• Support- framework that supports body and cradles its soft organs

• Protection- for delicate organs, heart, lungs, brain

• Movement- bones act as levers for muscles

• Mineral storage- calcium & phosphate

• Blood cell formation- hematopoiesis

The Skeletal System

Parts of the skeletal system·Bones (skeleton)·Joints·Cartilages·Ligaments (bone to bone)(tendon=bone tomuscle)

Divided into two divisions·Axial skeleton·Appendicular skeleton – limbs and girdle

Bones of the Human Body

The skeleton of an adult has 206 bones· Two basic types of bone tissue· Compact bone

· Homogeneous· Spongy bone

· Small needle-like pieces of bone· Many open spaces

275 bones12 weeks (6-9 inches long)

• Long Bones- metacarples, metatarsals, phalanges, humerus, ulna, radius, tibia, fibula

• Short Bones- carpals, tarsals

• Flat Bones- rib, scapula, skull, sternum

• Irregular Bones- vertebrae, some facial bones

• Sesamoid- patella

Classification of Bones

Long bones· Typically longer than wide· Have a shaft with heads at both ends· Contain mostly compact bone

•Examples: Femur, humerus

Classification of Bones

Short bones·Generally cube-shape·Contain mostly spongy bone

·Examples: Carpals, tarsals

Classification of Bones

Flat bones·Thin and flattened·Usually curved·Thin layers of compact bone around a layer of spongy bone

·Examples: Skull, ribs, sternum

Classification of Bones

Irregular bones·Irregular shape·Do not fit into other bone classification categories

·Example: Vertebrae and hip

Sesamoid bones

• Embedded within tendon where its passes over a joint

• Free surface covered with cartilage; the other part embedded within tendon; no periosteum

• Patella (within the tendon of m. quadriceps femoris)

Additional bones

• Mainly in the skull: ossa interfrontalis, coronalis, sagittalis, lambdoidalis, etc.

• Known also as ossa suturarum

Distalepiphysis

Proximal epiphysis

diaphysis

yellow marrow

epiphyseal line

periosteum

compact bone

spongy bone

Endosteum

hyaline cartilage

Sharpey’s fibers

Structures of a Long Bone

Periosteum·Outside covering ofthe diaphysis·Fibrous connectivetissue membrane

Sharpey’s fibers·Secure periosteum tounderlying bone

Arteries·Supply bone cellswith nutrients

Structures of a Long Bone

Articular cartilage· Covers the external surface of the epiphyses· Made of hyaline cartilage· Decreases friction at joint surfaces

Structures of a Long Bone

Medullary cavity·Cavity of the shaft·Contains yellow marrow (mostly fat) in adults·Contains red marrow(for blood cell formation) in infants

Components of Bone

• Cortical bone – Structural • Trabecular bone – Structural • Bone Marrow – Structural and RBC• Vessels – Nutritional and Innervation

Cortical Bone

• Osteon (Harvesian Canals)– Cylindrical tubes made of concentric lamellae– Central opening

• Blood vessels• Neural tissue• Lymphatic

• Periosteum– Fibrous tissue covering– Enables attachment of muscles and tendons

Cortical bone

• Lamellae– Concentric layers of

mineralized bone– Crisscross pattern at

90– Torsion and bending

strength• Osteoclasts

– Bone resorbing• Osteoblasts

– Bone forming

Trabecular Bone

• Cancellous or Spongy• Lattice structure• Pores filled with marrow• 20% Bone Mass• 80% Bone Surface

Trabecular Structure

• Plate and rod structure– Low loads - rod– Higher loads - plate

• Light yet spongy• Oriented in direction

of loads– “Wolff’s Law”

Bone Marrow

Consists of stroma, myeloid tissue, fat, lympatic tissuesRed marrow

Involved with the production of RBCConsists of haemopoetic tissueHighly vascularized

Yellow marrowNot as vascularized as red marrowLarge amount of fat cellsPercentage increases wrt red marrow with age (up

to20yrs)

Mechanisms of bone formation

• Membranous ossification how: direct differentiation of cells within mesenchymal condensations into bone forming cells (osteoblasts) flat bones of the skull, clavicle, periosteum

• Endrochondral ossification how: replacement of a cartilagenous template by bone endochondral bones:axial and appendicular skeleton, some bones in the skull

Membranous bone formation

Endochondral Ossification

cartilagecalcified cartilage

bone

epiphyseal plate

epiphyseal line

Endochondral Ossification

2o ossification center

Fetus: 1st 2 months

AdultChildhood

Just before birth

Types of bone cellsinvolved in bone homeostasis

How do cells look?

Origin of bone cells

hematoma callus bony callusbone remodeling

Diseases of the Skeletal System:

Osteoporosis- bone reabsorption outpaces bone deposit; bones become lighter and fracture easier

Factors: • age, gender (more in women)• estrogen and testosterone decrease• insufficient exercise (or too much)• diet poor in Ca++ and protein• abnormal vitamin D receptors• smoking

Osteoporosis

29 40 84 92

Rickets- vitamin D deficiency

Osteomalacia- soft bones, inadequate mineralization in bones, lack of vitamin D

Pagets Disease- spotty weakening in the bones, excessive and abnormal bone remodeling

Rheumatoid arthritis- autoimmune reaction

Diseases of the Skeletal System