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45 Cards in this Set
- Front
- Back
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Characteristics and Functions of Bone Tissue
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Bone is alive and continuously changing.
Bone Tissue and “Bones” bone tissue (osseous tissue) is a type of connective tissue with cells, vessels, nerves and a matrix hardened by minerals (mostly calcium phosphate) bones are units of the skeletal system individual bones are made up of bone tissue, marrow, cartilage and periosteum Functions of the skeletal system include support, protection, movement, blood formation, calcium and phosphate reservoir, pH balance |
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What are the diverse shapes of bones and their functions?
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Long Bones provide leverage for movement.
Short Bones provide limited motion in several directions. Flat Bones protect soft organs and provide broad surface for attachment of powerful muscles. Irregular Bones serve multiple, complex functions. |
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What are the different structural characteristics of Long Bone?
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Bone is surrounded by a vascular membrane called the periosteum.
The epiphysis is covered with articular cartilage (hyaline cartilage). The outer layer of bone is compact bone . Long bones are filled with spongy bone and marrow. The membrane lining the marrow cavity and all internal passages is the endosteum. |
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Features of long bones?
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Diaphysis (shaft) is a cylinder of compact bone containing the marrow cavity (medullary cavity)
the diaphysis covered with periosteum Marrow Cavity contains hematopoeitic tissue Epiphyses (enlarged ends) are spongy bone covered with a layer of compact bone enlarged ends strengthen joint and provide attachment of tendons and ligaments Joint Surface is covered with articular cartilage (smooth, low friction hyaline cartilage and synovial fluid) |
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What is the Periosteum? what is the endosteum?
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Periosteum is a layer of vascular, innervated, dense connective tissue surrounding the non-articular surfaces of bone. A thin layer of cell-rich connective tissue, the endosteum, lines the surface of the bone facing the marrow cavity. Both the periosteum and the endosteum possess osteogenic potency. During growth or following injury, cells in these layers may differentiate into bone forming osteoblasts. Perforating (Sharpey’s) fibers are collagen fibers that connect the periosteum to the bone matrix
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What is the structure of flat bone?
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External and internal surfaces of flat bone are composed of compact bone.
Middle layer is spongy bone (the diploe), but there is no marrow cavity. An impact to the skull may fracture the outer layer and crush the spongy bone, but not harm inner compact bone or the underlying brain. Periosteum covers the outer surface. Periosteum of inner surface blends with the meninges of the brain. |
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What are Osteogenic Cells?
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Osteogenic cells are in the endosteum and periosteum membranes.
arise from embryonic mesenchymal cells and become the only source of new osteoblasts multiply continuously and differentiate into osteoblasts in response to stress or fractures Osteoblasts produce an extracellular matrix of about 50% collagen (mostly type I) and 50% mineral (mostly calcium phosphate). Osteocytes are osteoblasts that have become completely surrounded by the matrix they produced. Spicules are isolated, newly formed bone fragments. |
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What are osteocytes?
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Osteocytes are arranged in rings in the bone matrix around a central canal that contains blood vessels and nerves.
Tips of the pseudopods of adjacent osteocytes are inter-connected with gap junctions. Osteocytes are completely surrounded by bone matrix. Lacunae are the pits in the matrix occupied by the cells. Filopods (pseudopods, cytoplasmic extensions, dendritic processes) are long thin extensions of the osteocytes that produce bone matrix around the cell. Filopods are within tiny canals in the matrix called canaliculi. Osteocytes use the gap junctions to communicate and to nourish each other. |
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What is the composition of Bone matrix and its functions?
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Bone Matrix is about 50% organic and 50% inorganic.
Organic matter consists of: type I collagen glycosaminoglycans (chondroitin sulfate) proteoglycans glycoproteins Inorganic matter consists of about: 85% hydroxyapatite (crystallized calcium phosphate salt) 10% calcium carbonate 5% other minerals (fluoride, sulfate, potassium, magnesium) Combination makes bones both strong and resilient minerals resist compression; collagen resists tension bone is similar to fiberglass: strong glass fibers embedded in a flexible polymer bone adapts to tension and compression by varying proportions of minerals and collagen fibers |
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What is Compact Bone?
