Chapter 7: Bone Structure & Function PDF

Chapter 7: Bone Structure & Function 7-1  Tissues and Organs of the Skeletal System Osteology—the study of bone Skeletal system—composed of bones, cartilages, and ligaments – Form strong, flexible framework of the body – Cartilage—...

Chapter 7: Bone Structure & Function 7-1  Tissues and Organs of the Skeletal System Osteology—the study of bone Skeletal system—composed of bones, cartilages, and ligaments – Form strong, flexible framework of the body – Cartilage—forerunner of most bones Covers many joint surfaces of mature bone Ligaments—hold bones together at the joints Tendons—attach muscle to bone – Aponeurosis is a thin flatten sheet of dense regular CT 7-2  Functions of the Skeleton Support—holds up the body, supports muscles, mandible and maxilla support teeth Protection—brain, spinal cord, heart, lungs Movement—limb movements, breathing, action of muscle on bone Electrolyte balance—calcium and phosphate ions Acid–base balance—buffers blood against excessive pH changes Blood formation—red bone marrow is the chief producer of blood cells 7-3  Bones and Osseous Tissue Bone (osseous tissue)—connective tissue with the matrix hardened by calcium phosphate and other minerals Mineralization or calcification—the hardening process of bone Individual bones consist of bone tissue, bone marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue Continually remodels itself and interacts physiologically with all of the other organ systems of the body Permeated with nerves and blood vessels, which attests to its sensitivity and metabolic activity 7-4  General Features of Bones Flat bones – Protect soft organs – Curved but wide and thin Long bones – Longer than wide – Rigid levers acted upon by muscles Short bones – Equal in length and width – Glide across one another in multiple directions Irregular bones – Elaborate shapes that do not fit into other categories 7-5  General Features of Bones Compact (dense) bone—outer shell of long bone Diaphysis (shaft)—cylinder of compact bone to provide leverage Medullary cavity (marrow cavity)—space in the diaphysis of a long bone that contains bone marrow Epiphyses—enlarged ends of a long bone – Enlarged to strengthen joint and attach ligaments and tendons Metaphysis—between diaphysis and epiphysis Spongy (cancellous) bone—covered by more durable compact bone – Skeleton three-fourths compact and one-fourth spongy bone by weight – Spongy bone in ends of long bones, and middle of nearly all others 7-6  General Features of Bones Articular cartilage—layer of hyaline cartilage that covers the joint surface where one bone meets another; allows joint to move more freely and relatively friction free Nutrient foramina—minute holes in the bone surface that allows blood vessels to penetrate 7-7  General Features of Bones Periosteum—external sheath that covers bone except where there is articular cartilage – Outer fibrous layer of dense irregular CT Some outer fibers continuous with the tendons that attach muscle to bone Perforating fibers—other outer fibers that penetrate into the bone matrix Strong attachment and continuity from muscle to tendon to bone – Inner osteogenic layer of bone- forming cells Important to growth of bone and healing of fractures Includes osteoprogenitor cells, osteoblasts, osteoclasts 7-8  General Features of Bones Endosteum—thin layer of reticular connective tissue lining marrow cavity – Has cells that dissolve osseous tissue and others that deposit it Epiphyseal plate (growth plate)—area of hyaline cartilage that separates the marrow spaces of the epiphysis and diaphysis – Enables growth in length – Epiphyseal line—in adults, a bony scar that marks where growth plate used to be 7-9 General Features of Bones Sandwich-like construction Two layers of compact bone enclosing a middle layer of spongy bone – Both surfaces of flat bone covered with periosteum Diploe—spongy layer in the cranium – Absorbs shock – Marrow spaces lined with endosteum 7-10  Bone Cells Bone is connective tissue that consists of cells, fibers, and ground substance Four principal types of bone cells – Osteoprogenitor cells; osteoblasts; osteocytes; 7-11 