Functions of the Skeletal System 1. Support 2. Protection 3. Attachment for muscles 4. Blood Cell Production 5. Mineral Storage

FUNCTIONS OF THE SKELETAL SYSTEM : Support Structural framework Support soft tissues Attachments for tendons of most skeletal muscles

Protection Cranium protects the brain Vertebrae protect the spinal cord Ribcage protects the heart and lungs Assistance in movement Bones provide a lever system used by the muscles

Mineral Homeostasis Bone stores calcium, phosphorous and other minerals Bone acts as a “mineral bank” and can release minerals into the blood to maintain homeostasis

Blood cell production Red bone marrow -Hematopoiesis -process of making blood cells *RBC, WBC, platelets -tissue is a network of reticular fibers with developing blood cells -other cells present: *adipocytes, macrophages, fibroblasts -all bone marrow is red in the new born, then decreases with age

Triglyceride storage Yellow bone marrow -the major portion of bone marrow in the adult -primarily adipocytes with a few blood cells

ANATOMY OF A BONE: Diaphysis Long, cylindrical main part of the bone Epiphysis Proximal and distal ends of the bone Metaphysis Region in mature bone where diaphysis joins the epiphysis Growing bone Epiphyseal plate (growth plate) -allows the diaphysis to grow in length but not in width -point where cartilage is replaced by bone

Articular Cartilage Thin layer of hyaline cartilage covering the epiphysis Located in articulations –bones join other bones Absorbs shock Reduces friction

Adjunct structures Periosteum Dense irregular CT Surrounds the bone (except at articular cartilages) Contains bone forming cells Allows bone to increase in diameter Protects bone Assists in fracture repairs Nourishes bone tissue Attachment point for tendons and ligaments

Medullary Cavity Or marrow cavity Space within diaphysis that contains the fatty yellow marrow Endosteum Membrane that contains bone forming cells Lines medullary cavity

HISTOLOGY OF BONE TISSUE: Matrix 25% water 25% protein 50% crystallized mineral salts -hydroxyapatite (calcium phosphate) -calcium carbonate -other minerals -bone hardness due to mineralized salts -deposited in the collagen fiber framework -bone flexibility due to collagen fibers -calcification is initiated by osteoblasts

Fig. 6-13, p. 197

Types of bone cells Osteogenic cells –osteum progenitor cells -mesenchymal stem cells -the only bone cells to undergo cell division -daughter cells become osteoblasts -found along the inner layer of the periosteum -endosteum and the bone canals that contain blood vessels

Osteoblasts -bone building cells -synthesize and secrete collagen fibers -initiate calcification

Osteocytes -mature bone cells -principal cells of bone tissue -osteoblasts that have become trapped in the matrix -no longer secrete matrix -maintain bone

Osteoclasts -huge cells -derived from the fusion of many (50) monocytes -concentrated in the endosteum -digest protein and mineral components of the bone

Compact Bone Tissue 80% of the skeleton external layer of all bones diaphyses of long bones provides protection and support

Bone Tissue arranged in units called osteons blood and lymph vessels and nerves enter bone through perforating canals central canal concentric lamellae lacunae canaliculi Osteons aligned with the lines of stress of the bone

Spongy Bone Tissue 20% of bone does not contain true osteons lamellae arranged in an irregular lattice – Trabeculae macroscopic spaces between trabeculae are filled with red bone marrow

Spongy bone Each trabecula has an osteocyte that lies in the lacunae Light and reduces overall bone weight Trabeculae support and protect red bone marrow Found in: hip bones, ribs, breastbone, backbones, and the ends of long bones Site of hematopoiesis in the adult

BLOOD AND NERVE SUPPLY OF BONE: Periosteal arteries Accompanied by nerves Enter diaphysis through perforating canals nutrient artery large artery in the center of the diaphysis

divides into proximal and distal branches that supply the diaphysis as far as the epiphyseal plates nutrient foramen hole that the nutrient artery goes through into the medullary canal

metaphyseal arteries enter metaphyses and supply red bone marrow with the nutrient artery epiphyseal arteries supply the red bone marrow and tissue of the epiphyses

nutrient veins take blood away epiphyseal veins metaphyseal veins periosteal veins lots of sensory nerves in the periosteum

BONE FORMATION Basics: embryo skeleton -fibrous connective tissue membranes -hyaline cartilage templates of future bones -ossification begins 6 th or 7 th week of life -two patterns of ossification

INTRAMEMBRANOUS OSSIFICATION - fewest steps -formation of flat bones of the skull and mandible and clavicle (fetal development) More common after birth. -1) formation of the center of ossification *mesenchymal cells form osteogenic cells first, then osteoblasts in fibrous CT membranes.

