Bone Formation, Growth and Remodeling

Bone formation

• The Skeleton is formed from two of the strongest and most supportive tissues in the body:

Cartilage and bone• In embryo: the skeleton is primarily make of

hyaline cartilage• In young child: most of the cartilage has been

replaced by bone

Bone formation• Except for flat bones, which form on fibrous

membranes, most bones develop using hyaline cartilage structure as their “models”

• Ossification: Bone formationStep 1. The hyaline cartilage model is completely covered with bone matrix by bone-forming cells called osteoblastsStep 2. The enclosed hyaline cartilage model is digested away, opening up a medullary cavity within the newly formed bone

Long bone growth• The epiphyseal plates provide for

longitudinal growth of the long bones during childhood:

“New” Cartilage is formed continuously on the external face of the articular cartilage and on the epiphyseal plate surface that is farther away from the medullary cavity

“Old” cartilage on the internal face of the articular cartilage and medullary cavity is broken down and replaced by bony matrix

Bone remodeling

• Bones are remodeled continually in response to changes in two factors:

1. Calcium levels in the blood (determines when to break down or form bones)

2. The pull of gravity and muscles on the skeleton (determines where to break down or form bones)

The Skeletal System

• Forming / Molding Bone:–Osteoblasts:• Cells that rebuild your

bones• Create new bone cells• Takes free floating Calcium

and converts it into bone

The Skeletal System

• Forming / Molding Bone:–Osteoclasts:• Cells that release calcium

from the bone into the blood system• Increase calcium in the

blood• Parathyroid Hormone

(PTH) activates Osteoclasts

Simple

• Bone breaks cleanly, but does not penetrate the skin

• Sometimes called a “closed fracture”

Compound

• Broken ends of the bone protrude through soft tissues and the skin

• Sometimes called an “open fracture”

Communited

• Bone breaks into many fragments

• Particularly common in the aged, whose bone are more brittle

Spiral

• Ragged break occurs when excessive twisting forces are applied to a bone

• Common sports fracture

Greenstick

• Bone breaks incompletely, much in the way a green twig breaks

Compression/Depressed/Impacted

• Compression: Bone is crushed

• Depressed: Broken bone portion is pressed inward

• Impacted: Broken bone ends are forced into each other

Stages in the healing of a bone fracture

1. Blood vessels rupture. A blood filled swelling called a hematoma forms. Bone deprived of nutrition die.

2. Break is splinted. Growth of new capillaries. Disposal of dead tissue.

3. Fibrocartilage is replaced by spongy bone called a bony callus.

4. Bony callus is remodeled over the next few months in response to stress.