The Skeletal System: Bone Tissue Chapter 6

Bone Skeletal system Functions: Defined: Includes all of the bones of the human body (total of 206), and their associated cartilages and joints. Functions: Support – supporting framework for body Protection – protects vital organs (brain and thoracic cavity) Movement / levers- allows movement and flexion as well as levers for different movements Mineral storage- calcium and phosphate Hematopoiesis- principal site for blood cell formation in red marrow of flat bones (e.g. sacrum, sternum, etc.) Electrolyte balance of calcium and phosphorus Acid/Base balance buffers the blood calcium phosphate Detoxification by absorbing heavy minerals

Classification of Bones by Shape

Bone Composition Two types of Bone: Compact Bone Bone is living tissue. solid network of cells and protein fibers surrounded by deposits of minerals. Components: 32% Organic materials (collagen & bone cells) 43% Minerals (calcium & phosphorous) 25% Water Two types of Bone: Compact Bone Spongy Bone http://images.google.com/imgres?imgurl=http://fire.biol.wwu.edu/lapsansk/348/bone_hist.jpg&imgrefurl=http://fire.biol.wwu.edu/lapsansk/bio348.htm&h=320&w=433&sz=25&hl=en&start=4&tbnid=KpCXAvv9AU5EIM:&tbnh=93&tbnw=126&prev=/images%3Fq%3Dbone%2Btissues%26svnum%3D10%26hl%3Den%26rls%3DGGLB,GGLB:1969-53,GGLB:en%26sa%3DN%20%20This%20site%20has%20a%20whole%20bone%20power%20point

Long Bone Anatomy (Humerus) Diaphysis Epiphysis Metaphysis -Epiphyseal growth plate Articular cartilage -Perforating fibers Periosteum- covers bone Medullary cavity Endosteum- lines bone

Cells in bone: Osteocytes = mature bone cells In lacunae Connected by canaliculi Osteoblasts synthesize new matrix Osteogenesis Osteoclasts dissolve bone matrix Osteolysis Osteoprogenitor cells differentiate into osteoblasts

Histology of Bone Tissue

Figure 6.3a

Matrix of Bone Inorganic mineral salts provide bone’s hardness Organic collagen fibers provide bone’s flexibility their tensile strength resists being stretched or torn Mineralization (calcification) is hardening of tissue when mineral crystals deposit around collagen fibers Bone is not completely solid since it has small spaces for vessels and red bone marrow spongy bone has many such spaces compact bone has very few

Bone Matrix If mineral removed, bone is too bendable If collagen removed, bone is too brittle

Histology of Bone Tissue Compact (Dense) Bone- resists stresses produced by weight & movement Components of compact bone are arranged into repeating structural units called osteons or Haversian systems Osteons consist of a central (Haversian) canal with concentrically arranged lamellae, lacunae, osteocytes, canaliculi

Compact bone terms Osteon/Haversian System: structural unit of compact bone. Oriented parallel to shaft and forming a group of hollow tubes through which an artery, vein and nerve pass into and through bone. Lacunae: small cavities (halo’s) containing osteocytes Osteocyte: true bone cell, spider shaped and found in lacunae at the junctions of the lamellae Lamellae: layers of the matrix with each layer of collagen fibers going in opposite direction to the adjacent layer ; maybe Concentric (forming rings like a tree)or Circumferential (encircling the entire bone structure). Canaliculi: Hair like canals that connect each lacunae and in turn connect to the central canal. Remove wastes and bring nutrients into osteocytes Volkman’s canal/Perforating canal: Canals running perpendicular to the Haversian canals, but connecting to them. They bring in the artery, vein and nerves to the bone structure.

Histology of Bone Tissue Osteon Canaliculi connect lacunae, forming a system of interconnected canals Providing routes for nutrients and oxygen to reach the osteocytes The organization of osteons changes in response to the physical demands placed on the skeleton

Histology of Bone Tissue Spongy Bone Lacks osteons Lamellae are arranged in a lattice of thin columns called trabeculae Spaces between the trabeculae make bones lighter Trabeculae support & protect the red bone marrow Hematopoiesis (blood cell production) occurs in spongy bone Found in ends of long bones and inside flat bones such as the hipbones, sternum, sides of skull, and ribs.