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Compact Bone does not have any hollow spaces in the bone matrix. Compact bone forms the thick-walled tube of the shaft (or diaphysis) of long bones, which surrounds the marrow cavity (or medullary cavity). A thin layer of compact bone also covers the epiphyses of long bones which is covered with hyaline cartilage.
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What is Trabecular bone?
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Trabecular Bone (also called cancellous or spongy bone) consists of a delicate network of trabeculae which branch and intersect to form a sponge-like tissue. The ends of long bones (or epiphyses) consist mainly of trabecular bone.
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What are the characteristics of Compact Bone?
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Osteon (Haversian System) is the basic structural unit of bone.
Osteons are cylinders of tissue formed from layers (concentric lamellae) of matrix arranged around a central (Haversian) canal. Central canal is lined with endosteum and contains a neurovascular bundle. Osteocytes are connected to each other and their blood supply by thin pseudopods within canaliculi. Perforating Canals (Volkmann’s canals) are perpendicular branches of the central (Haversian) canal. Circumferential Lamellae (Outer Lamellae) ring the outer circumference of the diaphysis. Bony trabeculae branch off the compact bone surrounding the marrow cavity, filling the cavity with spongy bone. |
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Describe the structure of compact bone.
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Bone matrix is produced by functional units called Haversian systems or osteons (O). Each osteon has a Haversian canal (HC) at the center that is surrounded by concentric layers of bone and osteocytes. Volkmann’s canals (VC) are lateral branches of Haversian canals. The outermost layers of bone just under the periosteum are circumferential lamellae (CL). Interstitial lamellae are remnants of old osteons.
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How does compact bone form?
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Osteoblasts develop from the endosteum membrane lining the Haversion canal (HC). Once osteoblasts are completely surrounded by bone matrix they become osteocytes. Concentric layers of bone matrix are added around the Haversian canal like growth rings of a tree. Lacunae (L) are cavities in the bone matrix that contain bone cells. Osteocytes have many thin processes (filopods) that radiate off the cell body. Bone matrix is secreted from the filopods and encases them in canaliculi (Ca).
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How are nutrients passed through the compact bone?
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Osteocytes surround themselves in a bony matrix of calcium salts and type I collagen fibers (1:1). The lacunae (La) is where the osteocyte was. The canaliculi are where the osteocyte filopods were. Nutrients are passed from capillaries in the Haversian canal (HC) to the innermost osteocytes and then to outer layers of osteocytes. Osteocytes in a Haversian system share nutrients through the tips of the filopods.
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Describe the characteristics of Spongy (trabecular or Cancellous Bone).
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Sponge-like appearance is formed by rods and plates of bone called trabeculae.
Spaces among the trabeculae are filled with bone marrow and adipose tissue. Trabecular bone is light but strong. |
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what are the Spongy bone structure and lines of stress?
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trabeculae in spongy bone develop along a bone’s load-bearing lines of stress and direct force to compact bone
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Bone Marrow
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Bone Marrow is a soft tissue that occupies the medullary cavity of long bones and the spaces among the trabeculae of spongy bone.
Red Marrow: looks like thick blood bone marrow is the adult hemopoietic tissue that produces blood cells and platelets found in vertebrae, ribs, sternum, pelvic girdle and proximal heads of femur and humerus in adults Yellow Marrow: fatty marrow in shaft of long bones in adults Gelatinous Marrow: yellow marrow is replaced with reddish jelly in old individuals |
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Intramembranous Ossification vs. Endochondral Ossification
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Intramembranous Ossification produces the flat bones of the skull, mandible and clavicle
Endochondral Ossification produces the bones of the trunk and extremities The principal difference between intramembranous bone formation and endochondral bone formation is what is replaced during development. The tissue replaced during development in intramembranous ossification is embryonic CT. The tissue replaced in endrochondral ossification is hyaline cartilage. The two processes can occur side by side, and do, in the case of fracture repairs. |
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Intramembranous Ossification
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Produces flat bones of skull and the clavicle
Steps of the process: mesenchyme condenses into a sheet of soft tissue that transforms into a network of soft trabeculae osteoblasts gather on the trabeculae to form osteoid tissue (uncalcified bone) calcium phosphate is deposited in the matrix and it hardens osteoblasts develop into osteocytes as they become surrounded by bone osteoclasts remodel the center to make marrow spaces osteoblasts remodel the surface to form compact bone the surfaces of the compact bone are covered with periosteum |
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Endochondral Ossification
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Bones developing from endochondral ossification begin as a hyaline cartilage model that is gradually replaced by bone.