Steps in Intramembranous ossification 1. Formation of the ossification center in the fibrous membrane. 2. Formation of the bone matrix within the fibrous membrane 3. Formation of woven bone and periosteum 4. Formation of compact bone plates and red bone marrow.

1)osteoblasts secrete matrix – become surrounded -osteoid= uncalcified matrix. 2) osteoblasts become osteocytes when matrix is complete. -osteocytes lie in lacunae -calcification begins

3) trabeculae develop from bone matrix -fuse to make spongy bone -blood vessels invade spaces -red bone marrow forms from blood vessel CT -4) Periosteum develops on the outside of the bone from mesenchyme -superficial layers of spongy bone replaced by compact bone

ENDOCHONDRAL OSSIFICATION Happens before birth. Replacement of cartilage by bone –most of the bones of the body Development of the cartilage model -chondroblasts form cartilage matrix in shape of future bone. -perichondrium (membrane) forms around hylaline cartilage model

Growth of the cartilage model - chondroblasts become chondrocytes when buried in cartilage model -model grows in length due to cell division of chondrocytes and secretion of matrix – interstitial growth

Development of the primary ossification center -nutrient artery penetrates perichondrium and cartilage model -osteogenic cells form osteoblasts -bone formation begins -perichondrium becomes periosteum -primary ossification center forms and enlarges toward the ends of the bone -proceeds inward from the external surface of the bone

Development of secondary ossification centers -diaphysis –cartilage replaced by compact bone with a red bone marrow core. -blood vessels enter epiphyses and form secondary ossification center -usually at birth -similar to primary ossification except -proceeds outward -spongy bone remains on the interior of the epiphyses

Formation of articular cartilage and epiphyseal plate -hyaline cartilage that covers epiphysis becomes articular cartilage -growth plate remains until adulthood -hyaline cartilage between the epiphysis and diaphysis

Fusion of Epiphyseal cartilages occurs at different ages Can be used to estimate age. Clavicle is the first to ossify (5 th week of pregnancy) and last to fuse (mid- twenties) Radius and ulna fuse about 20. Scapula – mid twenties Humerus – males 18 years, female 15 years.

appositional growth (thickness) -occurs with the addition of additional matrix under the perichondrium by osteoblasts. -mineralization occurs

BONE GROWTH Growth in Length Epiphyseal Plate has 4 zones 1. Zone of resting cartilage Chondrocytes anchor epiphyseal plate to the bone Cells do not function in bone growth

2. Zone of proliferating cartilage Larger chondrocytes Arranged like stacks of coins Replace those that die on diaphyseal side of growth plate

3. Zone of hypertrophic cartilage Chondrocytes larger and remain in columns Lengthening of diaphysis is a result of cell division in the zone of proliferation and maturation of cells in this zone

3. Zone of calcified cartilage - A few cells thick - Dead chondrocytes - Matrix around them has calcified - Osteoclasts dissolve matrix - Area invaded by osteoblasts and capillaries - Osteoblasts lay down bone matrix - Replace calcified cartilage -Diaphyseal border of epiphyseal plate cemented to diaphysis bone

Growth in Thickness: Bone grows in thickness only by appositional growth. 1. On the bone surface: Periosteal cells become osteoblasts. Osteoblasts secrete matrix until they are surrounded Become osteocytes Form a groove for the periosteal blood vessel.

2. Periosteum becomes endosteum. Ridges enclosing periosteal blood vessels fuse. 3. A new osteon is formed. New concentric lamellae formed by osteoblasts from the endosteum. Concentric lamellae proceed inward toward the periosteal blood vessel. Tunnel fills in to make a new osteon.

