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A bone is a Stiffness organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store , provide structure and support for the body, and enable mobility. Bones come in a variety of shapes and sizes and have complex internal and external structures. They are lightweight yet strong and hard and serve multiple functions.
Bone tissue (osseous tissue), which is also called bone in the mass noun sense of that word, is hard tissue, a type of specialised connective tissue. (2025). 9781315300030, CRC Press. ISBN 9781315300030 It has a honeycomb-like matrix internally, which helps to give the bone rigidity. Bone tissue is made up of different types of . and are involved in the formation and mineralisation of bone; are involved in the bone resorption of bone tissue. Modified (flattened) osteoblasts become the lining cells that form a protective layer on the bone surface. The mineralised matrix of bone tissue has an organic component of mainly collagen called ossein and an inorganic component of bone mineral made up of various salts. Bone tissue is mineralized tissue of two types, cortical bone and cancellous bone. Other types of tissue found in bones include bone marrow, endosteum, periosteum, , , and cartilage.
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, not counting numerous small . (2025). 9780761478829, Marshall Cavendish. ISBN 9780761478829 The largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear.
The Ancient Greek word for bone is ὀστέον (" osteon"), hence the many terms that use it as a prefix—such as osteopathy. In anatomical terminology, including the Terminologia Anatomica international standard, the word for a bone is (for example, short bone, long bone, sesamoid bone).
Bone matrix is 90 to 95% composed of elastic collagen fibers, also known as ossein, and the remainder is ground substance. The elasticity of collagen improves fracture resistance. The matrix is hardened by the binding of inorganic mineral salt, calcium phosphate, in a chemical arrangement known as bone mineral, a form of calcium apatite. It is the mineralization that gives bones rigidity.
Bone is actively constructed and remodeled throughout life by specialized bone cells known as osteoblasts and osteoclasts. Within any single bone, the tissue is woven into two main patterns: cortical and cancellous bone, each with a distinct appearance and characteristics.
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.
Type I collagen composes 90–95% of the organic matrix, with the remainder of the matrix being a homogenous liquid called ground substance consisting of 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 is produced when osteoblasts produce osteoid rapidly, which occurs initially in all fetus bones, but is later replaced by more resilient lamellar bone. In adults, woven bone is created after Bone fracture or in Paget's disease. 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,Salentijn, L. Biology of Mineralized Tissues: Cartilage and Bone, Columbia University College of Dental Medicine post-graduate dental lecture series, 2007 is stronger and filled with many collagen fibers parallel to other fibers in the same layer (these parallel columns are called osteons). In cross-section, the fibers run in opposite directions in alternating layers, much like in plywood, assisting in the bone's ability to resist torsion forces. 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 Micrometre per day. Lamellar bone also requires a relatively flat surface to lay the collagen fibers in parallel or concentric layers.
In order to mineralise the bone, the osteoblasts secrete alkaline phosphatase, some of which is carried by vesicles. This cleaves the inhibitory pyrophosphate and simultaneously generates free phosphate ions for mineralization, acting as the foci for calcium and phosphate deposition. Vesicles may initiate some of the early mineralization events by rupturing and acting as a centre for crystals to grow on. Bone mineral may be formed from globular and plate structures, and via initially amorphous phases.
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, are said to have a "head", "neck", and "body".
When two bones join, they are said to "articulate". If the two bones have a fibrous connection and are relatively immobile, then the joint is called a "suture".
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 .
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 Epiphysis 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. (2025). 9780781770552, Lippincott Williams & Wilkins. ISBN 9780781770552
The following steps are followed in the conversion of cartilage to bone:
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Bones protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs. Because of the way that bone is formed, bone has a high compressive strength of about , poor tensile strength of 104–121 MPa, and a very low shear stress strength (51.6 MPa). 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). While bone is essentially Brittleness, bone does have a significant degree of elasticity, contributed chiefly by collagen.
Mechanically, bones also have a special role in hearing. The ossicles are three small bones in the middle ear which are involved in sound transduction.
As well as creating cells, bone marrow is also one of the major sites where defective or aged red blood cells are destroyed.
