1 Osseous Tissue and Bone StructureChapter 6 Osseous Tissue and Bone Structure

2 Skeletal System: Bone TissueSkeletal system is not just composed of bone. Also includes: Cartilage- forerunner of bone; allows for resistance Dense connective tissue Adipose tissue Blood tissue Epithelial tissue Nervous tissue. Each bone is considered an organ Bone (osseous) tissue consists of cells and extracellular matrix. Two types of bone structure: COMPACT AND SPONGY Bone matrix is hardened by minerals Continually remodels itself

3 Skeletal System Bone functions: Support: composes framework of bodyProtection: protects vital organs (heart,lungs) Blood formation: contains red bone marrow where blood is formed. Red bone marrow found in developing bones of fetus and in the pelvis, ribs, sternum and ends of long bones in arm and thigh of the adult Movement: attachment site for skeletal muscle Mineral homeostasis: minerals are mobilized or stored in bones Lipid storage: contains yellow bone marrow where triglycerides can be stored

4 Classification of BonesSix Bone Shapes Sutural bones- between the flat bones of the skull Irregular bones-complex shapes with short, flat, notched, or ridged surfaces- vertebrae, pelvic Short bones: boxlike in appearance- wider than long–carpals, tarsals Flat bones- skull, ribs, sternum, scapula Long bones – longer than wide - femur, humerus, tibia, phalanges Sesamoid bones often develop inside tendons -patella

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6 Classification of BonesStructure of a Flat Bone Ex: the parietal bone of the skull Resembles a sandwich of spongy bone between two layers of compact bone Within the cranium, the layer of spongy bone between the compact bone is called the diploë

7 Bone Markings Depressions or grooves Elevations or projectionsAlong bone surface Tunnels Where blood and nerves enter bone Elevations or projections Where tendons and ligaments attach At articulations with other bones

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9 Bone Cells OSTEOPROGENITOR CELLS:Mesenchymal stem cells that divide to produce osteoblasts Located in endosteum, the inner cellular layer of periosteum; lines medullary cavity Assist in fracture repair; produce new bone matrix in a process called ossification. OSTEOBLASTS - Immature actively dividing bone cells secrete matrix compounds and collagen fibers; have extensive ER with ribosomes to produce fibers and hydroxyapatite OSTEOID — soft form of matrix produced by osteoblasts, but not yet calcified or mineralized to form bone Osteoblasts surrounded by hardened matrix become osteocytes Osteoid and cells are organic components of the matrix

10 Bone Cells Osteocytes Mature bone cells that maintain the protein and mineral content of the bone matrix; help in repair of damaged bone Live within lacunae organized around blood vessels Lie between layers (“lamellae”) of matrix Connect by cytoplasmic extensions through canaliculi in lamellae Form pathways for blood vessels Exchange nutrients and wastes Do not divide or undergo mitosis Regulates calcium, phosphorus concentrations

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12 Bone Cells OSTEOCLASTS - modified white blood cells (WBC) that secrete enzymes and acids used to dissolve bone (bone resorption) releasing calcium into blood (osteolysis). Giant, multinucleate cells develop in bone marrow by fusion of 3-50 stem cells which produce macrophages Create the medullary cavity of long bones Ruffled border increase surface area for bone resorption

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14 Bone (Osseous) Tissue Healthy Bone 50% the strength of steel in resisting COMPRESSION (directed PUSHING FORCES; when limit of compressive strength is reached, bone will be CRUSHED) 100% as strong as steel in resisting TENSION (stress applied while being STRETCHED OR PULLED) Specialized cells make up only 2% of bone mass Osteogenic (stem cells), Osteoblast, Osteocytes, Osteoclast Extracellular matrix- provides compression and tension ability Ground substance 2/3 of matrix varied consistency Produces solid matrix of calcium salt deposits around collagen fibers Contains calcium phosphate; “concrete”- can withstand considerable compression but shatters with sudden impacts, twisting. Reacts with calcium hydroxide, to form hydroxyapatite, which combines with other calcium salts and ions; most calcium deposits are in this form Fibers- (1/3) of bone matrix is collagen protein fibers, osteoid provides tensile strength-resist stretching and twisting “rebar” allows for flexibility; cannot withstand compression