4. Bone is further thickened by osteoblasts under the periosteum depositing a new outer circumferential lamellae. 5. Medullary cavity increases in diameter Bone tissue lining the medullary cavity is destroyed by osteoclasts in the endosteum

FACTORS AFFECTING BONE GROWTH Aequate diet of minerals and vitamins Calcium and phosphorus Fluoride Magnesium, iron, manganese Vitamin C -synthesis of collagen -differentiation of osteoblasts into osteocytes Vitamins K and B12 -protein synthesis Vitamin A stimulates osteoblast activity

Hormones IGFs –insulin growth factors -Stimulate cell division at epiphyseal plate and in the periosteum -Stimulate protein synthesis for new bone hGH Thyroid hormones -T3 and T4 Insulin

Sex Steriods -at puberty the secretion of these hormones -cause growth spurt -promote typical skeletal changes for each of the sexes -ultimately close epiphyseal plates – stop growth

BONE FRACTURES AND REPAIRS Definition: a fracture is any break in a bone

Types of Fractures: Open (compound) Broken ends protrude through the skin Simple (closed ) -Does not break the skin Comminuted -Bone splinters on impact leaving fragments between two major fragments

Greenstick -Occurs in children -A partial fracture -One side of the bone breaks -The other side of the bone bends Impacted -One end of the fractured bone is forcefully driven into the other Pott’s -Fracture of the distal end of the fibula, with serious injury of the distal tibial articulation

Colle’s Fracture of the distal end of the radius in which the distal fragment is displaced posteriorly Stress Fracture not visible Series of microscopic fissures

REPAIR OF A BONE FRACTURE: Formation of fracture hematoma Blood vessels break at fracture line Clot forms at fracture site 6-8 hours post injury Bone cells at fracture site die (no blood supply) Swelling and inflammation occur in response to dead bone cells Phagocytic cells and osteoclasts remove dead and damaged tissue May last up to several weeks

Fibrocartilaginous callus formation Infiltration of new blood vessels into fracture hematoma Organization of granulation tissue into procallus Fibroblasts invade the procallus Fibroblasts produce collagen fibers Connect broken ends of the bone

Phagocytes continue to remove debris Osteogenic cells develop into chondrocytes Chondrocytes produce fibrocartilage Procallus transformed into fibrocartilaginous callus Broken ends of bone bridged Fibrocartilaginous callus lasts 3 weeks

Bony callus formation In time fibrocartilage is converted to spongy bone Becomes a bony callus Bony callus lasts 3-4 months

Bone remodeling Dead portions of bone fragments are resorbed by osteoclasts Compact bone replaces spongy bone around periphery of the fracture

BONE’S ROLE IN CALCIUM HOMEOSTASIS Calcium facts: 99% of body calcium is in the bone nerve cells depend on calcium many enzymes require calcium as a cofactor calcium is required for blood clotting blood calcium range is between 9-11 mg/100 mL

Hormone Regulation of Blood Calcium PTH -Parathyroid Hormone -Secreted by the parathyroid glands -Negative feedback system to adjust blood calcium -If blood calcium falls –receptors in the parathyroid gland detect the change -Cyclic AMP turns on synthesis of PTH -PTH release into the blood -PTH increases the number and activity of osteoclasts -Increases bone resorption -Calcium and phosphorus ions are released from the bone into the blood -Calcium level returns to the normal range

Calcitonin When blood calcium levels are too high calcitonin is secreted by the parafollicular cells in the thyroid gland. Calcitonin inhibits osteoclasts Speeds up calcium uptake by bone Accelerates calcium deposition in bone Promotes bone formation When bone is placed under stress calcitonin production increases

AGING AND BONE TISSUE: Decrease of sex steroids with middle age results in a decrease in bone mass In old age the loss of bone through resorption occurs more rapidly than bone gain. Women’s bones are relatively smaller –so they are more susceptible. Two principal effects:

Loss of bone mass Demineralization Begins after age 30 in women and accelerates after 45 due to decrease in estrogen As much as 30% of bone calcium can be lost by age 70 8% of bone is lost every 10 years males calcium loss begins after age 60 in males only 3% of calcium is lost every 10 years

Brittleness Results from a lower rate of protein synthesis Organic portion of the matrix diminishes Collagen fibers

Osteoporosis Bone resorption outpaces bone deposition Bone mass becomes so depleted that bones fracture from stress of daily living 1,000,000 fractures / year affects the entire skeletal system shrinkage of vertebrae, height loss, hunched backs, bone pain

Affects: -post menopausal Caucasian or Asian women -some genetic correlation -low calcium or vitamin D diet -inactive life style -smoking

Therapy and prevention Therapy -Calcium supplements -Fosemax -inhibits osteoclasts -Hormone replacement Prevention -adequate calcium intake -weight bearing exercise