Endochondral Ossification proceeds as follows: a primary ossification center forms in the cartilage model cartilage becomes hypertrophic (chondrocytes swell as they form the primary ossification center). hypertrophic cartilage is removed by macrophages primary marrow space is formed as cartilage is removed Blood vessels and nerves grow into the marrow space osteogenic cells (mesenchymal-like fibroblasts) from the perichondrial/periosteal membrane invade the spaces created by the macrophages and transform into osteoblasts osteoblasts deposit osteoid tissue and calcified matrix |
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Describe the Process of Endochondral Ossification
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The primary ossification center forms from hypertrophic cartilage in the center of the cartilage model
Blood vessels, nerves and osteoblasts grow into the space created by removal of the hypertrophic cartilage. The same process begins in the epiphysis at each end forming a secondary ossification center that will develop into spongy bone. |
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Metaphysis
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The metaphysis is a transitional zone between epiphysis and diaphysis of a developing long bone.
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Different zone of metaphysis?
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Zone of Reserve Cartilage is a layer of resting hyaline cartilage
Zone of Cell Proliferation is a layer of chondrocytes that multiply forming columns of flattened lacunae Zone of Cell Hypertrophy is a region of swollen chondrocytes Zone of Calcification shows mineralization of the cartilage matrix and the chondrocytes die Zone of Bone Deposition replaces mineralized cartilage matrix. Voids are filled with osteoblasts and blood vessels forming haversian canals and osteons |
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Secondary Ossification Centers
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Secondary ossification centers begin to form in the epiphyses near time of birth.
Same stages occur as in primary ossification centers and result in the center of the epiphysis being transformed into spongy bone. Bone development does not proceed all the way to the end of the bone. Hyaline cartilage remains over epiphysis as the articular cartilage of the joint surface. The metaphysis is at the junction of the diaphysis and epiphysis and forms an epiphyseal plate (growth plate until growth ends). |
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Describe cranial formation of babies.
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Cranial ossification is incomplete at birth.
Soft regions of the skull called fontanels are regions of dense irregular connective tissue. There are usually 6 fontanels at birth. Most fontanels close by 12 months and all are usually closed by 24 months. |
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Bone Growth
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Bones grow and remodel throughout life
exercise or manual labor increases density and mass of bone Bones increase in length by interstitial growth at the epiphyseal plate Bones increase in width by appositional growth of new osteons http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter6/animation__bone_growth_in_width.html If one process outpaces the other, bone deformities can occur |
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Compact Bone Remodeling
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A new Haversian canal is excavated through old bone by a group of osteoclasts (OCL). The new canal becomes lined with osteoprogenitor cells that develop into osteoblasts (OBL) that begin to form new bone (osteoid) inside the canal. A blood vessel (BV) grows into the canal to provide the new tissue with nutrients. As osteoblasts become surrounded by bone matrix they turn into osteocytes (OCY).
Blood born macrophages migrate through capillary endothelial cells and merge to form osteoclasts. Osteoclasts form a “cutting cone” (region A) as they digest bone matrix and produce Howship’s lacunae. Behind the osteoclasts, preosteoblasts migrate onto the bone and differentiate into osteoblasts. Osteoblasts produce new osteoid that fills the cone with new bone (region B). |
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Bone Remodeling
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Mature osteons are eroded away by osteoclasts (OC). Osteoclasts are formed by the merger of several macrophages, and each contributes a nucleus to these large cells. Osteoclasts digest the bone matrix creating pits called Howships Lacunae (HL). In healthy bone, the rate of old bone erosion is matched by new bone deposition as new osteons grow into the Howships lacunae. Osteoclasts are also activated by constant pressure on bone.