Blood and Nerve Supply of Bone Bone is richly supplied with blood Periosteal arteries accompanied by nerves supply the periosteum and compact bone Epiphyseal veins carry blood away from long bones Nerves accompany the blood vessels that supply bones The periosteum is rich in sensory nerves sensitive to tearing or tension

Bone development and growth Ossification = converting other tissue to bone Intramembranous Ossification Endochondral Ossification Calcification = depositing calcium salts within tissues

Bone Formation Bone formation follows one of two patterns Formation of Bone in an Embryo (during 6th week) Bone formation follows one of two patterns Intramembranous ossification -formation of bone directly from mesenchymal cells. Flat bones of the skull and mandible are formed in this way “Soft spots” that help the fetal skull pass through the birth canal later become ossified forming the skull Endochondral ossification The replacement of cartilage by bone Most bones of the body are formed in this way including long bones

Intramembranous Bone Formation Mesenchymal cells become osteoprogenitor cells then osteoblasts. Osteoblasts surround themselves with matrix to become osteocytes. Matrix calcifies into trabeculae with spaces holding red bone marrow. Mesenchyme condenses as periosteum at the bone surface. Superficial layers of spongy bone are replaced with compact bone.

Intramembranous Ossification

Bone Formation Bone growth begins long before birth. 10 week fetus Cartilage bone of the skull Intramembranous ossification produces the roofing bones Primary centers of the diaphyses (skeleton of the lower limb) Future hip bone Bone Formation Bone growth begins long before birth. The basic shape of a long bone, such as an arm bone, is first formed as cartilage

Bone Formation 12 week fetus 16 week fetus Ossification begins to take place up to seven months before birth

Bone Formation Babies are born with 350 bones, many are composed almost entirely of cartilage. Latter the cartilage cells will be replaced by cells that form the bones. (ossification) The SOFT SPOT of a babie’s skull will fuse around age 2, but growth of the skull continues until adulthood. http://images.google.com/imgres?imgurl=http://www.esg.montana.edu/esg/kla/ta/devfoot.jpg&imgrefurl=http://www.esg.montana.edu/esg/kla/ta/&h=475&w=482&sz=25&hl=en&start=16&um=1&tbnid=ua5iFVQ59AR4hM:&tbnh=127&tbnw=129&prev=/images%3Fq%3Dnew%2Bborn%2Bx%2Brays%26svnum%3D10%26um%3D1%26hl%3Den%26rls%3DGGLB,GGLB:1969-53,GGLB:en%26sa%3DN Long bones develop and grow throughout childhood at the centers of ossification (growth plates)

Endochondral Bone Formation (1) Development of Cartilage model Mesenchymal cells form a cartilage model of the bone during development Growth of Cartilage model in length by chondrocyte cell division and matrix formation ( interstitial growth) in width by formation of new matrix on the periphery by new chondroblasts from the perichondrium (appositional growth)

Endochondral Bone Formation (2) Development of Primary Ossification Center nutrient artery penetrates center of cartilage model osteoblasts deposit bone matrix over calcified cartilage forming spongy bone trabeculae osteoclasts form medullary cavity

Endochondral Bone Formation (3) Development of Secondary Ossification Center blood vessels enter the epiphyses around time of birth spongy bone is formed but no medullary cavity Formation of Articular Cartilage cartilage on ends of bone remains as articular cartilage.

