HOME WEB NEWS IMAGES CLASSIFIEDS YELLOW PAGESPOLLS - SURVEYS WIKI COUNTRIES PHOTOS US UK INDIA
Cities Stocks Weather Movies Coupons Travel Blogs Health Horoscope Maps ASIA EUROPE Classifieds Yellow Pages
|
 2
|
This article is about the skeletal organs. For the tissue, see Osseous tissue. For other uses, see Bone (disambiguation).
Gray\'s Anatomy illustration of a human femur.
Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Because bones come in a variety of shapes and have a complex internal and external structure, they are lightweight, yet strong and hard, in addition to fulfilling their many other functions. One of the types of tissues that makes up bone is the mineralized osseous tissue, also called bone tissue, that gives it rigidity and honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage. There are 206 bones in the adult body and about 300 bones in the infant body.
Bones have eight main functions:
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 mineralized organic matrix that makes up the osseous tissue.
| Bones (Axial skeleton, Appendicular skeleton) | |
|---|---|
| VERTEBRAL COLUMN | vertebrae (cervical - thoracic - lumbar) - sacrum - coccyx |
| THORAX | sternum - rib |
| cranial bones of SKULL | occipital - parietal - frontal - temporal - sphenoid - ethmoid |
| facial bones of SKULL | nasal - maxilla - lacrimal - zygomatic - palatine - inferior nasal conchae - vomer - mandible - THROAT: hyoid (greater cornu, lesser cornu, body) |
| UPPER EXTREMITY | SHOULDER GIRDLE:clavicle - scapula - ARM: humerus - ulna - radius- HAND:carpals (scaphoid - lunate bone - triquetral - pisiform - trapezium - trapezoid - capitate - hamate) - metacarpals - phalanges (prox - int - dist) |
| LOWER EXTREMITY | PELVIS:pelvis (ilium, ischium, pubis) - LEG: femur - patella - fibula - tibia - FOOT: tarsals (calcaneus - talus - navicular - cuneiform - cuboid ) - metatarsals - phalanges (prox - int - dist) |
| MIDDLE EAR OSSICLES | malleus - incus - stapes |
| Bones of head and neck: the cranium of the skull | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Occipital |
| ||||||||||
| Parietal | Parietal eminence - Temporal line - Parietal foramen | ||||||||||
| Frontal |
| ||||||||||
| Temporal |
| ||||||||||
| Sphenoid |
| ||||||||||
| Ethmoid |
| ||||||||||
| Bones of head and neck: cranial sutures, fontanelles, and related regions | |
|---|---|
| Cranial sutures | Cranial: Frontoethmoidal - Frontal or Metopic (Frontal/Frontal) - Coronal (Frontal/Parietal) - Occipitomastoid (Occipital/Temporal) - Lambdoid (Parietal/Temporal) - Sagittal (Parietal/Parietal) - Sphenoethmoidal - Sphenofrontal - Sphenoparietal - Sphenosquamosal (Sphenoid/Temporal) - Sphenopetrosal (Sphenoid/Temporal) -Squamosal (Temporal/Parietal) - Petrosquamous (Temporal/Temporal)
Facial: Palatomaxillary suture Cranial-facial: Sphenozygomatic - Zygomaticotemporal - Zygomaticofrontal |
| Fontanelles | Anterior fontanelle - Posterior fontanelle - Sphenoidal fontanelle - Mastoid fontanelle |
| Foramina of multiple bones | Inferior orbital fissure - Foramen lacerum - Jugular foramen - Nasolacrimal canal |
| Fossae | Anterior cranial fossa - Middle cranial fossa - Posterior cranial fossa - Cranial cavity |
| Other compound structures | Cranial: Asterion - Pterion - Calvaria - Stephanion - Bregma - Lambda
Facial: Nasion Cranial-facial: Dacryon - Zygomatic arch - Temporal fossa - Infratemporal fossa - Pterygomaxillary fissure - Pterygopalatine fossa |
| Bones of upper limbs | |
|---|---|
| Pectoral girdle, clavicle | conoid tubercle - trapezoid line - costal tuberosity - subclavian groove |
| Scapula | fossae (subscapular, supraspinatous, infraspinatous) - suprascapular notch - glenoid cavity
tubercles (infraglenoid, supraglenoid) - spine of scapula - acromion - coracoid process borders (superior, lateral/axillary, medial/vertebral) - angles (superior, inferior, lateral) |
| Humerus | upper extremity: necks (anatomical, surgical) - tubercles (greater, lesser) - intertubercular sulcus
body: radial sulcus - deltoid tuberosity lower extremity: capitulum - trochlea - epicondyles (lateral, medial) - supracondylar ridges (lateral, medial) - fossae (radial, coronoid, olecranon) |
| Forearm | radius: upper extremity (head, tuberosity) - body - lower extremity (ulnar notch, styloid process) ulna: upper extremity (tuberosity, olecranon, coronoid process, radial notch, trochlear notch) - body - lower extremity (head, styloid process) |
| Hand | carpus: scaphoid - lunate - triquetral - pisiform - trapezium - trapezoid - capitate - hamate (hamulus)
metacarpus: 1st metacarpal - 2nd - 3rd - 4th - 5th phalanges of the hand: proximal - intermediate - distal |
| Bones of torso | |
|---|---|
| Sternum | Suprasternal notch, Manubrium, Sternal angle, Body of sternum, Xiphisternal joint, Xiphoid process |
| Rib | specific ribs (1, 2, 9, 10, 11, 12, false - 8-12, floating - 11-12) - parts (Angle, Tubercle, Costal groove, Neck, Head) |
| General vertebral structures | body of vertebra, vertebral arch (pedicle, lamina, vertebral notch), foramina (vertebral, intervertebral), processes (transverse, articular / zygapophysis, spinous) |
| Cervical vertebrae | C1 (anterior arch, posterior arch, lateral mass), C2 (dens), C3, C4, C5, C6, C7
anterior tubercle, posterior tubercle, foramen transversarium |
| Thoracic vertebrae | T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12
costal facets (superior, inferior, transverse) |
| Lumbar vertebrae | accessory process, mammillary process, L5 |
| Sacrum | pelvic surface (anterior sacral foramina), dorsal surface (posterior sacral foramina, median sacral crest, medial sacral crest, lateral sacral crest), lateral surface (sacral tuberosity), base, sacral hiatus - presacral space - sacral promontory - sacral canal - ala of sacrum - sacrovertebral angle |
