Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Because bones come in a variety of shapes and have a complex internal and external structure, they are lightweight, yet strong and hard, in addition to fulfilling their many other functions. One of the types of tissues that makes up bone is the mineralized osseous tissue, also called bone tissue, that gives it rigidity and honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage. There are two hundred six bones in the adult body.

Functions

Bones have eight main functions:

• Protection — Bones can serve to protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs.
• Shape — Bones provide a frame to keep the body supported.
• Blood production — The marrow, located within the medullary cavity of long bones and the interstices of cancellous bone, produces blood cells in a process called haematopoiesis.
• Mineral storage — Bones act as reserves of minerals important for the body, most notably calcium and phosphorus.
• Movement — Bones, skeletal muscles, tendons, ligaments and joints 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.
• 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.
• Sound transduction — Bones are important in the mechanical aspect of hearing.

Characteristics

The primary tissue of bone, osseous tissue, is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxylapatite (this is the osseous tissue that gives bones their rigidity). It has relatively high compressive strength but poor tensile strength, meaning it resists pushing forces well, but not pulling forces. While bone is essentially brittle, it does have a significant degree of elasticity contributed chiefly by collagen. All bones consist of living cells embedded in the mineralised organic matrix that makes up the osseous tissue.

Macrostructure

Bone is not a uniformly solid material, but rather has some spaces between its hard elements.

Compact bone

The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces. This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult skeleton. Compact bone may also be referred to as dense bone or cortical bone.

Trabecular bone

Filling the interior of the organ is the trabecular bone tissue (an open cell porous network also called cancellous or spongy bone) which is comprised of a network of rod- and plate-like elements that make the overall organ lighter and allowing 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.

Cellular structure

There are several types of cells constituting the bone;

• Osteoblasts are mononucleate bone-forming cells which descend from osteoprogenitor cells. They are located on the surface of osteoid seams and make a protein mixture known as osteoid, which mineralizes to become bone. Osteoid is primarily composed of Type I collagen. Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. They robustly produce alkaline phosphatase, an enzyme that has a role in the mineralisation of bone, as well as many matrix proteins. Osteoblasts are the immature bone cells.
• Bone lining cells are essentially inactive osteoblasts. They cover all of the available bone surface and function as a barrier for certain ions.
• Osteocytes originate from osteoblasts which have migrated into and become trapped and surrounded by bone matrix which they themselves produce. The spaces which they occupy are known as lacunae. Osteocytes have many processes which reach out to meet osteoblasts probably for the purposes of communication. Their functions include to varying degrees: formation of bone, matrix maintenance and calcium homeostasis. They possibly act as mechano-sensory receptors—regulating the bone's response to stress. They are mature bone cells.
• Osteoclasts are the cells responsible for bone resorption (remodeling of bone to reduce its volume). Osteoclasts are large, multinucleated cells located on bone surfaces in what are called Howship's lacunae or resorption pits. These lacunae, or resorption pits, are left behind after the breakdown of bone and often present as scalloped surfaces. Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with engulfment strategies similar to circulating macrophages. Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival, active enzymes, such as tartrate resistant acid phosphatase, are secreted against the mineral substrate.

Molecular structure

Matrix

The matrix is the major constituent of bone, surrounding the cells. It has inorganic and organic parts.

Inorganic

The inorganic is mainly crystalline mineral salts and calcium, which is present in the form of hydroxyapatite. The matrix is initially laid down as unmineralized osteoid (manufactured by osteoblasts). Mineralisation involves osteoblasts secreting vesicles containing alkaline phosphatase. 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.

Organic

The organic part of matrix is mainly Type I collagen. This is made intracellularly as tropocollagen and then exported. It then associates into fibrils. Also making up the organic part of matrix include various growth factors, the functions of which are not fully known. Other factors present include glycosaminoglycans, osteocalcin, osteonectin, bone sialo protein and Cell Attachment Factor. One of the main things that distinguishes the matrix of a bone from that of another cell is that the matrix in bone is hard.