1 Development of cartilage model primary ossification center the medullary cavity Growth of 2 3 4 Hyaline cartilage Calcified matrix Periosteum (covering compact bone) Uncalcified Medullary Nutrient artery and vein artery Perichondrium Proximal epiphysis Distal Diaphysis Primary ossification Secondary Epiphyseal artery and vein Development of secondary ossification center 5 Spongy bone 1 Articular cartilage Spongy bone Epiphyseal plate Secondary ossification center Nutrient artery and vein Uncalcified matrix Epiphyseal artery and vein Formation of articular cartilage and epiphyseal plate Development of secondary ossification center Development of cartilage model primary ossification the medullary cavity Growth of 2 3 4 5 6 Hyaline cartilage Calcified Periosteum (covering compact bone) Medullary artery Perichondrium Proximal epiphysis Distal Diaphysis Primary Spongy bone 1 Hyaline cartilage Calcified matrix Periosteum (covering compact bone) Uncalcified Medullary cavity Nutrient artery and vein artery Perichondrium Proximal epiphysis Distal Diaphysis Development of cartilage model primary ossification center the medullary Growth of Primary ossification 2 3 4 Spongy bone 1 Development of cartilage model primary ossification center Growth of 2 3 Hyaline cartilage Uncalcified matrix Calcified Nutrient artery Perichondrium Proximal epiphysis Distal Diaphysis Periosteum Primary ossification Spongy bone 1 Development of cartilage model Growth of 2 Hyaline cartilage Uncalcified matrix Calcified Perichondrium Proximal epiphysis Distal Diaphysis 1 Development of cartilage model Hyaline cartilage Perichondrium Proximal epiphysis Distal Diaphysis

Bone Growth During Infancy, Childhood and Adolescence Growth in Length (Interstitial Growth) The growth in length of long bones involves two major events: 1) Growth of cartilage on the epiphyseal plate 2) Replacement of cartilage by bone tissue in the epiphyseal plate

Zones of Growth in Epiphyseal Plate Zone of resting cartilage anchors growth plate to bone Zone of proliferating cartilage rapid cell division (stacked coins) Zone of hypertrophic cartilage cells enlarged & remain in columns Zone of calcified cartilage thin zone, cells mostly dead since matrix calcified osteoclasts removing matrix osteoblasts & capillaries move in to create bone over calcified cartilage Closure of epiphyseal plate (age 18-25) – bone stops growing – epiphyseal line

Bone Growth During Infancy, Childhood and Adolescence Growth in Thickness(Appositional Growth) Bones grow in thickness at the outer surface Remodeling of Bone Bone forms before birth and continually renews itself The ongoing replacement of old bone tissue by new bone tissue Old bone is continually destroyed and new bone is formed in its place throughout an individual’s life

Appositional Bone Growth

Factors That Affect Bone Growth and Maintenance 1. Hereditary 2. Nutrition 3. Hormones 4. Exercise or “stress”—for bones, exercise means bearing weight

Factors Affecting Bone Growth and Bone Remodeling Minerals Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling Vitamins Vitamin A stimulates activity of osteoblasts Vitamin C is needed for synthesis of collagen Vitamin D helps build bone by increasing the absorption of calcium from foods in the GI tract into the blood Vitamins K & B12 are needed for synthesis of bone proteins

Factors Affecting Bone Growth and Bone Remodeling Hormones During childhood, the hormones most important to bone growth are growth factors (IGFs), produced by the liver IGFs stimulate osteoblasts, promote cell division at the epiphyseal plate, and enhance protein synthesis Thyroid hormones also promote bone growth by stimulating osteoblasts Insulin promotes bone growth by increasing the synthesis of bone proteins Sex Hormones cause a dramatic effect on bone growth -Cause of the sudden “growth spurt” that occurs during the teenage year -promote changes in females, such as widening of the pelvis -Shut down growth at epiphyseal plates Parathyroid hormone, calcitriol, and calcitonin are other hormones that can affect bone remodeling

Fracture and Repair of Bone Fracture Types Open (compound) fracture The broken ends of the bone protrude through the skin Closed (simple) fracture Does not break the skin Comminuted fracture The bone is splintered, crushed, or broken into pieces Greenstick fracture A partial fracture in which one side of the bone is broken and the other side bends Impacted fracture One end of the fractured bone is forcefully driven into another Pott’s fracture Fracture of the fibula, with injury of the tibial articulation Colles’ fracture A fracture of the radius in which the distal fragment is displaced Stress fracture A series of microscopic fissures in bone

