Lecture 13 Bones. Function of Bones. Bones and Cartilages of the Human Body

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1 Lecture 13 Bones Function of Bones Support form the framework that supports the body and cradles soft organs Protection provide a protective case for the brain, spinal cord, and vital organs Movement provide levers for muscles Mineral storage reservoir for minerals, especially calcium and phosphorus Blood cell formation hematopoiesis occurs within the marrow cavities of bones Bones and Cartilages of the Human Body 1

2 Composition of Bone Organic Osteoblasts boneforming cells Osteocytes mature bone cells Osteoclasts large cells that resorb or break down bone matrix Inorganic Mineral salts Sixty-five percent of bone by mass Mainly calcium phosphates Responsible for bone hardness and its resistance to compression Gross Anatomy of Bones: Bone Textures Compact bone dense outer layer Spongy bone honeycomb of trabeculae filled with yellow bone marrow Microscopic Structure of Compact Bone Haversian system, or osteon: The structural unit of compact bone Lamella: weight-bearing, column-like matrix tubes composed mainly of collagen Haversian, or central canal: central channel containing blood vessels and nerves 2

3 Structure of Long Bone Long bones consist of a diaphysis and an epiphysis Diaphysis Tubular shaft that forms the axis of long bones Composed of compact bone that surrounds the medullary cavity Yellow bone marrow (fat) is contained in the medullary cavity Epiphyses Expanded ends of long bones Exterior is compact bone, and the interior is spongy bone Joint surface is covered with articular (hyaline) cartilage Epiphyseal line separates the diaphysis from the epiphyses Long Bone Growth and Remodeling Growth in length: cartilage continually grows and is replaced by bone as shown Remodeling: bone is resorbed and added by appositional growth as shown Two control loops regulate bone remodeling Hormonal mechanism maintains calcium homeostasis in the blood Mechanical and gravitational forces acting on the skeleton Structure of Short, Irregular, and Flat Bones Thin plates of compact bone on the outside with spongy bone (diploë) on the inside Have no diaphysis or epiphyses Contain Red bone marrow between the trabeculae Location of Hematopoietic Tissue (Red Marrow) In infants Found in the medullary cavity and all areas of spongy bone In adults Found in the diploë of flat bones, and the head of the femur and humerus 3

4 Importance of Ionic Calcium in the Body Calcium is necessary for: Transmission of nerve impulses Muscle contraction Blood coagulation Secretion by glands and nerve cells Cell division Hormonal Mechanism Rising blood Ca 2+ levels trigger the thyroid to release calcitonin Calcitonin stimulates calcium salt deposit in bone Falling blood Ca 2+ levels signal the parathyroid glands to release PTH PTH signals osteoclasts to degrade bone matrix and release Ca 2+ into the blood Response to Mechanical Stress Wolff s law: a bone grows or remodels in response to the forces or demands placed upon it Observations supporting Wolff s law include Long bones are thickest midway along the shaft (where bending stress is greatest) Curved bones are thickest where they are most likely to buckle Trabeculae form along lines of stress Large, bony projections occur where heavy, active muscles attach 4

5 Common Types of Fractures Homeostatic Imbalances Osteomalacia Bones are inadequately mineralized causing softened, weakened bones Main symptom is pain when weight is put on the affected bone Caused by insufficient calcium in the diet, or by vitamin D deficiency Rickets Bones of children are inadequately mineralized causing softened, weakened bones Bowed legs and deformities of the pelvis, skull, and rib cage are common Caused by insufficient calcium in the diet, or by vitamin D deficiency Osteoporosis Group of diseases in which bone reabsorption outpaces bone deposit Spongy bone of the spine is most vulnerable Occurs most often in postmenopausal women Bones become so fragile that sneezing or stepping off a curb can cause fractures A single gene that codes for vitamin D docking determines The tendency to accumulate bone mass early in life The risk for osteoporosis later in life Treatments Calcium and vitamin D supplements Increased weight-bearing exercise Hormone (estrogen) replacement therapy (HRT) slows bone loss Natural progesterone cream prompts new bone growth Statins increase bone mineral density Equal osteoblast and osteoclast activities Excess osteoclast activity 5

6 Paget s Disease Characterized by excessive bone formation and breakdown Pagetic bone with an excessively high ratio of woven to compact bone is formed Pagetic bone, along with reduced mineralization, causes spotty weakening of bone Osteoclast activity wanes, but osteoblast activity continues to work Usually localized in the spine, pelvis, femur, and skull Unknown cause (possibly viral) Treatment includes the drugs Didronate and Fosamax Developmental Aspects of Bones Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes and cartilages that form the embryonic skeleton The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms At birth, most long bones are well ossified (except for their epiphyses) During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone During puberty, testosterone and estrogens: Initially promote adolescent growth spurts Cause masculinization and feminization of specific parts of the skeleton Later induce epiphyseal plate closure, ending longitudinal bone growth By age 25, nearly all bones are completely ossified In old age, bone resorption predominates Axial and Appendicular Skeletons Axial skeleton bones of the skull, vertebral column, and rib cage Appendicular skeleton bones of the upper and lower limbs, shoulder, and hip 6