Woven or lamellar

Bone can be either woven or lamellar (layered). Woven bone is weak, with a small number of randomly oriented collagen fibers, but forms quickly and without a pre-existing structure during periods of repair or growth. Lamellar bone is stronger, formed of numerous stacked layers and filled with many collagen fibers parallel to other fibers in the same layer. The fibers run in opposite directions in alternating layers, assisting in the bone's ability to resist torsion forces. After a break, woven bone quickly forms and is gradually replaced by slow-growing lamellar bone on pre-existing calcified hyaline cartilage through a process known as "bony substitution."

Five types of bones

There are five types of bones in the human body: long, short, flat, irregular and sesamoid.

• Long bones are longer than they are wide, consisting of a long shaft (the diaphysis) plus two articular (joint) surfaces, called epiphyses. They are comprised mostly of compact bone, but are generally thick enough to contain considerable spongy bone and marrow in the hollow centre (the medullary cavity). Most bones of the limbs (including the three bones of the fingers) are long bones, except for the kneecap (patella), and the carpal, metacarpal, tarsal and metatarsal bones of the wrist and ankle. The classification refers to shape rather than the size.
• 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, as are the sesamoid 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.
• 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. The bones of the spine and hips are irregular bones.
• 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 force of the muscle is increased. Examples of sesamoid bones are the Patella and the Pisiform

Formation

The formation of bone during the fetal stage of development occurs by two methods: intramembranous and endochondral ossification.

Intramembranous ossification

Intramembranous ossification mainly occurs during formation of the flat bones of the skull; the bone is formed from mesenchyme tissue. The steps in intramembranous ossification are:

1. Development of ossification center
2. Calcification
3. Formation of trabeculae
4. Development of periosteum

Endochondral ossification

Endochondral ossification, on the other hand, occurs in long bones, such as limbs; the bone is formed from cartilage. The steps in endochondral ossification are:

1. Development of cartilage model
2. Growth of cartilage model
3. Development of the primary ossification center
4. Development of medullary cavity
5. Development of the secondary ossification center
6. Formation of articular cartilage and epiphyseal plate

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). When the child reaches 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).

Bone marrow

Bone marrow can be found in almost any bone that holds cancellous tissue. In newborns, all such bones are filled exclusively with red marrow (or hemopoietic marrow), but as the child ages it is mostly replaced by yellow, or fatty marrow. In adults, red marrow is mostly found in the flat bones of the skull, the ribs, the vertebrae and pelvic bones.

Remodeling

Remodeling or bone turnover is the process of resorption followed by replacement of bone with little change in shape and occurs throughout a person's life. Osteoblasts and osteoclasts, coupled together via paracrine cell signalling, are referred to as bone remodeling units.

Purpose

The purpose of remodeling is to regulate calcium homeostasis, repair micro-damaged bones (from everyday stress) but also to shape and sculpture the skeleton during growth.

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.

Repair

Repeated stress, such as weight-bearing exercise or bone healing, results in the bone thickening at the points of maximum stress (Wolff's law). It has been hypothesized that this is a result of bone's piezoelectric properties, which cause bone to generate small electrical potentials under stress.

Medical conditions related to bones

• Bone fracture
• Osteomyelitis
• Osteoporosis
• Osteonecrosis
• Osteosarcoma
• Osteogenesis imperfecta
• Arthritis

Osteology

The study of bones and teeth is referred to as osteology. It is frequently used in anthropology, archeology and forensic science for a variety of tasks. 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 maceration - boiling fleshed bones to remove large particles, then hand-cleaning.

Typically anthropologists and archeologists study bone tools made by Homo sapiens and Homo neanderthalensis. Bones can serve a number of uses such as projectile points or artistic pigments, and can be made from endoskeletal or external bones such as antler or tusk.

Alternatives to bony endoskeletons

There are several evolutionary alternatives to mammilary bone; though they have some similar functions, they are not completely functionally analogous to bone.

• Exoskeletons offer support, protection and levers for movement similar to endoskeletal bone. Different types of exoskeletons include shells, carapaces (consisting of calcium compounds or silica) and chitinous exoskeletons.
• A true endoskeleton (that is, protective tissue derived from mesoderm) is also present in Echinoderms. Porifera (sponges) possess simple endoskeletons that consist of calcareous or siliceous spicules and a spongin fiber network.