15 Ground Substance Fibers

16 Spongy Bone Compact Bone

17 COMPACT Bone 80% of bone massProvides support, movement, and resistance to weight stress in LIMITED range of motion or directions Accounts for most of diaphysis of long bone---below the periosteum Osteon is the basic unit Osteocytes are arranged in concentric (circular) lamellae ** Around a central canal containing blood vessels/nerves Perforating canals (Volkmann’s canals) Perpendicular to central canal Carry blood vessels into bone and marrow; connect osteons Circumferential Lamellae wrapped around the long bone; bind osteons together; produce bone growth ** Layers of matrix produce a target pattern

18 Compact Bone Collagen fibers “corkscrew”Rings of matrix Collagen fibers “corkscrew” down the matrix of the lamella giving it a helical arrangement Helices coil in one direction in one matrix lamella and in the opposite direction in the next matrix lamella for added strength

19 SPONGY Bone Sponge-like appearanceMatrix forms an open network of trabeculae (thin plates of bone) arranged in lamellae. Trabeculae have no central canals -nutrients reach osteocytes by diffusion along canaliculi that open onto the surface of the tracebulae. Blood vessels woven through open spaces in the trabeculae Between trabeculae are spaces that contain red bone marrow involved in hemopoiesis (blood cell formation). Spaces DECREASE WEIGHT OF BONE making it easier for muscle to move bones SPONGY Bone Spongy bone found in 1) Flat bones 2) Short bones 3) Irregular bones 4) Sesamoid bones 5) Epiphyses 6) Medullary cavity of long bones (except clavicle which has no medullary cavity)

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21 The Distribution of Forces Long BoneBones and stress Compact bone located where stresses are limited in direction The femur transfers weight from hip joint to knee joint Causing tension on the lateral side of the shaft And compression on the medial side Spongy bone located where stresses are MULTI-DIRECTIONAL “pulling forces” “pushing forces”

22 The Distribution of Forces – spongy boneTrabeculae provide maximum strength similar to braces used to support a building; oriented to follow the lines of stress and can realign if the direction of stress changes from several directions. Forces as much as 6x body weight are transmitted across the hip. The orientation of the trabeculae is affected by the mechanical stress to which the bone is exposed. Improper weight distribution can adversely affect nerve and spinal alignment. For example, sitting on a wallet. The wallet is pushing on the sciatic nerve and causing you to sit off center, so to sit up straight, you must curve your spine. This puts an uneven load on the sacroiliac joints and on the lower back.

23 Skeletal System: Bone structureLong bones contain the following structures: Diaphysis: the “shaft” of the bone Epiphyses: both ends of the bone Articulating cartilage: hyaline cartilage covering the epiphyses. Allows smooth articulation at joints. Metaphyses: between the epiphyses and diaphysis of bone Contain a epiphyseal plate at each end---hyaline cartilage growth allows for linear growth. Eventually replaced by osseous tissue, transforming plate into epiphyseal line.

24 Hold periosteum to bone matrix

25 Near joints periosteum becomes continuous with ligaments and tendonsA dense irregular connective tissue covering of the diaphysis. Isolates the bone from surrounding tissues Attachment of ligaments and tendons Contains cells involved in bone growth (width) Provides a route for blood (nutrients and gas exchange) and nerve supply. fibrous layer = dense irregular CT Cellular/osteogenic layer = nourish & help with repairs Sharpey’s perforating fibers penetrate the bone to attach periosteum to the matrix of the bone. Fibers are art of the outer fibrous layer of periosteum, entering into the outer circumferential and interstitial lamellae of bone

26 Bone Marrow Medullary cavity: cavity found in the diaphysis; consisting mostly of compact bone, walls composed of spongy bone and lined with a thin, vascular membrane the endosteum. bone marrow –composed of tissue stem cells which produce blood cells inside cavity of long bone and small spaces between trabeculae of spongy bone red marrow (myeloid tissue) hemopoietic tissue - produces white and red blood cells in nearly every bone in a child in adults, found in skull, vertebrae, ribs, sternum, part of pelvic girdle, and proximal heads of humerus and femur yellow marrow found in adults most red marrow turns into fatty yellow marrow no longer produces blood cells but fat, cartilage and bone Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