Fracture and Repair of Bone

The Major Types of Fractures Comminuted fractures

Displaced Colles’ Greenstick Compression Figure 6–16 (4 of 9)

Fracture and Repair of Bone Calcium and phosphorus needed to strengthen and harden new bone after a fracture are deposited only gradually and may take several months The repair of a bone fracture involves the following steps 1) Formation of fracture hematoma Blood leaks from the torn ends of blood vessels, a clotted mass of blood forms around the site of the fracture 2) Fibrocartilaginous callus formation Fibroblasts invade the fracture site and produce collagen fibers bridging the broken ends of the bone 3) Bony callus formation Osteoblasts begin to produce spongy bone trabeculae joining portions of the original bone fragments 4) Bone remodeling Compact bone replaces spongy bone

Spongy bone Osteoblast Osteoclast New compact bone Bony callus formation Bone remodeling Osteocyte 3 4 Compact bone Periosteum Fracture hematoma Fracture hematoma Bone fragment Red blood cell Blood vessel Formation of fracture hematoma Phagocyte Osteon 1 Fibroblast Fibrocartilaginous callus Collagen fiber Chondroblast Cartilage Fibrocartilaginous callus formation 2 Bony callus Bony callus Spongy bone Osteoblast Bony callus formation Osteocyte 3 Compact bone Periosteum Fracture hematoma Fracture hematoma Bone fragment Red blood cell Blood vessel Formation of fracture hematoma Phagocyte Osteon 1 Fibroblast Fibrocartilaginous callus Collagen fiber Chondroblast Cartilage Fibrocartilaginous callus formation 2 Phagocyte Osteoblast Fibroblast Fibrocartilaginous callus Collagen fiber Chondroblast Cartilage Fibrocartilaginous callus formation 2 Compact bone Spongy bone Periosteum Fracture hematoma Fracture hematoma Bone fragment Osteocyte Red blood cell Blood vessel Formation of fracture hematoma Osteon 1 Compact bone Spongy bone Periosteum Fracture hematoma Fracture hematoma Bone fragment Osteocyte Red blood cell Blood vessel Formation of fracture hematoma Phagocyte Osteon 1

Bone’s Role in Calcium Homeostasis Bone is the body’s major calcium reservoir Levels of calcium in the blood are maintained by controlling the rates of calcium resorption from bone into blood and of calcium deposition from blood into bone Both nerve and muscle cells depend on calcium ions to function properly Blood clotting also requires Ca2+ Many enzymes require Ca2+ as a cofactor

Bone’s Role in Calcium Homeostasis Actions that help elevate blood Ca2+ level Parathyroid hormone (PTH) –from parathyroid gland -regulates Ca2+ exchange between blood and bone tissue PTH increases the number and activity of osteoclasts PTH acts on the kidneys to decrease loss of Ca2+ in the urine PTH stimulates formation of Calcitriol, a hormone that promotes absorption of calcium from foods in the gastrointestinal tract Calcitonin – from thyroid gland - effects opposite that of PTH

Exercise and Bone Tissue Bone tissue alters its strength in response to changes in mechanical stress Under stress, bone tissue becomes stronger through deposition of mineral salts and production of collagen fibers by osteoblasts Unstressed bones diminishes because of the loss of bone minerals and decreased numbers of collagen fibers The main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of gravity Weight-bearing activities help build and retain bone mass

Aging and Bone Tissue The level of sex hormones diminishes during middle age, especially in women after menopause A decrease in bone mass occurs Bone resorption by osteoclasts outpaces bone deposition by osteoblasts Female bones generally are smaller and less massive than males Loss of bone mass in old age has a greater adverse effect in females

Aging and Bone Tissue There are two principal effects of aging on bone tissue: 1) Loss of bone mass Results from the loss of calcium from bone matrix→osteoporosis 2) Brittleness Results from a decreased rate of protein synthesis Collagen fibers gives bone its tensile strength Loss of tensile strength→ very brittle bones & susceptible to fracture

Spinal Osteoporosis

Compact Bone

Spongy Bone (Cancellous)

Hyaline Cartilage

Adipose Tissue