Skeletal System Fragility fracture Sign of low bone density A fracture that occurs after a fall from less than standing height. BMD = Bone Mineral Density Osteopenia Means “low bone mass” BMD at least 1 – 2.5 standard deviations below the mean for young adults. Osteoporosis BMD at least 2.5 standard deviations below the mean for young adults.

Bone Classification Bones are classified according to their shapes: Long bones Longitudinal axes Expanded ends Examples: forearm and thigh (most bones of the appendicular skeleton)

Bone Classification Short bones Cubelike Lengths and widths roughly equal Examples: bones of the wrists and ankles

Bone Classification Flat bones Platelike structures Broad surfaces Examples: ribs, scapulae, some bones of the skull

Bone Classification Irregular bones Variety of shapes Usually connected to several other bones Examples: vertebrae of the backbone and many facial bones

Bone Classification Sesamoid bones Usually small and nodular Embedded with tendons adjacent to joints Examples: patella (knee cap) Some individuals that have more than 206 bones will have sesamoid bones in the elbow.

Parts of a Long Bone Epiphysis – expanded portions found at the ends of long bones Proximal epiphysis is closer to center of the body Distal epiphysis is further from center of the body Articular cartilage – hyaline cartilage that coats the ends of the epiphyses Diaphysis – shaft of long bone, found between epiphyses Periosteum – tough vascular covering of entire bone except where the articular cartilage is found.

Parts of a Long Bone Periosteum (cont.) Firmly attached to bone Fibers continuous with ligaments and tendons Helps form and repair bone Bony projections called processes provide sites where ligaments and tendons attach Medullary Cavity Hollow chamber found inside the diaphysis Continuous with spaces of spongy bone

Parts of a Long Bone Medullary Cavity (cont.) Thin layer of cells lining the medullary cavity called the endosteum Soft connective tissue called marrow fills the endosteum. Two types of marrow: –Red marrow: makes new blood cells, including red blood cells, white blood cells, and platelets. –Yellow marrow: inactive, not producing new blood cells, but maintains the ability to become active when needed.

Compact & Spongy Bone Compact Bone Tightly packed tissue Makes up the wall of the diaphysis and epiphysis Also called cortical bone Continuous matrix with no spaces Spongy Bone Also called cancellous bone Found in the epiphyses Contains numerous branching bony plates Irregular spaces reduce bone weight

Microscopic Structure of a Long Bone Osteocytes – mature bone cells Lacunae – small bony chambers that contain osteocytes Osteons – (also called Haversian systems) cylinder shaped units including osteocytes that surround a central canal Central canals – (also called Haversian or osteonic canals) tiny channels in bone tissue that house blood vessels and nerves Canaliculi – microscopic canals that connect lacunae of bone tissue and allow osteocytes to communicate

Microscopic Structure of a Long Bone Extracellular matrix of bone tissue is largely composed of the inorganic salt calcium phosphate (Ca 3 (PO 4 ) 2 ) Inorganic salts make bone hard and resistant to crushing Collagen gives strength and resilience to bone. Many osteons cemented together make up compact bone Central canals extend longitudinally through bone tissue

Microscopic Structure of a Long Bone Perforating canals also called Volkmann’s canals connect central canals transversely contain larger blood vessels and nerves that connect to smaller vessels and nerves in the central canals help central canals communicate with the surface of bone and the medullary cavity.

Microscopic Structure of a Long Bone Spongy Bone Composed of osteocytes and extracellular matrix Osteocytes do not aggregate around a central canal Osteocytes lie within trabeculae Trabeculae form a meshwork of intercommunicating spaces in spongy bone that are filled with bone marrow Spongy bone gets its nutrients from substances that diffuse into canaliculi that lead to the trabeculae.

Bone Development and Growth Bones begin to form during the first few weeks of prenatal development, Bones form by replacing existing connective tissue in one of two ways: –Intramembranous bones originate between sheetlike layers of connective tissue, –Endochondral bones begin as masses of cartilage that are later replaced by bone tissue.

Intramembranous Bones Found in the broad, flat bones of the skull, Steps in development : 1.Membrane-like layers of unspecialized connective tissues appear at the sites of future bones. 2.Partially differentiated cells enlarge and further differentiate into bone-forming cells called osteoblasts. 3.Osteoblasts become active in the layers of connective tissue and deposit bony matrix around themselves. 4.Spongy bone forms in all directions within the layers of connective tissues. 5.When matrix completely surrounds osteoblasts, they are called osteocytes. 6.Periosteum forms, followed by compact bone over the spongy bone.

Endochondral Bones Make up most of the bones of the skeleton Steps in development: 1.Develop from masses of hyaline cartilage shaped like future bony structures. 2.In long bones, changes begin in the center of the diaphysis, where cartilage breaks down and disappears. 3.A periosteum forms from connective tissue that encircles the diaphysis. 4.Blood vessels and osteoblasts from the periosteum invade the disintegrating cartilage and spongy bone forms there. 5.Bone develops from the middle to the ends of the bones in an area called the primary ossification center. 6.Osteoblasts from periosteum deposit a thin layer of compact bone around the primary ossification center. 7.Epiphyses remain cartilaginous and continue to grow.