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Factors affecting bone
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20 or more hormones, vitamins and growth factors affect bone and not all the mechanisms are known.
Bone growth is especially rapid at puberty resulting in a “growth spurt”: growth hormone and sex hormones stimulate proliferation of osteogenic cells and chondrocytes in growth plate adolescent girls grow faster than boys and reach their full height earlier (estrogen has strong effect on bone growth) males grow for a longer time resulting in larger average stature due to testosterone Growth ceases when epiphyseal plate “closes” anabolic steroids may cause premature closure of growth plate producing short adult stature |
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Growth hormone and bone growth
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Growth hormone (GH) is a protein hormone released from the anterior pituitary gland under the control of the hypothalamus. In children, GH has growth-promoting effects on the body. It stimulates the secretion of somatomedins from the liver, which are a family of insulin-like growth factor (IGF) hormones. These, along with GH and thyroid hormone, stimulate linear skeletal growth in children. In adults, GH stimulates protein synthesis in muscle and the release of fatty acids from adipose tissue (anabolic effects). It inhibits uptake of glucose by muscle while stimulating uptake of amino acids. The amino acids are used in the synthesis of proteins, and the muscle shifts to using fatty acids as a source of energy. GH secretion occurs in a pulsatile (short, concentrated secretion) and sporadic manner. Thus, a single test of the GH level is usually not performed.
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Gigantism (growth hormone disorder)
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Hypersecretion of growth hormone during childhood can lead to gigantism.
Robert Pershing Wadlow, with his mother at his side, had grown to a record height of 8’11” by the time of his death at age 22. |
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Acromegaly (growth hormone disorder)
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Hypersecretion of growth hormone in old age leads to acromegaly.
symptoms of acromegaly include thickening of hand, foot, jaw and brow bones and growth of soft tissues particularly in the nose and ears. |
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Dwarfism (growth hormone disorder)
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Achondroplastic Dwarfism is a genetic disorder resulting in short stature but normal-sized head and trunk because the long bones of the limbs stop growing in childhood but other bones are unaffected.
Pituitary Dwarf has lack of growth hormone and is short in stature with normal proportions. |
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Other dwarfisms
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There are over 200 different types of dwarfism, all of which involve bone growth disorders (osteodysplasia) that result in short stature (adult height less than 4 ft. 10 in. tall).
Primordial Dwarfism is a group of disorders in which growth is proportional but severely delayed, beginning in the womb. This results in some of the smallest people in the world. The individual pictured has Majewski osteodysplastic primordial dwarfism (MOPD) Type II. Only about 100 individuals worldwide have been identified as having MOPD type II. Both males and females of all ethnic backgrounds are affected. Some families have more than one child with MOPD Type II, which suggests that the disorder is inherited in an autosomal recessive manner. Majewski osteodysplastic primordial dwarfism type II (MOPD II): Natural History and Clinical Findings. Hall, Flora, Scott, Pauli, Tanaka, Am J. Med Genet A. 2004 Sep 15 ; 130A(1):55-72. |
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Mineral deposition
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Mineralization is a process in which ions (mostly calcium and phosphate) are removed from blood plasma and are deposited in bone tissue matrix.
Steps of the mineralization process: osteoblasts produce collagen fibers fibers become encrusted with minerals |
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Mineral resorption
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Bone is eroded by the osteoclasts to re-model bone and to release minerals (calcium, phosphate and others) into the blood
Bone erosion takes place at the “ruffled border” of osteoclasts hydrogen ion pumps in the cell membrane secrete hydrogen ions into the space between the osteoclast and the bone and chloride ions follow forming HCl hydrochloric acid with a pH of 4 dissolves bone minerals an enzyme is also released (acid phosphatase) that digests the collagen osteoclasts are stimulated by increased pressure osteoblasts are stimulated by decreased pressure Dental braces reposition teeth by creating greater pressure on the bone on one side of the tooth and less on the other side to remodel jaw bone |
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Osteoclasts
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Osteoclasts develop from bone marrow stem cells that have a common bone marrow stem cell lineage as monocytes/macrophages. In the bone, 3-50 of these pre-osteoclasts can merge into giant, multinucleated osteoclasts.