Exposed bone

Bone penetrating the skin and being exposed to the outside can be both a natural process in some animals, and due to injury:

• A deer's antlers are composed of bone
• The extinct predatory fish Dunkleosteus, instead of teeth, had sharp edges of hard exposed bone along its jaws
• A compound fracture occurs when the edges of a broken bone punctures the skin
• Though not strictly speaking exposed, a bird's beak is primarily bone covered in a layer of keratin

Terminology

Several terms are used to refer to features and components of bones throughout the body:

Bone feature	Definition
articular process	A projection that contacts an adjacent bone.
articulation	The region where adjacent bones contact each other—a joint.
canal	A long, tunnel-like foramen, usually a passage for notable nerves or blood vessels.
condyle	A large, rounded articular process.
crest	A prominent ridge.
eminence	A relatively small projection or bump.
epicondyle	A projection near to a condyle but not part of the joint.
facet	A small, flattened articular surface.
foramen	An opening through a bone.
fossa	A broad, shallow depressed area.
fovea	A small pit on the head of a bone.
labyrinth	A cavity within a bone.
line	A long, thin projection, often with a rough surface. Also known as a ridge.
malleolus	One of two specific protuberances of bones in the ankle.
meatus	A short canal.
process	A relatively large projection or prominent bump.(gen.)
ramus	An arm-like branch off the body of a bone.
sinus	A cavity within a cranial bone.
spine	A relatively long, thin projection or bump.
suture	Articulation between cranial bones.
trochanter	One of two specific tuberosities located on the femur.
tubercle	A projection or bump with a roughened surface, generally smaller than a tuberosity.
tuberosity	A projection or bump with a roughened surface.

Several terms are used to refer to specific features of long bones:

Bone feature	Definition
diaphysis	The long, relatively straight main body of a long bone; region of primary ossification. Also known as the shaft.
epiphysis	The end regions of a long bone; regions of secondary ossification.
epiphyseal plate	Also known as the growth plate or physis. In a long bone it is a thin disc of hyaline cartilage that is positioned transversely between the epiphysis and metaphysis. In the long bones of humans, the epiphyseal plate disappears by twenty years of age.
head	The proximal articular end of the bone.
metaphysis	The region of a long bone lying between the epiphysis and diaphysis.
neck	The region of bone between the head and the shaft.

References

• Marieb, E.N. (1998). Human Anatomy & Physiology, 4th ed. Menlo Park, California: Benjamin/Cummings Science Publishing.
• Netter, Frank H. (1987), Musculoskeletal system: anatomy, physiology, and metabolic disorders, Summit, New Jersey: Ciba-Geigy Corporation.
• Tortora, G. J. (1989), Principles of Human Anatomy, 5th ed. New York: Harper & Row, Publishers.

See also

• List of bones of the human skeleton
• Terms for anatomical location
• Orthopaedics

External links

• Review (including references) of piezoelectricity and bone remodelling
• A good basic overview of bone biology from the Science Creative Quarterly
• Bone Health at Got Bones?

•  • [ e] Bone and cartilage
Cartilage	chondroblast, chondrocyte, perichondrium, types (hyaline, elastic, fibrous), fibrocartilage callus, metaphysis
Bone	ossification (intramembranous, endochondral, epiphyseal plate), cycle (osteoblast, osteoid, osteocyte, osteoclast), types (cancellous, cortical), regions (epiphysis, diaphysis), structure (osteon/Haversian system, Haversian canals, endosteum, periosteum, Sharpey's fibres, enthesis, lacunae, canaliculi, trabeculae, medullary cavity, bone marrow), shapes (long, short, flat, irregular, sesamoid)