Determined by the species, age, and the type of bone, bone cells make up to 15 percent of the bone. Growth factor storage—mineralized bone matrix stores important growth factors such as insulin-like growth factors, transforming growth factor, bone morphogenetic proteins and others. (1988). 9780470513637 ISBN 9780470513637
The issue of having fragile bones during our childhood leads to an increase in certain disorders and conditions such as juvenile osteoporosis, though it is less common to see, the necessity for a healthy routine especially when it comes to bone development is essential in our youth. Children that naturally have lower bone mineral density have a lower quality of life and therefore lead a life that is less fulfilling and uncomfortable. Factors such as increases in Calcium intake has been shown to increase BMD stores. Studies have shown that increasing calcium stores whether that be through supplementation or intake via foods and beverages such as leafy greens and milk have pushed the notion that prepuberty or even early pubertal children will see increases in BMD with the addition of increase Calcium intake.
Another research study goes on to show that long-term calcium intake has been proven to significantly contribute to overall BMD in children without certain conditions or disorders. This data shows that ensuring adequate calcium intake in children reinforces the structure and rate at which bones will begin to densify. Further detailing how structuring a strong nutritional plan with adequate amounts of Calcium sources can lead to strong bones but also can be a worth-while strategy into preventing further damage or degradation of bone stores as we age.
The connection between Calcium intake & BMD and its effects on youth as a whole is a very world-wide issue and has been shown to affect different ethnicities in a variety of differing ways. In a recent study, there was a strong correlation between calcium intake and BMD across a variety of diverse populations of children and adolescence ultimately coming to the conclusion that fundamentally, achieving optimal bone health is necessary for providing our youth with the ability to undergo hormonal changes as well. They found in a study of over 10,000 children ages 8–19 that in females, African Americans, and the 12–15 adolescent groups that at 2.6–2.8g/kg of body weight, they began to see a decrease in BMD. They elaborate on this by determining that this is strongly influenced by a lower baseline in calcium intake throughout puberty. Genetic factors have also been shown to influence lower acceptance of calcium stores.
Ultimately, the window that youth have for accruing and building resilient bones is very minimal. Being able to consistently meet calcium needs while also engaging in weight-bearing exercise is essential for building a strong initial bone foundation at which to build upon. Being able to reach our daily value of 1300 mg for ages 9–18 is becoming more and more necessary and as we progress in health, the chance that osteoporosis and other factors such as bone fragility or potential for stunted growth can be greatly reduced through these resources, ultimately leading to a more fulfilling and healthier lifestyle.
The action of osteoblasts and osteoclasts are controlled by a number of chemical 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. For example, the rate at which osteoclasts resorb bone is inhibited by calcitonin and osteoprotegerin. Calcitonin is produced by parafollicular cells in the thyroid gland, and can bind to receptors on osteoclasts to directly inhibit osteoclast activity. Osteoprotegerin is secreted by osteoblasts and is able to bind RANK-L, inhibiting osteoclast stimulation.
Osteoblasts can also be stimulated to increase bone mass through increased secretion of osteoid and by Enzyme inhibitor the ability of osteoclasts to break down osseous tissue. Increased secretion of osteoid is stimulated by the secretion of growth hormone by the pituitary, thyroid hormone and the sex hormones ( and ). These hormones also promote increased secretion of osteoprotegerin.
When a doctor sees a patient, a history and exam will be taken. Bones are then often imaged, called radiography. This might include ultrasound X-ray, CT scan, MRI scan and other imaging such as a Bone scan, which may be used to investigate cancer. Other tests such as a blood test for autoimmune markers may be taken, or a synovial fluid aspirate may be taken.
Fractures and their underlying causes can be investigated by , CT scans and . 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.
Bone tissue that is destroyed or altered as a result of cancers is distorted, weakened, and more prone to fracture. This may lead to compression of the spinal cord, destruction of the marrow resulting in bruising, bleeding and immunosuppression, and is one cause of bone pain. 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 cancer staging, prognosis, and what symptoms they cause. Many primary cancers of bone are treated with radiotherapy. Cancers of bone marrow may be treated with chemotherapy, and other forms of targeted therapy such as immunotherapy may be used. Palliative care, which focuses on maximising a person's quality of life, may play a role in management, particularly if the likelihood of survival within five years is poor.