Endochondral Bones Steps in development (continued): 8. A secondary ossification center appears in the epiphyses. 9. Spongy bone is deposited in the diaphysis and in the epiphysis. 10. As a result, a band of cartilage called the epiphyseal plate or metaphysis, remains between the primary and secondary ossification centers.

Types of Bone Cells Osteoblasts –Bone-forming cells Osteocytes –Mature bone cells surrounded by matrix Osteoclasts –Large, multinucleated cells that break down matrix –Bone destroying cells –Secrete an acid that dissolves the inorganic part of the matrix –Abnormally active osteoclasts destroy bone tissue in many bone cancers.

Epiphyseal Plate Also called a growth plate Found between the primary and secondary ossification centers of long bones Bone continues to grow as long as the epiphyseal plate is made of cartilage Bone stops growing when the epiphyseal plate ossifies (turns to bone) Growth hormone keeps the plate cartilaginous Sex hormones cause the plate to ossify.

Epiphyseal Plate If an epiphyseal plate is damaged before it ossifies, Elongation of the bone may stop prematurely, or Bone growth may be uneven. The epiphysis that was damaged may need to be surgically altered to equalize the growth rate of bones that are developing at different rates.

Homeostasis of Bone Bone continues to be resorbed by osteoclasts and remodeled by osteoblasts throughout life. As a result, total bone mass for adults remains nearly constant, even though 3-5% of bone calcium is exchanged each year. Vitamin D is necessary for the proper absorption of calcium in the small intestine. Physical exercise pulling on bones causing bone tissue to thicken and strengthen.

Fractures A break in a bone, Can be classified as traumatic, spontaneous, or pathologic, Can be classified by the nature of the break (greenstick, etc.) When exposed to the outside by an opening in the skin, is termed as compound (or open)

Types of Fractures (See p. 136) Greenstick - incomplete break –Break occurs on the convex surface of the bone. Fissured – incomplete break –Longitudinal break Comminuted – complete break –Fragments the bone Transverse – complete break –Break occurs at right angle to the axis of the bone Oblique – complete break –Occurs at angle other than right angle Spiral – complete break –Caused by excess twisting of bone

Healing of Fractures (See p. 137)

Bone Function Shape body structures –Give shape to the head, face, thorax (chest), and limbs. Support body structures –Bones of the lower limbs, pelvis, and backbone support the body’s weight. Protect body structures –Bones of the skull protect the eyes, ears and brain. –The rib cage and shoulder girdle protect the heart and lungs. –The bones of the pelvis protect the lower abdominal region and internal reproductive organs.

Bone Function (cont.) Body movement –Interact with muscles as simple mechanical devices called levers –Components of a lever: 1.Rigid bar or rod, 2.Fulcrum or pivot, 3.Object moved against resistance 4.Force that supplies energy to move bar. –Bone/muscle as a lever: 1.Forearm is rigid rod 2.Elbow is fulcrum 3.Object lifted against force 4.Muscle supplies force

Bone Function (cont.) Blood cell formation –Called hematopoiesis, –Hemoglobin in red blood cells carries oxygen around the body, –Red marrow actively produces blood cells (red blood cells, white blood cells, and platelets), –Yellow marrow stores fat, but can become red marrow if blood cells needed.

Bone Function (cont.) Storage of inorganic salts –Bone stores calcium mainly in the form of calcium phosphate (Ca 3 (PO 4 ) 2 ), –Low blood calcium results in calcium being pulled from bone matrix by osteoclasts and deposited into blood circulation. –High blood calcium results in calcium being removed from blood and deposited into bone matrix by osteoblasts. –Bone can accumulate other harmful metals such as radium, lead, or strontium when ingested.

Bone Function (cont.) Two major portions of skeletons: Axial skeleton Consists of bony and cartilaginous parts that support and protect the organs of the head, neck, and trunk. Skull: 8 cranial & 14 facial bones Middle ear bones: 3 bones per ear Hyoid bone: 1 bone Vertebral column: 26 vertebrae Thoracic cage: 25 bones Bone Function (cont.) Skeletal Organization (See Table 7.1, p. 141)

Bone Function (cont.) Two major portions of skeletons: Appendicular Skeleton Consists of the bones of the upper and lower limbs and the bones that anchor the limbs to the axial skeleton. Pectoral girdle(shoulder): 2 bones per shoulder Upper limbs(arms and hands): 30 bones per arm and hand Pelvic girdle (hips): 1 bone per hip Lower limbs (legs and feet): 30 bones per leg and foot Bone Function (cont.) Skeletal Organization (See Table 7.1, p. 141)