•  • [ e] Bones of head and neck: the face
maxilla	Body of maxilla - Maxillary sinus - surfaces of body Anterior (Incisive fossa, Canine fossa, Infraorbital foramen, Anterior nasal spine) - Infratemporal (Alveolar canals, Maxillary tuberosity) - Orbital (Infraorbital groove, Infraorbital canal) - Nasal (Pterygopalatine canal) - processes Zygomatic process - Frontal process (Agger nasi, Anterior lacrimal crest) - Alveolar process - Palatine process (Incisive foramen, Incisive canals, Foramina of Scarpa, Premaxilla, Anterior nasal spine)
lacrimal	Posterior lacrimal crest - Lacrimal groove
zygomatic	Orbital process, Zygomaticofacial foramen - Zygomaticotemporal foramen - Zygomaticorbital foramina
palatine	Pterygopalatine fossa - Pterygoid fossa - Horizontal plate (Posterior nasal spine) - Perpendicular plate (Pterygopalatine canal, Sphenopalatine foramen, Pyramidal process) processes (Orbital - Sphenoidal)
mandible	body (Symphysis menti, Mental protuberance, Mental foramen, Mylohyoid line) - Ramus (Mandibular foramen, Mylohyoid groove, Mandibular canal, Angle, Coronoid process, Condyloid process, Mandibular notch)
others	nasal bone - inferior nasal conchae (ethmoidal process, maxillary process) - vomer

•  • [ e] Bones of head and neck: the cranium
Occipital	Foramen magnum - Squama occipitalis (Inion - Nuchal lines - Planum occipitale - Planum nuchale - Cruciform eminence - Internal occipital protuberance - Sagittal sulcus - Internal occipital crest) Lateral parts (Occipital condyle - Hypoglossal canal - Condyloid fossa - Condylar canal - Jugular process - Jugular tubercle) - Basilar part (Pharyngeal tubercle)
Parietal	Parietal eminence - Temporal line - Parietal foramen
Frontal	Squama frontalis (Frontal suture - Frontal eminence - Superciliary arches - Glabella - Supraorbital foramen - Zygomatic process - Sagittal sulcus - Frontal crest - Foramen cecum) - Pars orbitalis (Ethmoidal notch - Lacrimal fossa - Trochlear fovea - Posterior ethmoidal foramen - Anterior ethmoidal foramen - Frontal sinus - Frontonasal duct)
Temporal	Squama temporalis (Articular tubercle - Suprameatal triangle - Mandibular fossa - Petrotympanic fissure) - Mastoid portion (Mastoid foramen - Mastoid process - Mastoid notch - Occipital groove - Sigmoid sulcus - Mastoid antrum) - Petrous portion (Facial canal - Hiatus of the facial canal - Internal auditory meatus - Subarcuate fossa - Carotid canal - Aqueduct of cochlea - Jugular fossa - Inferior tympanic canaliculus - Mastoid canaliculus - Styloid process - Stylomastoid foramen - Petrosquamous suture) - Tympanic part (Suprameatal spine) - Zygomatic process
Sphenoid	Body Superior surface (Ethmoidal spine, Chiasmatic groove, Optic foramen, Sella turcica, Fossa hypophyseos, Middle clinoid process, Dorsum sellae, Posterior clinoid processes, Petrosal process, Clivus) - Lateral surface (Carotid groove - Sphenoidal lingula) - Anterior surface (Sphenoidal sinuses) - Great wings (Spine, Foramen rotundum, Foramen ovale, Foramen Vesalii, Foramen spinosum, Infratemporal crest, Sulcus tubae auditivae) - Small wings (Superior orbital fissure, Anterior clinoid process, Optic foramen) - Pterygoid processes (Pterygoid fossa, Scaphoid fossa, Lateral pterygoid plate, Medial pterygoid plate, Pterygoid canal, Pterygoid hamulus) - Sphenoidal conchae
Ethmoid	Cribriform plate (Crista galli) - Perpendicular plate - Labyrinth (Ethmoid sinus) - Lateral surface Lamina papyracea - Uncinate process - Medial surface Middle nasal concha - Superior meatus - Superior nasal concha - Middle meatus

•  • [ e] Bones of head and neck: cranial sutures, fontanelles, and related regions
Cranial sutures	Coronal - Lambdoid - Occipitomastoid - Sphenofrontal - Sphenoparietal - Sphenosquamosal - Sphenozygomatic -Squamosal - Zygomaticotemporal - Zygomaticofrontal - Frontal/Metopic - Sagittal - Frontoethmoidal - Petrosquamous - Sphenoethmoidal - Sphenopetrosal
Fontanelles	Anterior fontanelle - Posterior fontanelle
Foramina of multiple bones	Inferior orbital fissure - Foramen lacerum - Jugular foramen - Nasolacrimal canal
Fossae	Anterior cranial fossa - Middle cranial fossa - Posterior cranial fossa - Cranial cavity
Other compound structures	Asterion - Nasion - Pterion - Dacryon - Zygomatic arch - Calvaria - Temporal fossa - Infratemporal fossa - Stephanion - Pterygomaxillary fissure - Pterygopalatine fossa

