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Functions Long bones can be connected to skeletal muscles via tendons. Bones connect at joints by ligaments. The interaction between bone and muscle is studied in biomechanics. The red bone marrow, located in the spongy bone matrix, allows for bones to produce red blood cells (hemopoiesis). Also, the bone marrow located at the bones diaphysis allow for white blood cells to be produced. Structure
Bone cells Osteoblasts are typically viewed as bone forming cells. They are located near to the surface of bone and their functions are to make osteoid (a protein mixture which is secreted by osteoblasts. When it mineralizes, it becomes bone. Osteoid is primarily composed of Type I collagen.) and manufacture hormones, such as prostaglandins, which act on bone itself. Osteoblasts are mononucleate. Active osteoblasts are situated on the surface of osteoid seams. They robustly produce alkaline phosphatase, a chemical that has a role in the mineralisation of bone, as well as many matrix proteins. Bone lining cells (BLCs) share a common lineage with osteogenic (bone forming) cells. They function as a barrier for certain ions, induced osteogenetic cells. They are flattened, mononucleate cells which line bone. Osteocytes originate from osteoblasts which have migrated into and become trapped and surrounded by bone matrix which they themselves produce. The space which they occupy is known as a lacuna. 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 bones' response to stress. Osteoclasts are the cells responsible for bone resorption. Osteoclasts are large, multinucleated cells located on bone surfaces in what are called Howship's lacunae. 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. The process of bone resorption 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, resorption actively unfixes it thereby increasing circulating calcium levels. These processes occur in tandem at site-specific locations and are known as bone turnover or remodeling. Osteoblasts and osteoclasts, coupled together via paracrine cell signalling, are referred to as bone remodeling units. The iteration of remodeling events at the cellular level is influential on shaping and sculpting the skeleton both during growth as well as after. Matrix The matrix comprises the other major constituent of bone. It has inorganic and organic parts. 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 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. 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 GAGs, osteocalcin, osteonectin, bone sialo protein and Cell Attachment Factor. Formation The formation of bone during the fetal stage of development occurs by two methods: intramembranous and endochondral ossification. Intramembranous ossification Endochondral ossification 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). 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 marrow (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 is the process of resorption followed by replacement of bone with little change in shape and occurs throughout a person's life. Its purpose is the release of calcium and the repair of micro-damaged bones (from everyday stress). Repeated stress results in the bone thickening at the points of maximum stress. It has been hypothesized that this is a result of bone's piezoelectric properties, which cause bone to generate small electrical potentials under stress. Bone pathologies One of the most common bone illnesses is a bone fracture. Bones heal by natural processes, but untended and unsupported can lead to misgrown bone. Other illnesses are for example osteoporosis, osteonecrosis and bone cancer (osteosarcoma). The joints can be affected by arthritis. Exposed bone Bone penetrating the skin and being exposed to the outside, apart from the effect of injury, occasionally is a normal feature:- Terminology Several terms are used to refer to features and components of bones: Several terms are used to refer to specific features of long bones: Several terms are used to describe the orientation of bones with respect to axes of the body, generally those anatomical terms of location, especially: Notes See also | ||||||||||||
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