osteoclasts  Bone Cells Osteoprogenitor cells—stem cells found in endosteum, periosteum, and in central canals – Arise from embryonic mesenchymal cells; multiply continuously to produce new osteoblasts – Located in periosteum and endosteum Osteoblasts—bone-forming cells – Line up as single layer of cells under endosteum and periosteum – Forms osteoid-semisolid bone matrix – Nonmitotic – Synthesize soft organic matter of matrix which then hardens by mineral deposition – Become entrapped within the matrix 7-12  Bone Cells Osteocytes—former osteoblasts that have become trapped in the matrix they have deposited – Lacunae—tiny cavities where osteocytes reside – Canaliculi—little channels that connect lacunae – Cytoplasmic processes reach into canaliculi – Maintain the bone matrix by signaling osteoblast – When stressed, produce biochemical signals that regulate bone remodeling – Contribute to homeostatic mechanism of bone density and calcium and phosphate ions 7-13  Bone Cells Copyright © The McGraw-Hill Companies, Inc. 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Osteocyte Osteoclast Osseous tissue Stem cells Periosteum Resorption bay Nuclei Osteoclast Fusion Ruffled Lysosomes border (b) Osteoclast development Osteoclasts—bone-dissolving cells found on the bone surface – Osteoclasts develop from same bone marrow stem cells that give rise to blood cells – Different origin from rest of bone cells – Unusually large cells formed from the fusion of several stem cells Typically have 3 to 4 nuclei, may have up to 50 7-14  Bone Cells – Ruffled border—side facing bone surface Several deep infoldings of the plasma membrane which increases surface area and resorption efficiency – Resorption bays—pits on surface of bone where osteoclasts reside – Remodeling—results from combined action of the bone-dissolving osteoclasts and the bone- depositing osteoblasts 7-15  The Matrix Matrix of osseous tissue is, by dry weight, about one- third organic and two-thirds inorganic matter Organic matter—synthesized by osteoblasts – Collagen, carbohydrate–protein complexes, such as glycosaminoglycans, proteoglycans, and glycoproteins – Provides strength by resisting stretching Inorganic matter – 85% hydroxyapatite (crystallized calcium phosphate salt) – 10% calcium carbonate – Other minerals (fluoride, sodium, potassium, magnesium) – Harden matrix 7-16  Histology of Osseous Tissue Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bone marrow T Pelvic bone Head of femur Spongy bone Compact bone (a) (c) Lamella Lacunae Canaliculi Central canal (d) 20 m 7-17 a,c: © Dr. Don W. Fawcett/Visuals Unlimited; d: Visuals Unlimited Histology of Osseous Tissue Perforating (Volkmann) canals – Perpendicular to central canals Central canals – vertical canals Concentric lamellae – surround central canal Circumferential lamellae – Rings of bone on entire bone Internal & external Interstitial lamellae – Between osteons – Usually old fragments  Spongy Bone Spongelike appearance Spongy bone consists of: – Slivers of bone called spicules – Thin plates of bone called trabeculae – Spaces filled with red bone marrow No osteons – All osteocytes close to bone marrow Provides strength with minimal weight – Trabeculae develop along bone’s lines of stress 7-19 Bone Marrow Bone marrow—general term for soft tissue that occupies the marrow cavity of a long bone and small spaces amid the trabeculae of spongy bone 7-20  Bone Marrow Red marrow (myeloid tissue) – In nearly every bone in a child – Hemopoietic tissue—produces blood cells and is composed of multiple tissues in a delicate, but intricate arrangement that is an organ to itself – 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 7-21  Bone Development Ossification or osteogenesis—the formation of bone In the human fetus and infant, bone develops by two methods – Intramembranous ossification – Endochondral ossification 7-22  Intramembranous Ossification Figure 7.10 Produces flat bones of skull, clavicle, mandible Endochondral Ossification 7-24  Endochondral Ossification During infancy and childhood, the epiphyses fill with spongy bone Cartilage limited to the articular cartilage covering each joint surface, and to the epiphyseal plate – A thin wall of cartilage separating the primary and secondary marrow cavities – Epiphyseal plate persists through childhood and adolescence By late teens to early 20s, all remaining cartilage in the epiphyseal plate is ossified – Gap between epiphyses and diaphysis closes – Bone can no longer grow in length 7-25 The Fetal Skeleton at 12 Weeks Copyright © The McGraw-Hill Companies, Inc. 