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illustration of a human Femur .]] Bones are rigid Organs that form part of the Endoskeleton of Vertebrate s. Bones function to move, support, and protect the body, produce Red and White Blood Cell s and store minerals. Bones come in a variety of shapes and have a complex internal and external structure, allowing them to be lightweight yet strong and hard, while fulfilling their many other functions. One of the types of tissues that makes up bones is the mineralized Osseous Tissue , also called bone tissue, that gives bones their rigidity and honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include Marrow , Endosteum and Periosteum , Nerve s, Blood Vessel s and Cartilage . FUNCTIONS Bones have eight main functions:
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. The human bone is 5 times stronger than steel. 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. Periosteum The exterior of bones (except where they interact with other bones through Joint s) is covered by the Periosteum , which has an external fibrous layer, and an internal Osteogenic layer. The periosteum is richly supplied with blood, lymph and nerve vessels, attaching to the bone itself through Sharpey's Fibres . OSTEONAL STRUCTURE See Also: Osteon Osteons are the predominant structure found in some compact bone. Their main features are a haversian canal in the centre, surrounded by lamellae, in which, in turn, osteocytes are interspersed. The osteocytes are connected by canaliculi, enabling transport of nutrients. CELLULAR STRUCTURE There are several types of cells constituting the bone;
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 Fibril s. Also making up the organic part of matrix include various growth factors, the functions of which are not fully known. Other factors present include Glycosaminoglycan s, 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.
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: # Development of ossification center # Calcification # Formation of trabeculae # 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: # Development of cartilage model # Growth of cartilage model # Development of the primary ossification center # Development of medullary cavity # Development of the secondary ossification center # 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 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 Tool s 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 Evolution ary alternatives to mammilary bone; though they have some similar functions, they are not completely functionally analogous to bone.
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:
TERMINOLOGY Several terms are used to refer to features and components of bones throughout the body: Several terms are used to refer to specific features of long bones: REFERENCES
SEE ALSO EXTERNAL LINKS
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