27 Red and Yellow Bone Marrow

28 Bone marrow transplant

29 External / Internal Bone structureEndosteum: membrane lining of the medullary cavity An incomplete cellular layer: Lines the medullary (marrow) cavity Covers trabeculae of spongy bone Lines central canals Contains osteoblasts, osteoprogenitor cells, and osteoclasts Active in bone growth and repair

30 Bone Formation and GrowthBONE IS NEVER FORMED AS A PRIMARY TISSUE. IT ALWAYS REPLACES AN EXISTING TISSUE SUCH AS CARTILAGE OR DENSE CT. Human bones can grow until about age 25 Ossification: Bone formation Two main forms of ossification Endochondral ossification involves the replacement of a hyaline cartilage “skeleton” with osseous tissue. Most bones originate as hyaline cartilage Intramembranous ossification Occurs in the dermis Produces dermal (flat) bones such as mandible (lower jaw) and clavicle (collarbone) and skull No cartilage present Injured bones use this process when healing Calcification the depositing of calcium salts Good animation https://www.youtube.com/watch?v=p-3PuLXp9Wg

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32 Bone Formation and GrowthIntramembranous Ossification Also called dermal ossification because it occurs in the dermis Produces dermal flat bones such as mandible (lower jaw) and clavicle (collarbone); skull and is also responsible for remodeling or repairing injuries to bone Process: Mesenchymal stem cells cluster and differentiate into osteoblast Osteoblast secrete soft matrix called osteoid As calcium phosphate and other mineral deposits move into osteoid it hardens the matrix. With the help of osteoclasts bony interconnected trabeculae form and thicken effectively creating spaces in the bone. The hardened matrix traps the osteoblasts in cavities called lacunae. The osteoblasts are no longer able to reproduce and become osteocytes. Blood vessels move into between the trabeculae forming the red bone marrow. Osteoblasts form compact bone at surface; surface mesenchyme produces periosteum on the exterior of the bone surface.

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34 Intramembranous Ossification1) In mesoderm embryonic germ layer mesenchymal cells (connective tissue stem cells) form clusters and differentiate into osteoblast which secerete a soft organic matrix – the osteoid. 2) Mineral salts are then deposited and begins to form bony spicules called trabeculae 3) The osteoblasts get trapped in the matrix and become osteocytes embedded in lacunae. Calcification occurs with the increased accumulation of calcium and mineral salts

35 As calcium phosphate (non-living-inorganic) is deposited it hardens the matrix forming bony interconnected trabeculae which thicken forming spaces in the bone. Osteoclasts create marrow cavity. Blood vessels grow into the matrix between the trabeculae and differentiate into the red bone marrow (produces 500 billion blood cells per day) Osteoblasts form compact bone at surface; surface mesenchyme produces periosteum

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37 Endochondral OssificationThis process uses hyaline cartilage as the model for long bone formation. Most bones are formed by this process. MSC stem cells differentiate into chondroblast chondroblast form a hyaline cartilage model Hyaline model is surrounded by the perichondrium -layer of dense irregular connective tissue perichondrium becomes periosteum- containing a undifferentiated cells (osteoprogenitor cells) which later become osteoblasts. Expanding cartilage matrix encases chondrocytes in lacunae.

38 Endochondral Ossification2) Collar formation: About the 3rd month of development periosteum (dense irregular connective tissue) forms around hyaline cartilage Nutrient artery penetrates the perichondrium and invade the inner cavity of the cartilage model. The hole that the vessels poke through are called the nutrient foramen. Fibroblasts migrate from the artery and differentiate into osteogenic cells (stem cells) which then become osteoblasts. The osteoblasts secrete osteoid (un-mineralized bone). against the shaft of the cartilage model (Appositional Growth). This serves as support for the new bone. The end result is a bony collar on the outside of the cartilage.