| Bones of lower limbs | |||||||
|---|---|---|---|---|---|---|---|
| Femur |
| ||||||
| Tibia |
| ||||||
| Fibula | head · body · lateral malleolus | ||||||
| Tarsus | calcaneus (sustentaculum tali, trochlear process) · talus (body, neck, head) · navicular · cuboid · cuneiform (medial, intermediate, lateral) | ||||||
| Metatarsus | 1st metatarsal · 2nd · 3rd · 4th · 5th | ||||||
| Other | patella · phalanges of the foot | ||||||
| Bones of pelvis/pelvic cavity | |
|---|---|
| General | sacrum, coccyx, hip bone |
| Ilium | Body (Arcuate line) Wing: gluteal lines (Posterior, Anterior, Inferior) - Fossa - iliac spines (Anterior superior, Anterior inferior, Posterior superior, Posterior inferior) - Crest - Tuberosity |
| Ischium | Body (Ischial spine, Lesser sciatic notch) - Superior ramus (Tuberosity of the ischium) - Inferior ramus |
| Pubis | Body - Superior ramus (Pubic tubercle, Pubic crest, Obturator crest) - Inferior ramus (Pectineal line) |
| Compound | Acetabulum (Acetabular notch) - Iliopubic eminence/Iliopectineal line - Linea terminalis - Ischiopubic ramus/Pubic arch
Obturator foramen - Greater sciatic foramen/Greater sciatic notch - Lesser sciatic foramen Lesser pelvis (Pelvic inlet, Pelvic brim, Pelvic outlet) - Greater pelvis |
Bone is not a uniformly solid material, but rather has some spaces between its hard elements.
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.
Filling the interior of the organ is the trabecular bone tissue (an open cell porous network also called cancellous or spongy bone) which is composed 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.
There are several types of cells constituting the bone;
The matrix is the major constituent of bone, surrounding the cells. 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 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.
The organic part of matrix is mainly composed of Type I collagen. This is synthesised 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 glycosaminoglycans, 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.
Bone is first deposited as woven bone, in a disorganized structure with a high proportion of osteocytes in young and in healing injuries. Woven bone is weaker, with a small number of randomly oriented collagen fibers, but forms quickly. It is replaced by lamellar bone, which is highly organized in concentric sheets with a low proportion of osteocytes. Lamellar bone is stronger and filled with many collagen fibers parallel to other fibers in the same layer. The fibers run in opposite directions in alternating layers, much like plywood, 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."
There are five types of bones in the human body: long, short, flat, irregular and sesamoid.
The formation of bone during the fetal stage of development occurs by two processes: intramembranous and endochondral 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:
Endochondrial 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:
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).
There are two types of bone marrow, yellow and red, most commonly seen is 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 , 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 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.
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.
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.
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. Netter, p. 187-189, A scholarly review.
The action of osteoblasts and osteoclasts are contolled by a number of chemical factors which either promote or inhibit the activity of the bone remodelling 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.
Osteoblasts can be stimulated to increase bone mass through increased secretion of osteoid and by inhibiting the ability of osteoclasts to break down osseous tissue.
Bone building through increased secretion of osteoid is stimulated by the secretion of growth hormone by the pituitary, thyroid hormone and the sex hormones (estrogens and androgens). These hormones also promote increased secretion of osteoprotegerin.Boulpaep, Emile L.; Boron, Walter F. (2005). Medical physiology: a cellular and molecular approach. Philadelphia: Saunders, p.1089-1091. ISBN 1416023283. Osteoblasts can also be induced to secrete a number of cytokines that promote reabsorbtion 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 reabsorbtion 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.
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.
There are many disorders of the skeleton. One of the more prominent is osteoporosis.
Osteoporosis is a disease of bone - leading to an increased risk of fracture. In osteoporosis, the bone mineral density (BMD) is reduced, bone microarchitecture is disrupted, and the amount and variety of non-collagenous proteins in bone is altered. Osteoporosis is defined by the World Health Organization (WHO) in women as a bone mineral density 2.5 standard deviations below peak bone mass (20-year-old sex-matched healthy person average) as measured by DXA; the term "established osteoporosis" includes the presence of a fragility fracture.WHO (1994). "Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. Report of a WHO Study Group". World Health Organization technical report series 843: 1–129. PMID 7941614. Osteoporosis is most common in women after the menopause, when it is called postmenopausal osteoporosis, but may develop in men and premenopausal women in the presence of particular hormonal disorders and other
