Lecture exam 2-Skeletal system 2

What is the Zone of Resting Cartilage?

This zone anchors the growth plate to the epiphysis. The chondrocytes are resting (not dividing yet).

What is the Zone of Proliferation?

In this zone, chondrocytes divide rapidly and form stacks/columns of cells, adding new cartilage on the epiphyseal side.

What is the Zone of Hypertrophy?

Here, the chondrocytes mature and enlarge (hypertrophy), pushing the epiphysis and diaphysis farther apart, which lengthens the bone.

What is the Zone of Calcification?

The cartilage matrix becomes calcified, and the chondrocytes die, leaving behind a scaffold for bone to replace.

What is the Zone of Ossification (Ossified Bone)?

In this zone, the calcified cartilage is replaced by bone on the diaphyseal side — this is where new bone actually forms.

Rabbits Play Hard Cause Ossification

Resting → Proliferation → Hypertrophy → Calcification → Ossification

How does a long bone grow in length?

Cartilage is first made (proliferation), then enlarged (hypertrophy), then calcified, and finally replaced by bone (ossification).

What starts the growth process in the epiphyseal plate?

Chondrocytes divide and enlarge (replication + hypertrophy), causing the cartilage to grow.

How does this cartilage growth affect the bone?

Interstitial cartilage growth increases the length of the bone.

What happens next on the diaphyseal side of the plate?

The calcified cartilage is replaced by bone (ossification), adding new bone to the shaft.

Does the epiphyseal plate get thicker during growth?

No — the thickness stays the same because new cartilage is added at the same rate that old cartilage is replaced by bone.

What is appositional growth?

Appositional growth is growth in the width (thickness) of a bone.

Where does appositional growth occur?

It occurs beneath the periosteum by osteoblasts laying down new bone.

What happens first during appositional growth?

Osteoblasts form ridges of bone around a blood vessel, leaving a groove.

What happens when the ridges grow and meet?

The groove becomes a tunnel around the blood vessel. The periosteum on the groove is now called the endosteum of the tunnel.

What happens inside the new tunnel?

Osteoblasts from the endosteum build concentric lamellae, starting a new osteon

How is the osteon completed?

Additional concentric lamellae fill in the tunnel and finish forming the osteon (a structural unit of compact bone).

Why is bone width important?

Bone width is a major factor in bone strength — thicker bones = stronger bones.

Which cells make the bone wider?

Osteoblasts from the periosteum lay down new bone on the outside surface.

What happens to the medullary cavity during width growth?

Osteoclasts in the endosteum break down bone on the inside, which enlarges the medullary cavity.

Why do osteoclasts enlarge the medullary cavity?

This keeps the bone from becoming too heavy while still increasing strength.

What is the first step of bone repair?

Hematoma formation — blood forms a clot around the break.

What is a hematoma and why is it important?

A hematoma is a blood clot that stabilizes the fracture and brings immune cells + nutrients needed for healing.

What happens in the callus formation stage?

A soft callus made of fibers and cartilage forms first, then a hard callus made of woven bone forms around the break.

What is the difference between the internal and external callus?

• Internal callus: inside break → fibers + cartilage + woven bone

• External callus: outside break → mostly cartilage + woven bone

What happens in callus ossification?

The soft cartilage in the callus is replaced by woven bone.

What is the final stage of bone healing?

Bone remodeling — woven bone is remodeled into lamellar (mature) bone and the bone regains shape and strength.