Osteoclasts release from their ruffled border many lysosomes that contain enzymes and acids that erode bone matrix and produce pits in the matrix called “resorption bays” or Howship’s lacunae. |
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Calcium balance
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Calcium and phosphate homeostasis are tightly regulated in normal ranges by numerous factors including calcitriol, calcitonin and parathyroid hormone (PTH)
Calcium levels outside of normal values can cause neuromuscular problems: Neuron membrane potentials are influenced by extracellular ion concentrations. Many ions contribute to the total overall charge inside and outside the cell. At rest there is more positive charge outside the cell relative to inside the cell. Sodium ions are normally the most important in establishing this charge gradient but extracellular calcium (a +2 cation) also contributes to the net positive charge outside the cell. Hypocalcemia (deficiency of blood calcium) causes excessive excitability of nervous system leading to muscle spasms, tremors or tetany and laryngospasm may cause suffocation Hypocalcemia increases neuron membrane excitability by decreasing membrane threshold. Low extracellular Ca++ makes the role of Na more important in membrane depolarization because extracellular Na+ ions account for most the positive charge outside the cell. Therefore, small changes in Na flow across the membrane will more easily depolarize the membrane. Hypercalcemia (excessive of blood calcium) depresses nervous system activity Hypercalcemia decreases membrane excitability. When ligand-gated Na+ channels open and Na rushes in, Ca ++ stays in the extracellular fluid outside the cell. Excessive extracellular Ca ++ diminishes the depolarizing effect of the Na+ inflow. |
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Calcium imbalance causes:
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Calcium levels outside of normal values causes:
hypocalcemia (deficiency of blood calcium) causes excessive excitability of nervous system leading to muscle spasms, tremors or tetany laryngospasm may cause suffocation hypercalcemia (excessive of blood calcium) depresses nervous system activity Calcium phosphate homeostasis depends on calcitriol, calcitonin and parathyroid hormone (PTH) |
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Calcitriol (activated Vitamin D)
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Calcitriol is produced by:
UV radiation of epidermal keratinocytes converts a cholesterol derivative (7-dehydrocholesterol) into Vitamin D3 (cholecalciferol) liver adds -OH to convert D3 into calcidiol kidney adds -OH to convert calcidiol to calcitriol Actions of the hormone Calcitriol include: stimulates intestine to absorb calcium and phosphate promotes urinary reabsorption of calcium ions promotes osteoclast activity to release calcium from bone into blood |
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Calcitonin
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Secreted by C cells of the thyroid gland when plasma calcium concentration rises too high
Functions: reduces osteoclast activity by as much as 70% in 15 minutes increases the number and activity of osteoblasts Calcitonin has a powerful effect in children, but it is less effective in adults calcitonin deficiency is not known to cause any disease in adults, but it may be useful in reducing bone loss in osteoporosis |
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Parathyroid Hormone (PTH)
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PTH is secreted by the parathyroid glands when plasma calcium level is too low.
Functions: stimulates osteoblasts to release osteoclast-stimulating factor that stimulates osteoclast multiplication and activity inhibits collagen synthesis and bone deposition by osteoblasts promotes calcium resorption by the kidneys promotes calcitriol synthesis |
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Osteoporosis
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Most common bone disease.
Bones lose mass and become brittle due to loss of both organic matrix and minerals: increases risk of fracture of hip, wrist and vertebral column widow’s (dowager’s) hump results from deformed spine Best treatment is prevention: exercise and a recommended calcium intake of 1000 mg/day between ages 25 and 40. |
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Healing of fractures
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Normally healing takes 8 - 12 weeks (longer in elderly)
Stages of healing: see next slide 1. fracture hematoma is a blood clot resulting from broken blood vessels 2. soft callus of fibrocartilage is formed by fibroblasts and infiltrated by capillaries at site of break 3. a soft callus is gradually replaced by a hard callus of spongy bone in about 6 weeks 4. remodeling occurs over 6 months as spongy bone is replaced with compact bone |