•  • [ e] Bones of upper limbs
Pectoral girdle, clavicle	conoid tubercle - trapezoid line - costal tuberosity - subclavian groove
Scapula	spine of scapula - suprascapular notch - acromion - glenoid cavity - glenoidal labrum - coracoid process - fossae (subscapular, supraspinatous, infraspinatous) - borders (superior, lateral/axillary, medial/vertebral) - angles (superior, inferior, lateral) - tubercles (infraglenoid, supraglenoid)
Humerus	upper extremity - necks (anatomical, surgical) - tubercles (greater, lesser) - intertubercular groove - body - radial sulcus - deltoid tuberosity lower extremity - capitulum - trochlea - epicondyles (lateral, medial) - supracondylar ridges (lateral, medial) - fossae (radial, coronoid, olecranon)
Forearm	ulna: upper extremity (olecranon, coronoid process, tuberosity, radial notch, semilunar notch) - body of ulna - lower extremity (styloid process) radius: upper extremity (tuberosity) - body - lower extremity (ulnar notch, styloid process)
Hand	carpus: scaphoid - lunate - triquetral - pisiform - trapezium - trapezoid - capitate - hamate (hamulus) - metacarpus: 1st metacarpal - 2nd - 3rd - 4th - 5th - phalanges of the hand

Spine [ edit]

general structures: body of vertebra, vertebral arch (pedicle, lamina, vertebral notch), foramina (vertebral, intervertebral), processes (transverse, articular, spinous) cervical vertebrae: C1 (anterior arch, posterior arch, lateral mass), C2 (dens), C7, posterior tubercle, foramen transversarium thoracic vertebrae: costal facets (superior, inferior, transverse) lumbar vertebrae: accessory process, mammillary process sacrum/coccyx: pelvic surface (anterior sacral foramina, dorsal surface (posterior sacral foramina, median sacral crest, medial sacral crest, lateral sacral crest), lateral surface, base, sacral hiatus

•  • [ e] Bones of lower limbs
femur	head of femur (fovea capitis femoris) - neck of femur - greater trochanter - trochanteric fossa - lesser trochanter - tubercle of the femur - intertrochanteric line - intertrochanteric crest - linea quadrata - body of femur (linea aspera, third trochanter, pectineal line, adductor tubercle) - lower extremity of femur (lateral condyle, medial condyle, lateral epicondyle, medial epicondyle, patellar surface)
tibia	upper extremity - medial condyle - lateral condyle - intercondyloid eminence - tuberosity of the tibia - posterior intercondyloid fossa - anterior intercondyloid fossa body of tibia - soleal line - lower extremity - medial malleolus
fibula	head of fibula - body of fibula - lateral malleolus
tarsus	calcaneus (sustentaculum tali, trochlear process) - talus - cuboid - medial cuneiform - intermediate - lateral
metatarsus	1st metatarsal - 2nd - 3rd - 4th - 5th
other	patella - phalanges of the foot

•  • [ e] Bones of pelvis/pelvic cavity
General	sacrum, coccyx, hip bone
Ilium	Arcuate line - Wing - gluteal lines (Posterior, Anterior, Inferior) - Fossa - Tuberosity - Crest - iliac spines (Anterior superior - Anterior inferior - Posterior superior - Posterior inferior)
Ischium	Body (Ischial spine, Lesser sciatic notch) - Superior ramus (Tuberosity of the ischium) - Inferior ramus
Pubis	Superior ramus (Pubic tubercle, Pubic crest, Obturator crest) - Inferior ramus (Pectineal line)
Compound	Obturator foramen - Acetabulum - Acetabular notch - Greater sciatic notch - Iliopectineal eminence - Ischiopubic ramus - Pubic arch - Lesser pelvis (Pelvic inlet, Pelvic brim, Pelvic outlet) - Greater pelvis