Learning Outline

Skeletal System

A & P 1

The specific structures of the gross anatomy of the skeleton, such as names of specific bones and bone features, are covered in the lab course—not the lecture course. They will appear on the lecture course tests only insofar as they serve as examples of the concepts discussed here. If you want an optional overview of the bones of the skeleton click here.

Functions of the skeletal system

Support

Framework of body, holding other organs in place

Movement

Attachment sites for skeletal muscles

Movable joints; leverage for movement

Protection

Hard covering of thoracic organs, brain, spinal cord, other soft structures

Protection as in bodily defense against injury

Mineral and fat storage

Calcium & phosphorous salts stored in bone tissue

Yellow fat (yellow bone marrow) stored in bone cavities

Blood cell production

Hematopoiesis (hemato = "blood" poiesis = "making")

Red bone marrow is blood-forming tissue inside some bones

Human skeletal system (female left, male right)
click either image to see expanded views
from Bernhard Ungerer used by permission

 

Bone organs (gross structure)

Bone number

206 bones is standard / typical (but nearly everyone has more/fewer)

Regions of skeleton

Axial skeleton—forms "axis" of body

• Includes bones of skull, vertebral column, thorax
  

Appendicular skeleton—forms appendages (arms, legs)

• Includes bones of shoulder/arm/hand and hip/leg/foot
  

Bone categories

Long bones

• Examples: radius, ulna, tibia, fibula, humerus, femur, metacarpal bones, metatarsal bones
  

Short bones

• Examples: carpal bones, tarsal bones
  

Flat bones

• Examples: sternum, ribs, scapula
  

Irregular bones

• Examples: vertebrae, pelvic bones, skull bones of face

Sesamoid bones

• Example: patella
  

Long bone structure

Epiphyses (sing. epiphysis) are end regions

• Usually have spongy bone on inside, compact bone on outside
  

Diaphysis is middle "shaft" region (pl. diaphyses)

• Usually compact bone on outside, cavity on inside
  
  
• Medullary cavity contains yellow marrow
  
  
  • Lined with thin membrane called endosteum
    

Bone covered with periosteum (dense fibrous sheet) and articular (joint) cartilage

Long bone structure

Flat bone structure

Short and irregular bones have a similar structure to flat bones

Internal and external table

• Thick sheet of compact bone that forms the hard outer shell of bone
  
  
• Covered with periosteum

Diploe (diploë)

• Cancellous (spongy) bone filled with red marrow forming inner region of bone

 

Bone tissue (microscopic structure)

Compact bone

Hard bone forming outer shell of all bone organs

Bone matrix is collagen fibers with apatite mineral (calcium/phosphorus) crystals encrusted on the fibers

• Always starts out as fibrous membrane or cartilage, then turns to bone
  
  
  • Endochondral ossification — cartilage becomes bone
    
    
    • Epiphyseal plate is cartilage between epiphysis and diaphysis as they grow together
      
      
  • Intramembranous ossification — membrane becomes bone
    
    
    • Fontanels are "soft spots" in infant skull where ossification is not complete
      
      
      • Allow for deformation of skull during childbirth
        

Osteon (haversian system) is a tapered, cylindrical unit that makes up compact bone tissue

• Central canal (haversian or osteonal canal) surrounded by concentric lamellae (layers) of hard bone matrix
  
  
  • Transverse (Volkmann) canals connect central canals side to side, forming blood networks
    
    
• Osteoblasts ("bone makers") make matrix, then are trapped and now "retired" and called osteocytes ("bone cells")
  
  
  • Lacuna (pl. lacunae) are the spaces in which osteocytes are found
    
    
  • Osteocytes may "come out of retirement" during remodeling
    
    
• Canaliculi ("tiny canals") connect the lacunae (each housing one osteocyte) to each other and to the central canal
  
  
• Lamellae (sing. lamella)
  
  
  • Concentric lamellae form osteons
    
    
  • Intersititial lamellae, found between osteons, are left over from preliminary osteons and woven bone
    
    
  • Circumferential lamellae form continuous boundaries at the outer and inner surfaces of the compact bone (around the inner and outer circumference)

Around 21 million osteons in adult skeleton Each osteon is 100-400 µm in diameter (1 inch = 25,400 µm) A medium osteon has about 30 lamellae (each about 3 µm) The central canal is around 50 µm in diameter

Cancellous (spongy) bone

Made up of irregular, fractal trabeculae of hard bone surrounded by red bone marrow (liquid)

• Cancellous bone is also called trabecular bone
  

Red marrow is myeloid tissue (myelo = "marrow")

• Hematopoiesis

Compact Bone (osteon highlighted in green)
Click image to enlarge it

Osteon (closer view)
Click image to enlarge it

Remodeling

Bone is constantly being torn down and built up—this is remodeling

Role of bone cells

• Osteoblasts make new bone matrix (using Ca⁺⁺ from blood)
  
