurs.
For this reason, DEXA scans are often done in people with one or more risk factors, who have developed osteoporosis and are at risk of fracture.
Osteoporosis treatment includes advice to stop smoking, decrease alcohol consumption, exercise regularly, and have a healthy diet.
Main article: Osteopathic medicine in the United States
Osteopathic medicine is a school of medical thought originally developed based on the idea of the link between the musculoskeletal system and overall health, but now very similar to mainstream medicine.
As of 2012, over 77,000 physicians in the United States are trained in osteopathic medical schools.
The study of bones and teeth is referred to as osteology.
This can include determining the nutritional, health, age or injury status of the individual the bones were taken from.
Preparing fleshed bones for these types of studies can involve the process of maceration.
Bones can serve a number of uses such as projectile points or artistic pigments, and can also be made from external bones such as antlers.
Bird skeletons are very lightweight.
Their bones are smaller and thinner, to aid flight.
Among mammals, bats come closest to birds in terms of bone density, suggesting that small dense bones are a flight adaptation.
Many bird bones have little marrow due to their being hollow.
The extinct predatory fish Dunkleosteus had sharp edges of hard exposed bone along its jaws.
Many animals possess an exoskeleton that is not made of bone.
The proportion of cortical bone that is 80% in the human skeleton may be much lower in other animals, especially in marine mammals and marine turtles, or in various Mesozoic marine reptiles, such as ichthyosaurs, among others.
This is presumably carried out in order to replenish lacking phosphate.
Many bone diseases that affect humans also affect other vertebrates—an example of one disorder is skeletal fluorosis.
Society and culture
Bones from slaughtered animals have a number of uses.
They have further been used in bone carving, already important in prehistoric art, and also in modern time as crafting materials for buttons, beads, handles, bobbins, calculation aids, head nuts, dice, poker chips, pick-up sticks, ornaments, etc. A special genre is scrimshaw.
Bone glue can be made by prolonged boiling of ground or cracked bones, followed by filtering and evaporation to thicken the resulting fluid.
Historically once important, bone glue and other animal glues today have only a few specialized uses, such as in antiques restoration.
Essentially the same process, with further refinement, thickening and drying, is used to make gelatin.
Broth is made by simmering several ingredients for a long time, traditionally including bones.
Its name originates from oracle bones, which were mainly ox clavicle.
Various cultures throughout history have adopted the custom of shaping an infant's head by the practice of artificial cranial deformation.
A widely practised custom in China was that of foot binding to limit the normal growth of the foot.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Bone.