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Cranial bones Mandible Humerus Vertebrae Radius Ulna Scapula Ribs Femur Pelvis 7-26 © Biophoto Associates/Photo Researchers, Inc.  Bone Elongation Epiphyseal plate—a region of transition from cartilage to bone – Functions as growth zone where the bones elongate – Consists of typical hyaline cartilage in the middle – With a transition zone on each side where cartilage is being replaced by bone – Metaphysis is the zone of transition facing the marrow cavity Interstitial growth—bones increase in length – Epiphyses close when cartilage is gone—epiphyseal line 7-27  Zones of the Metaphysis Zone 1 Zone 5 Zone of resting cartilage 1 Typical histology of resting hyaline cartilage; secures epiphysis to epiphyseal place 2 Zone of cell proliferation Multiplying Chondrocytes multiplying and chondrocytes lining up in rows of small flattened lacunae Enlarging chondrocytes 3 Zone of cell hypertrophy Cessation of mitosis; enlargement of chondrocytes and thinning of lacuna walls Breakdown of lacunae 4 Zone of calcification Temporary calcification of cartilage matrix between Calcifying columns of lacunae cartilage 5 Zone of bone deposition Bone Breakdown of lacuna walls, marrow leaving open channels; death of chondrocytes; bone deposition by osteoblasts, Osteoblasts forming trabeculae of spongy bone Osteocytes Trabeculae of spongy bone Victor Eroschenko 7-28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Bone Widening and Thickening Appositional growth— bones increase in width throughout life – Deposition of new bone at the surface – Osteoblasts on deep side of periosteum deposit osteoid tissue Become trapped as tissue calcifies – Lay down matrix in layers parallel to surface Forms circumferential lamellae over surface – Osteoclasts of endosteum enlarge marrow cavity 7-29  Bone Remodeling Bone remodeling occurs throughout life—10% per year – Repairs microfractures, releases minerals into blood, reshapes bones in response to use and disuse – Wolff’s law of bone: architecture of bone determined by mechanical stresses placed on it and bones adapt to withstand those stresses Remodeling is a collaborative and precise action of osteoblasts and osteoclasts Bony processes grow larger in response to mechanical stress 7-30  Calcium Homeostasis Calcium homeostasis depends on a balance between dietary intake, urinary and fecal losses, and exchanges between osseous tissue Calcium is required for – Initiation of muscle contraction – Exocytosis of molecules from cells, including neurons – Stimulation of the heart by pacemaker cells – Blood clotting Calcium homeostasis is regulated by three hormones: – Calcitriol, calcitonin, and parathyroid hormone 7-31  Calcitriol Synthesis and Action Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-dehydrocholesterol Ultraviolet light HO Vitamin D3 (cholecalciferol) CH2 HO Bone resorption Calcidiol Reduced excretion OH of Ca2+ CH2 Calcitriol HO OH Absorption of Ca2+ and CH2 phosphate HO OH 7-32  Calcitriol Calcitriol—a form of vitamin D produced by the sequential action of the skin, liver, and kidneys Calcitriol behaves as a hormone that raises blood calcium concentration – Increases calcium absorption by small intestine – Increases calcium resorption from the skeleton – Promotes kidney reabsorption of calcium ions, so less lost in urine Necessary for bone deposition—need adequate calcium and phosphate Abnormal softness of bones in children (rickets) and in adults (osteomalacia) without adequate vitamin D 7-33  Calcitonin Calcitonin—secreted by C cells (clear cells) of the thyroid gland when calcium concentration rises too high Lowers blood calcium concentration in two ways – Osteoclast inhibition Reduces osteoclast activity as much as 