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40 Endochondral Ossification3) Cavity formation: As the bony collar is forming the cartilage is enlarging, the chondocytes increase in size. The matrix is reduced to strut-like projections in the center of the model that will ossify/harden to form bone. The calcification/hardening of the center makes the inner cartilage impermeable to the diffusion of nutrients. Which results in the massive death of the chondrocytes. As the cartilage starts deteriorates a cavity is formed. The remaining cartilage is broken down by osteoclasts forming the marrow cavity. The osteoblasts along with fibroblasts from the invading blood vessels begin formation of trabeculae (spongy bone) This centered cartilage is called the primary ossification center. https://i0.wp.com/lifenews.wpengine.netdna-cdn.com/wp-content/uploads/2014/11/ultrasound4d55.png

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42 Endochondral Ossification4) Elongation: as blood vessels, osteoclasts, and osteocytes continue to invade the bone the shaft (diaphysis) starts to elongate. The medullary cavity forms and the diaphysis will slowly continue to lengthen during embryonic development. Also, vessels bud into the hyaline cartilage at the ends (epiphysis) of the long bones forming what are called secondary ossification centers. Secondary ossification centers and marrow cavities form in ends of bone Osteogenic cells differentiate into osteoblast laying down bony matrix. Spongy bone formation proceeds outward towards the periphery. No formation of medullary cavity Hyaline cartilage is left on the ends of the bones (called articular cartilage) and the epiphyseal plates (growth plates) are also formed. The articular cartilage and epiphyseal plates are the only remains of the original hyaline cartilage model Cartilage is replaced by bone. Growth plates provide for increase in length of bone during childhood and adolescence By early twenties, growth plates are gone and primary and secondary marrow cavities united

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45 Bone Formation and GrowthInterstitial growth for increasing length Results from the deposition of bony matrix on the diaphysis side of the epiphyseal plate. Cartilage cells closest to the epiphysis divide, increasing the thickness of the cartilage. At the diaphyseal side the cartilage is replaced with bony tissue resulting in longer bone. Epiphyseal plate Continues to allow interstitial growth until the age of 18 for females and 21 for males. Cartilage stops dividing under the influences of hormones and ossifies into an epiphyseal line Presence of epiphyseal line indicates that linear bone growth is no longer possible. Premature ossification of epiphyseal plate of one bone in the extremities may result in unequal bone lengths

46 Bone Growth at an Epiphyseal Cartilage

47 Bone Formation and GrowthAppositional Growth Results in increased thickness (width) of bone Differentiation of cells at the periosteum into osteoblasts that begin to deposit collagen fibers and extracellular matrix; forming bony ridges Osteoblast become osteocytes as they become surrounded by bony matrix. Extracellular matrix ridges surround periosteal blood vessels and periosteum becomes endosteum forms circumferential lamellae Animation: https://www.youtube.com/watch?v=X6E5Rz9tOKE

48 Appositional Bone GrowthFigure 6.10a

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50 Bone Tissue = Homeostasis/RemodelingHomeostasis is controlled by a balance of hormonal actions influenced by diet, exercise, and physical health stasis Bone building (by osteoblasts) and bone recycling (by osteoclasts) must balance. Bone continually remodels, recycles, and replaces If deposition is greater than removal, bones get stronger If removal is faster than replacement, bones get weaker More breakdown than building, bones become weak Exercise, particularly weight-bearing exercise, causes osteoblasts to build bone- when a bone is placed under stress bone cells migrate to the stressed area and begin laying down new bone. Cells secrete collagen which is deposited between cells. Mineralization follows. Heavily stressed bones become thicker and stronger Bone degenerates quickly. Up to 1/3 of bone mass can be lost in a few weeks of inactivity Vitamin C required for collagen synthesis, osteoblast differentiation Vitamin A stimulates osteoblast activity Vitamins K and B12 help synthesize bone proteins

51 Exercise, Hormones, and NutritionGrowth hormone and thyroxine (T4) stimulate bone growth Before Puberty most bone growth is stimulated by: Human growth hormone (hGH) produced by anterior pituitary Estrogens and androgens stimulate osteoblasts After Puberty bone growth is stimulated/regulated by: Sex hormones estrogen and androgen stimulate bone growth; Estrogen promotes the programmed death of osteoclast; therefore is protective in the adult female The Skeleton is a Calcium Reserve Bones store calcium and other minerals Calcium is the most abundant mineral in the body. The hormone calcitriol (Vitamin D); formed in the kidneys necessary for absorption of Ca and P from digestive tract. Calcium ions are vital to: Blood coagulation Nervous system function Muscular contraction Enzymatic function

52 Result of hypersecretion of human growth hormone (hGH)characterized by excessive growth and height significantly above average.

