Bone
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. The largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear. Anatomists classify these structures into five distinct shapes based on their appearance and function. Long bones feature a shaft called the diaphysis that is much longer than its width. They also possess an epiphysis, which is a rounded head at each end of the shaft. Most bones of the limbs fall into this category, including those of the fingers and toes. Exceptions include the eight carpal bones of the wrist and the seven articulating tarsal bones of the ankle. Short bones are roughly cube-shaped and have only a thin layer of compact bone surrounding a spongy interior. These provide stability and support as well as some limited motion. Flat bones are thin and generally curved with two parallel layers of compact bone sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones, as is the sternum. Sesamoid bones are embedded within tendons to increase muscle leverage. Examples include the patella and the pisiform. Irregular bones do not fit into the above categories and often contain bony sinuses. The bones of the spine, pelvis, and some bones of the skull are irregular bones.
Bone tissue is metabolically active and composed of several types of cells working in tandem. Osteoblasts are mononucleate bone-forming cells located on the surface of osteon seams. They make a protein mixture known as osteoid which mineralizes to become bone. The osteoid seam is a narrow region of newly formed organic matrix that is not yet mineralized. Osteoblasts also manufacture hormones such as prostaglandins to act on the bone itself. When an osteoblast finishes its work it becomes trapped inside the hardening bone and transforms into an osteocyte. Osteocytes are cells of mesenchymal origin that originate from osteoblasts that have migrated into and become surrounded by a bone matrix they produced themselves. These cells occupy spaces called lacunae while their processes occupy channels called canaliculi. Osteoclasts are very large multinucleate cells responsible for the breakdown of bones through resorption. New bone is then formed by the osteoblasts to replace what was removed. Bone is constantly remodeled by this cycle of resorption followed by replacement with little change in shape. Approximately 10% of the skeletal mass of an adult is remodelled each year.
Bones serve multiple metabolic functions beyond simple structural support. Bones act as reserves of minerals important for the body, most notably calcium and phosphorus. The cancellous part of bones contains bone marrow where blood cell production occurs. This process is called hematopoiesis. Every day over 2.5 billion red blood cells and platelets are produced in this way. White blood cells including granulocytes number between 50 and 100 billion daily. As well as creating cells, bone marrow is also one of the major sites where defective or aged red blood cells are destroyed. Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts. This helps regulate acid-base balance throughout life. Bone tissue can store heavy metals and other foreign elements removing them from the blood. These stored toxins can later be gradually released for excretion. Bone controls phosphate metabolism by releasing fibroblast growth factor 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 and fat deposition.
The formation of bone is called ossification and occurs during the fetal stage through two processes. Intramembranous ossification involves the formation of bone from connective tissue rather than from cartilage. This mainly occurs during formation of the flat bones of the skull but also the mandible and maxilla. Endochondral ossification involves the development of bone from cartilage. This process includes the development of a cartilage model its growth and development. Secondary ossification occurs after birth and forms the epiphyses of long bones. The diaphysis and both epiphyses of a long bone are separated by a growing zone of cartilage known as the epiphyseal plate. At skeletal maturity between 18 to 25 years of age all of the cartilage is replaced by bone. This fuses the diaphysis and both epiphyses together in a process called epiphyseal closure. Woven bone is produced when osteoblasts produce osteoid rapidly initially in all fetal bones. It is later replaced by more resilient lamellar bone which has a regular parallel alignment of collagen into sheets. Lamellar bone makes its first appearance in humans in the fetus during the third trimester.
A number of diseases can affect bone including arthritis fractures infections osteoporosis and tumors. When a doctor sees a patient bones are often imaged using radiography. This might include ultrasound X-ray CT scan MRI scan and other imaging such as a Bone scan. 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 or DEXA scans. Osteoporosis is most common in women after menopause when it is called postmenopausal osteoporosis. One of the most important risk factors for osteoporosis is advanced age. Accumulation of oxidative DNA damage in osteoblastic and osteoclastic cells appears to be a key factor in age-related osteoporosis. Fractures occur when there is significant force applied or repetitive trauma over a long time. 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.
Bird skeletons are very lightweight with bones smaller and thinner than those of mammals to aid flight. Among mammals bats come closest to birds in terms of bone density suggesting small dense bones are a flight adaptation. Many bird bones have little marrow due to them being hollow. The proportion of cortical bone that is 80% in the human skeleton may be much lower in other animals especially marine mammals and marine turtles. In early whales and pinnipeds this proportion often increases as they return to an aquatic lifestyle. Subsequently pelagic taxa acquire spongy bone but aquatic taxa living in shallow water retain thick compact bones. Sea cows exhibit pachyostotic or osteosclerotic bones if they move slowly. The extinct predatory fish Dunkleosteus had sharp edges of hard exposed bone along its jaws. A deer's antlers are composed of bone which is an unusual example of bone being outside the skin once the velvet is shed. Many animals particularly herbivores practice osteophagy the eating of bones to replenish lacking phosphate.
In prehistoric times bones were used for making tools and carving art. They have further been used in modern time as crafting materials for buttons beads handles bobbins dice and arrows. Bone glue can be made by prolonged boiling of ground or cracked bones followed by filtering and evaporation. 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 would write their questions on the oracle bone and burn the bone where it cracked would be the answer. The wishbones 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 may make a wish. To point the bone at someone is considered bad luck in some cultures such as Australian aborigines. Various cultures throughout history adopted the custom of shaping an infant's head by artificial cranial deformation. A widely practised custom in China was that of foot binding to limit the normal growth of the foot.
Common questions
How many bones are in the adult human body?
The adult human body contains a total of 206 separate bones. This number results from approximately 300 bones present at birth fusing together during development.
What is the largest bone in the human body and where is it located?
The femur or thigh-bone is the largest bone in the human body. It functions as a long bone with a shaft called the diaphysis and rounded heads known as epiphyses.
When does skeletal maturity occur and what happens to the epiphyseal plate?
Skeletal maturity occurs between 18 to 25 years of age when all cartilage is replaced by bone. This process fuses the diaphysis and both epiphyses together in an event called epiphyseal closure.
Why do birds have hollow bones compared to mammals?
Bird skeletons are very lightweight with bones smaller and thinner than those of mammals to aid flight. Many bird bones contain little marrow because they are hollow structures designed for aerial movement.
How was oracle bone script used in ancient China?
Oracle bone script was a writing system used in ancient China based on inscriptions made mainly on ox clavicle bones. The Ancient Chinese would write questions on the bone and burn it so cracks would provide answers.