7 The Axial Skeleton Forms the longitudinal axis of the body 80 bones Skull: 8 cranial bones 14 facial bones 6 auditory ossicles Hyoid bone Vertebral column: 24 vertebrae Sacrum Coccyx Thoracic cage: 24 ribs Sternum Supports and protects organs in body cavities Attaches to muscles of: Head, neck, and trunk Respiration Appendicular skeleton The Skull The skull protects: The brain Entrances to respiratory system Entrance to digestive system 22 bones: 8 cranial bones: Form the braincase or cranium 14 facial bones: Separate the oral and nasal cavities Form the nasal septum Sinuses Cavities which decrease the weight of the skull: lined with mucus membranes protect the entrances of the respiratory system The Vertebral Column Formed from 26 irregular bones (vertebrae) connected in such a way that a flexible curved structure results Cervical vertebrae 7 bones of the neck Thoracic vertebrae 12 bones of the torso Lumbar vertebrae 5 bones of the lower back Sacrum bone inferior to the lumbar vertebrae that articulates with the hip bones Posteriorly concave curvatures cervical and lumbar Posteriorly convex curvatures thoracic and sacral 7

8 Abnormal spine curvatures Scoliosis (abnormal lateral curve), Kyphosis (hunchback), and Lordosis (swayback) Damage to Intervertebral Discs Slipped disc: bulge in anulus fibrosus invades vertebral canal Herniated disc: nucleus pulposus breaks through anulus fibrosus presses on spinal cord or nerves The Appendicular Skeleton 8

9 Synovial Joints: General Structure Synovial joints all have the following 1. Articular cartilage 2. Joint (synovial) cavity 3. Articular capsule 4. Synovial fluid 5. Reinforcing ligaments Joints in which the articulating bones are separated by a fluidcontaining joint cavity All are freely movable diarthroses Examples all limb joints, and most joints of the body Injuries Sprains The ligaments reinforcing a joint are stretched or torn Partially torn ligaments slowly repair themselves Completely torn ligaments require prompt surgical repair Cartilage Injuries The snap and pop of overstressed cartilage Common aerobics injury Repaired with arthroscopic surgery Dislocations Occur when bones are forced out of alignment Usually accompanied by sprains, inflammation, and joint immobilization Caused by serious falls and are common sports injuries Subluxation partial dislocation of a joint Inflammatory and Degenerative Conditions Bursitis An inflammation of a bursa, usually caused by a blow or friction Symptoms are pain and swelling Treated with anti-inflammatory drugs; excessive fluid may be aspirated Tendonitis Inflammation of tendon sheaths typically caused by overuse Symptoms and treatment are similar to bursitis Arthritis More than 100 different types of inflammatory or degenerative diseases that damage the joints Most widespread crippling disease in the U.S. Symptoms pain, stiffness, and swelling of a joint Acute forms are caused by bacteria and are treated with antibiotics Chronic forms include osteoarthritis, rheumatoid arthritis, and gouty arthritis 9

10 Osteoarthritis (OA) Most common chronic arthritis; often called wear-and-tear arthritis Affects women more than men 85% of all Americans develop OA More prevalent in the aged, and is probably related to the normal aging process Course OA reflects the years of abrasion and compression causing increased production of metalloproteinase enzymes that break down cartilage As one ages, cartilage is destroyed more quickly than it is replaced The exposed bone ends thicken, enlarge, form bone spurs, and restrict movement Joints most affected are the cervical and lumbar spine, fingers, knuckles, knees, and hips Treatment OA is slow and irreversible Treatments include: Mild pain relievers, along with moderate activity Magnetic therapy Glucosamine sulfate decreases pain and inflammation Rheumatoid Arthritis (RA) Chronic, inflammatory, autoimmune disease of unknown cause, with an insidious onset Usually arises between the ages of 40 to 50, but may occur at any age Signs and symptoms include joint tenderness, anemia, osteoporosis, muscle atrophy, and cardiovascular problems The course of RA is marked with exacerbations and remissions Course RA begins with synovitis of the affected joint Inflammatory chemicals are inappropriately released Inflammatory blood cells migrate to the joint, causing swelling Inflamed synovial membrane thickens into a pannus Pannus erodes cartilage, scar tissue forms, articulating bone ends connect The end result, ankylosis, produces bent, deformed fingers Treatment Conservative therapy aspirin, long-term use of antibiotics, and physical therapy Progressive treatment anti-inflammatory drugs or immunosuppressants The drug Enbrel, a biological response modifier, neutralizes the harmful properties of inflammatory chemicals Gouty Arthritis Deposition of uric acid crystals in joints and soft tissues, followed by an inflammation response Typically, gouty arthritis affects the joint at the base of the great toe In untreated gouty arthritis, the bone ends fuse and immobilize the joint Treatment colchicine, nonsteroidal anti-inflammatory drugs, and glucocorticoids 10