Skeletal system

206 bones

General overview• Skeleton-Greek for dried up body• Divided into 2 divisions– Axial skeleton• Forms the longitudinal axis of the body

– ~80 bones » Skull» Bones associated with skull» Vertebral column» Throacic cage

– Appendicular skeleton• Bones of the limbs and girdles

– 126 bones

General overview cont.

• Hardest material in the body– From calcium salts deposited in the matrix

• Relatively lightweight• Remarkable ability to resist tension and other

forces acting on it– Organic parts, especially the collagen fibers give

flexibility

Bone functions1. Support

-structural support for the whole body framework for attachment of soft tissues and organs, gives shape to head, face, thorax, and limbs

2. Protection-protect soft body organs

3. Movement-acts as leversMovements range from motion of fingertip to changing position of the whole body

4. Storage-fat is stored in internal cavities(yellow marrow in adults)-the bone itself stores inorganic salts mostly in the form of calcium and phosphorus

5. Hematopoiesis-blood cell formationOccurs in red marrowProduces red blood cells, white blood cells, and platelets

skull, ribs, sternum, clavicles, vertebrae, and pelvis

classification

• 2 types of bone tissue1. compact- dense, smooth, homogenous2. Spongy- numerous branching bony plates and

lots of open spaces

Bone shapes• Long bones- long and slender– All bones of limbs except wrist and ankles

• Flat bones- have thin, roughly parallel surfaces provide protection for underlying soft tissues and extensive surface area for attachment of skeletal muscles– Roof of skull, sternum, ribs, and scapula

• Sutural bones(Womian bones)– Small, flat, irregularly shaped bones between flat

bones of the skull• Individual variations of number, shape, and location

Bone shapes cont.• Irregular bones- complex shapes with short,

flat, notched, or rigid surfaces– Spinal vertebrae and bones of the pelvis– Several skull bones

• Short bones- small, cube shaped– Mostly spongy bone– Carpal and tarsal

• Sesmoid bones-small and flat– Develop inside tendon– Located near joints• patella

bone structure• Diaphysis- tubular shaft

– Most of bone’s length– Compact bone– Covered with periosteum

• Marrow cavity/medullar cavity– Central space that stores marrow– Usually yellow marrow in adults

• Epiphysis- expanded area at each end– Spongy bone– Covered with articular cartilage

• Epiphyseal line- remnant of epiphyseal plate– Causes length growth of long

bone

Microscopic anatomy of compact boneOsteocytes- mature bone cellsFound in lacunae Lacunae are within concentric circles called lamellae and Haversian

canalsHaversian canals

run lengthwisecarry blood and nerves throughout the bone

Osteon- central canal and matrixCanaliculi- radiate outward and connect all bone cells to nutrients

well nourishedheal quickly

Volkmann’s canal- communication pathway between exterior and interior of the bonerun into compact bone at right angles to the shaft

Bone formation, growth and remodeling

• Skeleton formed from cartilage and bone• In embryos skeleton primarily hyaline cartilage• In young child hyaline cartilage has been

replaced with bone• Cartilage remains only in isolated areas such

as the bridge of the nose, parts of the ribs, and the joints

Ossificationbone formation

• Hyaline cartilage model covered with bone matrix by bone forming cells osteoblast

• Then cartilage model is digested away opening up a medullary cavity within the newly formed bone– Begins in center of diaphysis– Primary ossification center • Bone tissue develops from here toward the ends of the

cartilaginous structure

Ossification cont.

• By birth most of the cartilage has been converted to bone except the articular cartilage and the epiphyseal plate

• Articular cartilage- covers bone ends and persist for life

• Epiphyseal plate- provide for longintudinal growth

Secondary ossification

• Epiphysis remains cartilage to grow• Secondary ossification appears in epiphysis and spongy

bone forms in all directions• Bones must also widen while lengthening • Osteoblast add bone tissue to face of diaphysis• Osteoclast break down bone from the inside to form the

medullary cavity• Appositional growth- process by which bone increases in

diameter– Growth controlled by hormones– Once the ossification centers meet lengthening is no longer

possible in that end of the bone• Ends during adolescence• Epiphyseal disk where ossification centers meet

Bone remodeling

• Continually remodeled based on two factors1. Calcium levels in the blood2. The pull of gravity and muscles on the skeleton

Blood calcium levels

• When blood levels drop:– Parathyroid glands release parathyroid hormone– Activates osteoclast that break down the bone

matrix and release calcium• When blood levels rise:– Calcium is deposited in the bone as hard calcium

salts

Bone remodeling

• Essential for bones to retain normal proportions and strength during long bone growth as body increases in size and weight

• Bones are thicker and form large projections to increase their strength in areas with bulky muscles