53 Calcium Homeostasic ControlParathyroid Hormone (PTH) Produced by parathyroid glands in neck Increases calcium ion levels by: Stimulating osteoclasts – causing bone breakdown increasing intestinal absorption of calcium Decreasing calcium excretion at kidneys Increases production of vitamin D (calcitriol) Calcitonin Secreted by C cells (parafollicular cells) in thyroid Decreases calcium ion levels: Inhibits osteoclast activity Increases calcium excretion at kidneys Lowers the amount of calcium absorption from intestines Increases the amount of bone formation by increasing numbers of osteoblasts which then utilize the excess calcium for mineralization of new bone matrix

54 Bone Fracture Cracks or breaks in bone caused by physical stress.Common bone fractures: Closed (simple)- simple bone break Open (compound)- fracture where bone punctures skin Comminuted- ends of broken bones splinter leaving fragments between broken ends Greenstick- Partial fracture on one side of the bone as the bone BENDS rather than breaking cleanly; seen in children Impacted- broken end of one bone driven into the other

55 Fractures Transverse fractures- breaking of a bone shaft across its axis Displaced fractures –produce new and abnormal bone arrangements Compression fractures –occur in vertebrae (hard landing on buttock) Spiral fractures- twisting stresses Common bone fractures: Pott’s fracture- fracture of distal fibula, usually resulting in damage to medial aspect of ankle joint Colle’s fracture- fracture of distal radius often seen upon falling on outstretched arm. Stress fractures- microscopic fractures seen with repeated stress to bones.

56 Fracture Repair Bleeding/Inflammation Soft callus formationExtensive bleeding causes swelling and redness, increase in WBC activity Produces a clot (fracture hematoma) Bone cells in the area die; removed by osteoclasts/macrophages Soft callus formation Fibroblasts migrate from endosteum and periosteum forming soft CALLUS=mass of tissue that forms at fracture site; connects the broken ends of the bone together External callus of CARTILAGE AND BONE surrounds and stabilizes the break Internal callus develops in medullary cavity at the ends of the broken bones as a network of SPONGY BONE unites the inner surface depositing collagen forming granulation tissue.

57 Fracture Repair OsteoblastsCartilage of external callus is replaced by bone and spicules of spongy bone help to unite the broken ends of the bone forming a bony matrix- HARD CALLUS. Osteoblasts and osteocytes remodel the fracture for up to a year Reducing bone calluses Swelling may mark the area for some time but will eventually fade. Often the new bone is stronger than previous older bone.

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59 Steps in the Repair of a Fracture

60 Bone Repair Obstacles Weight bearing too soon – moves bone fragments before restored Smoking – constricts blood vessels decreasing circulation Health conditions: Diabetes, hormone problems, vascular disease Medications that inhibit immune process: Ex: corticosteroids Infection Advanced age Poor nutrition and metabolism

61 Bone and Aging Bone deposition (deposit) outpaced by Resorption (breakdown) at middle- age. With increased age demineralization of bone and brittleness occurs Demineralization: the loss of bone mass resulting from the loss of calcium and other minerals. Results in fragile limbs; reduction in height; tooth loss Menopausal women are at greater risk for demineralization due to the rapidly decreased levels of estrogen. Males at lower risk due to continued production of testosterone late into life Decreased protein synthesis results in brittleness of bone Fewer osteoblasts and osteocytes

62 Bone Disorders OsteoporosisResults from decrease bone mass; bones more prone to fracture Predominantly seen in menopausal women due to decreased estrogen levels Estrogen INCREASES OSTEOBLAST activity and STIMULATES bony matrix deposition Estrogen INHIBITS OSTEOCLAST activity preventing bone loss. Decreases in estrogen levels lead to an increase in osteoclast activity that can produce bone loss. Can also be seen in female elite athletes that have stopped menstruating due to low body fat (unable to produce adequate levels of estrogen) Risk increases with family history, thin/small build, diet poor in calcium or vitamin D, sedentary life style, smoking, drinking alcohol and Caucasian or Asian ethnicity.

63 Osteoporosis head of the femurSpinal Osteoporosis Osteoporosis head of the femur