  
• Osteoclasts ("bone breakers") dissolve bone matrix (releasing Ca⁺⁺ to blood)
  

Role of hormones

• Calcitonin (CT; from thyroid) increases Ca++ storage (out of blood)
  
  
• Parathyroid hormone (PTH) gets Ca++ out of storage (into blood)
  

Age effects

• More bone is made than is lost until age 25 (usually rapid until puberty, then slows)
  
  
• About as much bone is made as is lost 25-50 (can vary)
  
  
• More bone is lost than is made 50-120 (usually slight)
  

Stress effects

• Mechanical stress can affect bones (fractures, pressure, exercise, etc.)
  
  
• Stress increases bone density (therefore, gravity and exercise increase bone density)
  
  
• Usually accounts for changes in bone density in old age

Skeletal variations

Sex

• Male — heavier, larger; more defined markings; deep pelvis
  
  
• Female — lighter, smaller; less defined markings; shallow/broad pelvis
  

Age (see section above)

Environment

• Toxins
  
  
• Stress (including fractures)—see above

Required—please read the A&P Connect article Skeletal Variations online at the Evolve website

A good place to study normal variations in human skeletons is in the underground ossuary (place for bones) in Paris—also known as the "Paris catacombs." Skeletal remains of thousands of people buried in 18th century cemeteries were moved to abandoned chalk quarries under the streets of Paris and can be visited today. Click each photo to enlarge it. Curious about the Paris catacombs? Click here.

One frontal bone or two? Figure A is a photograph from the Paris catacomb showing a skull with a sagittal suture separating the frontal part of the skull into a left frontal bone and right frontal bone. Figure B is a photograph of a nearby skull that is "standard" in that it has no sagittal suture dividing the frontal bone—it has a single frontal bone. It is very likely that neither individual was aware of these facts while they were alive. This is an example of how the human skeleton can vary from one person to another.

Joints

See Tables 9-1, 9-2 and 9-3 in textbook

Definition

Joint is where two or more bones come together (join)

Arthro = joint

• Arthritis = joint inflammation (many causes / types)
  

Ligaments

• Fibrous structures that connect one bone to another
  

Structural categories

Fibrous — bones are joined by fibrous tissue

Cartilaginous — bones are joined by cartilage

Synovial — bones are joined at a fluid-filled space lined with synovial membrane

Functional categories

Immovable — bones don't move relative to one another

• Synarthroses
  

Slightly movable — bones can move, but not much

• Amphiarthroses
  

Freely movable — bones have significant movement

• Diarthroses
  

Fibrous

Fibrous joints are synarthrotic joints

Syndesmoses

• Fibrous bands called ligaments connect the bones
  
  
• Example: joints between the distal radius and ulna
  

Sutures

• Fibrous tissue connects flat bones that fit together like jigsaw puzzle pieces
  
  
• Example: joints between flat bones of skull
  

Gomphoses

• Fibrous tissue connects root of tooth to socket of jaw bone
  

Cartilaginous

Cartilaginous joints are amphiarthrotic)

Synchondroses

• Hyaline cartilage between bones
  
  
• Example: first rib-sternum (costosternal) joint
  

Symphyses

• Fibrocartilage between bones
  
  
• Examples:
  
  • between BODIES of vertebra
    
  • pubic symphysis
    

Synovial

Synovial joints are diarthrotic

Uniaxial — single axis of movement

• Hinge
  
  
• Pivot
  

Biaxial — two axes of movement

• Saddle
  
  
• Condyloid
  

Multiaxial — multiple axes of movement

• Ball and socket
  
  
• Gliding
  

Bursa

 

Types of movements

Angular movements

Angular movements increase or decrease the angle of a joint

Flexion — decreases angle of joint

• Ankle flexion — special case
  
  
  • Plantar flexion — moves toes inferiorly
    
    
  • Dorsiflexion — moves toes superiorly
    

Extension — increases angle of joint

• Hyperextension — goes beyond anatomical position
  

Abduction — moves part away from midline of body or region

Adduction — moves toward from midline of body or region

Circular movements

Circular movements move body parts in a circle

Rotation — pivots part on its axis

Circumduction — moves distal end of part in a circle-like path

Supination — twisting of limb (e.g. arm or leg) away from median

Pronation — twisting of limb toward median

Gliding movement

Special movements

Ankle movements

• Inversion — move sole toward median
  
  
• Eversion — move sole away from median
  

Forward and back movements

• Protraction — move part anteriorly
  
  
• Retraction — move part posteriorly
  

Up and down movements

• Elevation — move part superiorly
  
  
• Depression — move part inferiorly