• Osteoblasts become trapped in bone matrix and become osteocytes

Bone remodeling summary

• PTH determines when bone is broken down in response to calcium

• The stresses of muscle pull and gravity acting on skeleton determine where bone matrix is broken down or formed so that the skeleton can remain strong and vital as possible

Bone fractures

• Treated by reduction– the realignment of the broken bone ends– Closed reduction– the bone ends are coaxed back into

their normal positions by the physician’s hands– Open reduction– surgery is performed and the bone

ends are secured together with pins or wires. It is then immobilized by a cast or traction to allow the healing process

– The healing time for a simple fracture is 6 to 8 weeks, but can be much longer for larger bones and the elderly

Repair of factures1. hematoma is formed

1. Blood vessels are ruptured and form blood filled swelling2. Bone cells deprived of nutrition die

2. The break is splinted by a fibrocartilage callus1. Growth of new capillaries and disposal of dead tissue by phagocytes2. Connective tissue form a mass of repair tissue called fibrocartilage

callus that acts as a splint for the broken bone3. The bony callus is formed

1. Osteoblast and osteoclasts gradually replace the fibrocartilage with spongy bone

4. Bone remodeling occurs1. Over the next weeks to months the bony callus is remodeled in

response to mechanical stresses so that it forms a strong permanent patch at the fracture site

Fetal skull• ¼ as long as total body• Thin and somewhat flexible and less easily fractured than

adult bones• Skeleton still unfinished especially in skull• Fontanels (“soft-spots”)- fibrous membranes connecting

cranial bones– 2 frontanels

• Anterior- larger and diamond shaped• Posterior – smaller triangular shaped

• Allow fetal skull to be compressed slightly during birth• Allow brain to grow during later pregnancy and early infancy • Gradually converted to bone

– Usually by age 2

Vertebral column• Axial support• Extends from skull to pelvis• 26 irregular bones• Surrounds and protects spinal cord• Before birth spine consist of 33 separate vertebrae• Single vertebrae are separated by invertebral

discs(fibrocartilage) that cushion and absorb shock while allowing flexibility – In young people high water content and are spongy and

compressible– Water amount decreases with age

• Discs and S-shaped structure of vertebral column work together to prevent shock to the head and make trunk flexible

Spinal curvatures

• Primary curvature– Present when born– Thoracic and sacral

• Secondary curvature– Develop later• Cervical – when baby begins raising head• Lumbar- when baby begins to walk

Female vs. male pelvis

• Female inlet is larger and more circular• Female pelvis as a whole is shallower, and the

bones are lighter and thinner• The female ilia flare more laterally• The female sacrum is shorter and less curved• The female ischial spines are shorter and farther

apart; thus the outlet is larger• The female pubic arch is more rounded because

the angle of the pubic arch is greater

Joints

• Functional junctions• Vary in structure and function• Classified according to degree of movement

that they make possible– Immovable, slightly immovable, freely movable

• Grouped by type of tissue– Fibrous, cartilaginous, or synovial

Fibrous

• Lie between bones that closely contact one another

• Thin layer of dense connective tissue joins the bones at joints

• No appreciable movement takes place at these joints– Ex. Sutures between flat bones in skull– Distal ends of fibula have limited movement

Cartilaginous joints

• Allow limited movement• Disk of fibrocartilage or hyaline cartilage

connect bones– Ex. Joints of this type separate the vertebrae of

the vertebral column– symphysis pubis and the first rib with the sternum

Synovial joints• Most joints of the skeletal system• Allow free movement• Articular ends of bones covered with hyaline

and capsule of dense connective tissue holds them together– Composed of outer layer of ligaments and inner

lining that is synovial membrane• Allows lubrication

• May also have bursae which aid in movement of tendons– Bursae– fluid filled sacs

Types of synovial joints• Ball and socket joint

– Consist of ball-shaped head that articulates with cup-shaped cavity of another bone

– Allows a wider range of motion• Allows movement in all planes as well as rotational movement around a central axis• Ex. Shoulders and hipd

• Condyloid joint– Oval shaped condyle fits into an elliptical plane of another bone– Permits a variety of movements in different planes but does not allow for

rotational movement– Between metacarpals and phalanges

• Gliding joint– Nearly flat or slightly curved – Sliding and twisting movements– Ex. Most within the wrist and ankle

Types of synovial joints• Hinge joint

– The convex surface of one bone fits into the concave surface of another bone

– Resembles the joint of a door-permits movement in only one plane– Ex. Elbow and phalanges

• Saddle joint– Between bones whose articulating surface have both concave and convex

regions– The surface of one bone fits the complimentary surface of the other bone– Variety of movements– Joint between the carpal and metacarpal of the thumb

• Pivot joint– The cylindrical surface of one bone rotates within a ring formed of bone

and ligament– Limited to rotation around a central axis– Joint between proximal ends of the radius and ulna