Children makeup approximately 85% of Type 1 Diabetes cases and in America there was an average 22% rise in cases over the first 24 months of the COVID-19 Pandemic. With the increase of developing some form of diabetes across all ranges continually growing the health impacts on bone development and bone health in these populations are still being researched. Most evidence suggests that diabetes, either Type 1 and Type 2, inhibits osteoblastic activity and causes both lower BMD and BMC in both adults and children. The weakening of these developmental aspects is thought to lead to an increased risk of developing many diseases such as osteoarthritis, osteoporosis, osteopenia and fractures. Development of any of these diseases is thought to be correlated with a decrease in ability to perform in athletic environments and activities of daily living.
Focusing on therapies that target molecules like osteocalcin or AGEs could provide new ways to improve bone health and help manage the complications of diabetes more effectively.
One of the most important risk factors for osteoporosis is ageing. Accumulation of oxidative DNA damage in osteocyte and cells appears to be a key factor in age-related osteoporosis.Chen Q, Liu K, Robinson AR, et al. DNA damage drives accelerated bone aging via an NF-κB-dependent mechanism. J Bone Miner Res. 2013;28(5):1214-1228.
Osteoporosis treatment includes advice to stop smoking, decrease alcohol consumption, exercise regularly, and have a healthy diet. Calcium and trace mineral supplements may also be advised, as may Vitamin D. When medication is used, it may include , Strontium ranelate, and hormone replacement therapy.
Typically anthropologists and archeologists study made by Homo sapiens and Homo neanderthalensis. Bones can serve a number of uses such as projectile points or artistic pigments, and can also be made from external bones such as .
A bird's beak is primarily made of bone as projections of the which are covered in keratin.
Some bones, primarily formed separately in subcutaneous tissues, include headgears (such as bony core of horns, antlers, ossicones), osteoderm, and os penis/os clitoris. A deer's are composed of bone which is an unusual example of bone being outside the skin of the animal once the velvet is shed.
The extinct predatory fish Dunkleosteus had sharp edges of hard exposed bone along its jaws.
The proportion of cortical bone that is 80% in the human skeleton may be much lower in other animals, especially in and marine turtles, or in various Mesozoic , such as Ichthyosauria, among others. This proportion can vary quickly in evolution; it often increases in early stages of returns to an aquatic lifestyle, as seen in early and , among others. It subsequently decreases in pelagic taxa, which typically acquire spongy bone, but aquatic taxa that live in shallow water can retain very thick, Pachyostosis, Osteosclerosis, or pachyosteosclerotic bones, especially if they move slowly, like Sirenia. In some cases, even marine taxa that had acquired spongy bone can revert to thicker, compact bones if they become adapted to live in shallow water, or in Hypersaline lake (denser) water.
Many animals, particularly , practice osteophagy—the eating of bones. 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.
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.
Bone char, a porous, black, granular material primarily used for filtration and also as a black pigment, is produced by charring mammal bones.
Oracle bone script was a writing system used in ancient China based on inscriptions in bones. Its name originates from oracle bones, which were mainly ox clavicle. The Ancient Chinese (mainly in the Shang dynasty), would write their questions on the oracle bone, and burn the bone, and where the bone cracked would be the answer for the questions.
To point the bone at someone is considered bad luck in some cultures, such as Australian aborigines, such as by the Kurdaitcha.
The Furcula of fowl have been used for divination, and are still customarily used in a tradition to determine which one of two people pulling on either prong of the bone may make a wish.
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.
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(2025). 9781416023289, Saunders. ISBN 9781416023289 Osteoblasts can also be induced to secrete a number of 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.
Volume
Bone volume is determined by the rates of bone formation and bone resorption. 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. Cancellous bone volume in postmenopausal osteoporosis may be determined by the relationship between the total bone forming surface and the percent of surface resorption.
Clinical significance
A number of diseases can affect bone, including arthritis, fractures, infections, osteoporosis and tumors. Conditions relating to bone can be managed by a variety of doctors, including Rheumatology for joints, and orthopedic surgeons, who may conduct surgery to fix broken bones. Other doctors, such as rehabilitation specialists may be involved in recovery, radiology in interpreting the findings on imaging, and in investigating the cause of the disease, and may play a role in preventing complications of bone disease such as osteoporosis.
Fractures
In normal bone, Bone fracture 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 and , 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.