70% Less calcium liberated from bones – Osteoblast stimulation Increases the number and activity of osteoblasts Deposits calcium into the skeleton 7-34  Calcium Homeostasis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blood Ca2+ Blood Ca2+ excess returns to normal Calcitonin secretion Reduced Less bone osteoclast resorption activity Increased More bone osteoblast deposition activity 7-35 (a) Correction for hypercalcemia  Parathyroid Hormone Parathyroid hormone (PTH)—secreted by the parathyroid glands which adhere to the posterior surface of thyroid gland PTH released with low calcium blood levels PTH raises calcium blood level by four mechanisms – Binds to receptors on osteoblasts Simulating them to secrete RANKL which raises the osteoclast population – Promotes calcium reabsorption by the kidneys, less lost in urine – Promotes the final step of calcitriol synthesis in the kidneys, enhancing calcium-raising effect of calcitriol – Inhibits collagen synthesis by osteoblasts, inhibiting bone deposition 7-36  Calcium Homeostasis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blood Ca2+ Blood Ca2+ deficiency returns to normal Parathyroid hormone secretion Increased More bone osteoclast resorption activity Reduced Less bone osteoblast deposition activity More urinary Prevention of phosphate hydroxyapatite excretion formation Less urinary Conservation calcium of calcium excretion 7-37 (b) Correction for hypocalcemia  Factors Affecting Bone At least 20 or more hormones, vitamins, and growth factors affect osseous tissue Bone growth especially rapid in puberty and adolescence – Surges of growth hormone, estrogen, and testosterone occur and promote ossification – These hormones stimulate multiplication of osteogenic cells, matrix deposition by osteoblasts, and chondrocyte multiplication and hypertrophy in metaphyses – Girls grow faster than boys and reach full height earlier Estrogen stronger effect than testosterone on bone growth – Males grow for a longer time and taller 7-38 Factors Affecting Bone 7-39  Fractures and Their Repair Stress fracture—thin break caused by abnormal trauma to a bone – Falls, athletics, and military combat Pathological fracture—break in a bone weakened by disease – Bone cancer or osteoporosis Fractures classified by structural characteristics – Direction of fracture line – Break in the skin – Multiple pieces 7-40 Types of Bone Fractures 7-41  Healing of Fractures Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Marrow cavity Fibrocartilage Hard callus Hematoma Soft callus Spongy bone New blood vessels Compact bone 1 Hematoma formation 2 Soft callus formation 3 Hard callus formation 4 Bone remodeling The hematoma is converted Deposition of collagen and Osteoblasts deposit a temporary Small bone fragments are to granulation tissue by invasion fibrocartilage converts granulation bony collar around the fracture to removed by osteoclasts, while of cells and blood capillaries. tissue to a soft callus. unite the broken pieces while osteoblasts deposit spongy ossification occurs. bone and then convert it to compact bone. Figure 7.18 7-42  Bone Disorders Osteoporosis—the most common bone disease – Severe loss of bone density Bones lose mass and become brittle due to loss of organic matrix and minerals – Subject to pathological fractures of hip, wrist, and vertebral column – Kyphosis (widow’s hump) —deformity of spine due to vertebral bone loss 7-43  Osteoporosis Estrogen maintains density in both sexes; inhibits resorption by osteoclasts – Testes and adrenals produce estrogen in men – In women, rapid bone loss after menopause Osteoporosis is common in young female athletes with low body fat causing them to stop ovulating and ovarian estrogen secretion is low Treatments – Estrogen replacement therapy (ERT) slows bone resorption, but increases risk of breast cancer, stroke, and heart disease Drugs Fosamax, Actonel destroy osteoclasts – PTH slows bone loss if given as daily injection Forteo (PTH derivative) increases density by 10% in 1 year; promotes bone growth – May promote bone cancer so use is limited to 2 years – Best treatment is prevention: exercise and a good bone- building diet