bone bone a bone is a rigid 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 minerals, 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. bonea bone dating from the pleistocene ice age of an extinct species of elephanta scanning electronic micrograph of bone at 10,000× magnificationidentifiersmeshd001842ta98a02.0.00.000ta2366, 377thh3.01.00.0.00001 fma5018anatomical terminology bone tissue (osseous tissue), which is also called bone in the uncountable sense of that word, is hard tissue, a type of specialized connective tissue. it has a honeycomb-like matrix internally, which helps to give the bone rigidity. bone tissue is made up of different types of bone cells. osteoblasts and osteocytes are involved in the formation and mineralization of bone; osteoclasts are involved in the resorption of bone tissue. modified (flattened) osteoblasts become the lining cells that form a protective layer on the bone surface. the mineralized 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, nerves, blood vessels and cartilage. in the human body at birth, there are approximately 300 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 largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear. the 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 os (for example, os breve, os longum, os sesamoideum). structure 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. 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 hydroxylapatite. it is the mineralization that gives 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, each with a different appearance and characteristics. cortex cross-section details of a long bone 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. each column is multiple layers of osteoblasts and osteocytes around a central canal called the haversian canal. 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. cortical bone is covered by a periosteum on its outer surface, and an endosteum on its inner surface. 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. trabeculae further information: trabecula § bone trabecular micrograph of cancellous bone cancellous bone, or spongy bone, also known as trabecular bone, is the internal tissue of the skeletal bone and is an open cell porous network that follows the material properties of biofoams. cancellous bone has a higher surface-area-to-volume ratio than cortical bone and it is less dense. 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. cancellous bone is highly vascular and often contains red bone marrow where hematopoiesis, the production of blood cells, occurs. 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. as far as short bones are concerned, trabecular alignment has been studied in the vertebral pedicle. thin formations of osteoblasts covered in endosteum create an irregular network of spaces, known as trabeculae. within these spaces are bone marrow and hematopoietic stem cells that give rise to platelets, red blood cells and white blood cells. 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. marrow bone marrow, also known as myeloid tissue in red 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 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. vascular supply bone receives about 10% of cardiac output. blood enters the endosteum, flows through the marrow, and exits through small vessels in the cortex. in humans, blood oxygen tension in bone marrow is about 6.6%, compared to about 12% in arterial blood, and 5% in venous and capillary blood. cells bone cells bone is metabolically active tissue composed of several types of cells. these cells include osteoblasts, which are involved in the creation and mineralization of bone tissue, osteocytes, and osteoclasts, which are involved in the reabsorption of bone tissue. osteoblasts and osteocytes are derived from osteoprogenitor cells, but osteoclasts are derived from the same cells that differentiate to form macrophages and monocytes. within the marrow of the bone there are also hematopoietic stem cells. these cells give rise to other cells, including white blood cells, red blood cells, and platelets. osteoblast light micrograph of decalcified cancellous bone tissue displaying osteoblasts actively synthesizing osteoid, containing two osteocytes. osteoblasts are mononucleate bone-forming cells. they are located on the surface of osteon seams and make a protein mixture known as osteoid, which mineralizes to become bone. the osteoid seam is a narrow region of a newly formed organic matrix, not yet mineralized, located on the surface of a bone. osteoid is primarily composed of type i collagen. osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. 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 osteoblast then deposits calcium phosphate which is hardened by hydroxide and bicarbonate ions. 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. osteocyte osteocytes are cells of mesenchymal origin and originate from osteoblasts that have migrated into and become trapped and surrounded by a 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. osteoclast osteoclasts are very large multinucleate cells that are responsible for the breakdown of bones by the process of bone resorption. 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. because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with phagocytic-like mechanisms 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. the reabsorption of bone by osteoclasts also plays a role in calcium homeostasis. composition main article: extracellular matrix bones consist of living cells (osteoblasts and osteocytes) embedded in a mineralized organic matrix. the primary inorganic component of human bone is hydroxyapatite, the dominant bone mineral, having the nominal composition of ca10(po4)6(oh)2. the organic components of this matrix consist mainly of type i collagen—"organic" referring to materials produced as a result of the human body—and inorganic components, which alongside the dominant hydroxyapatite phase, include other compounds of calcium and phosphate including salts. approximately 30% of the acellular component of bone consists of organic matter, while roughly 70% by mass is attributed to the inorganic phase. the collagen fibers give bone its tensile strength, and the interspersed crystals of hydroxyapatite give bone its compressive strength. 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 be found. 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 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. transmission electron micrograph of decalcified woven bone matrix displaying characteristic irregular orientation of collagen fibers 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. in adults, woven bone is created after fractures 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, is stronger and filled with