Tumors
Tumor that can affect bone in several ways. Examples of benign include osteoma, osteoid osteoma, osteochondroma, osteoblastoma, enchondroma, giant-cell tumor of bone, and aneurysmal bone cyst.
Cancer
Cancer can arise in bone tissue, and bones are also a common site for other cancers to spread (metastasise) to. Cancers that arise in bone are called "primary" cancers, although such cancers are rare. Metastases within bone are "secondary" cancers, with the most common being breast cancer, lung cancer, prostate cancer, thyroid cancer, and kidney cancer. Secondary cancers that affect bone can either destroy bone (called a "lytic cycle" cancer) or create bone (a "sclerotic" cancer). Cancers of the bone marrow inside the bone can also affect bone tissue, examples including leukemia and multiple myeloma. Bone may also be affected by cancers in other parts of the body. Cancers in other parts of the body may release parathyroid hormone or parathyroid hormone-related peptide. This increases bone reabsorption, and can lead to bone fractures.
Diabetes
Type 1 diabetes is an autoimmune disease in which the body attacks the insulin-producing pancreas cells causing the body to not make enough insulin. In contrast type 2 diabetes in which the body creates enough Insulin, but becomes resistant to it over time.
Other painful conditions
Osteoporosis
Osteoporosis is a disease of bone where there is reduced bone mineral density, increasing the likelihood of bone fracture. Osteoporosis is defined in women by the World Health Organization as a bone mineral density of 2.5 standard deviations below peak bone mass, relative to the age and sex-matched average. This density is measured using dual energy X-ray absorptiometry (DEXA), with the term "established osteoporosis" including the presence of a fragility fracture. 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 tobacco smoking and medications, specifically . 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.
Osteopathic medicine
Osteopathic medicine is a school of medical thought that links the musculoskeletal system to overall health. , over 77,000 physicians in the United States are trained in osteopathic medical schools.
Bone health
Bone health is vastly important all throughout life due to a number of reasons, some of those being, without strong healthy bones we are more at risk for different chronic diseases, and fractures as well as day-to-day function being more difficult with poor bone health. Developing strong bones as a child is one of the most important steps to having healthy bones all throughout life because this is when a strong foundation is built, which will make it much easier to maintain musculoskeletal health in later years. Adolescence offers a window to really develop bones in either a positive or negative way. It is estimated that diet and exercise during these years can impact peak bone mass as an adult nearly 20–40%. One study done on children with developmental coordination disorder found an increase in bone mass up to 4% and 5% in the cortical areas of the tibia alone from a 13-week training period, which is truly significant when considering how participants only participated in the multimodal workouts twice per week, and it would be reasonable to expect these increases to be greater if workouts were more frequent, especially in youth without developmental coordination disorder. Peak bone mass occurs between the second and third decade of most people's lives, and with this being the case if we can really stockpile as much bone mass and increase our BMD and BMC by living healthy active lives, and having good diets that consume adequate calcium and vitamin D then we will truly have a leg up in our later lives as well as actively decreasing risks of certain chronic diseases such as osteoporosis.
Osteology
The study of bones and teeth is referred to as osteology. It is frequently used in anthropology, archeology and forensics 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 the process of maceration.
Other animals
Bird skeletons are very lightweight. Their bones are smaller and thinner, to aid flight. Among mammals, 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 them being hollow.
Society and culture
Bones from slaughtered animals have a number of uses. In prehistoric times, they have been used for making . They have further been used in bone carving, already important in prehistoric art, and also in modern time as crafting materials for , , , , calculation aids, , dice, , pick-up sticks, , scrimshaw, ornaments, etc.
See also
Sources
(2025). 9780702030857, Churchill Livingstone/Elsevier. ISBN 9780702030857
(2025). 9780721602400, W.B. Saunders. ISBN 9780721602400
(2025). 9780443068508, Churchill Livingstone/Elsevier. ISBN 9780443068508
Further reading
(2025). 9780471689348, Wiley. ISBN 9780471689348
(2025). 9780071476928, McGraw-Hill Medical. ISBN 9780071476928
(2025). 9780805359091, Benjamin Cummings. ISBN 9780805359091
(2013). 9783642023996, Springer. ISBN 9783642023996
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