Ossification

Question 1

Types of ossification

Answer 1

Endochondral and intramembranous ossification

Question 2

Endochondral ossification

Answer 2

• Bones form from a hyaline cartilage template and form long bones

Stages:

1 - Cartilage cells die, periosteum produce osteoblasts that lay down a bony collar

2 - Cartilage matrix continues to deteriorate, increasing size of bone, chondrocytes in bone multiply to further increase size

3 - Blood vessels penetrate, carrying osteoprogenitor cells with them that can transform into osteoblasts, setting up primary ossification centre

4 - At brith, capillaries penetrate 2nd ossification centre, osteoprogenitor cells differentiate to form osteoblasts that can lay down osteoid

5 - Ossification complete and cartilage remains only in epiphyseal growth plate

Question 3

Intramembranous ossification

Answer 3

• Form bones from embryonic CT (meschenchymal cells)
• Form flat bones and often occurs in the skull

Stages:

1 - Osteoprogenitor cells differentiate to form osteoblasts, these lay down osteoid and form an ossification centre

2 - Mineralisation occurs producing a section of bone

3 - Islands join to form tuberculae with BV between

4 - Bone formation on outer and inner surfaces form compact bone

Question 4

Chondrocytes

Answer 4

Cells that have secreted cartilage and become embedded in it

Question 5

osteoprogenitor cells

Answer 5

Progenitor cells that originate from mesenchymal cells and can differentiate into osteoblasts or chondrocytes

Question 6

Growth of bone cells - Epiphyseal plate

Answer 6

1. Resting zone - Maintains growth plate by secreting PTHrp

Proliferative zone - Chondrocytes multiply

Hypertrophic zone - Chondrocytes increase in size and secret alkaline phosphate

Calcification zone - Chondrocytes die, leaving cavities that will later be invaded by osteoblasts

Ossification zone - Osteoprogenitor cells invade, differentiate into osteoblasts which help create calcified cartilage