Bone growth + adaptation

Question 1

What are the two different types of ossification?

Answer 1

1) Endochondral ossification = bone development from a hyaline cartilage template
2) Intramembranous ossification = Bone development within fibrous connective tissue.

Question 2

What is the name of the membrane that wraps around the hyaline cartilage?

Answer 2

The perichondrium.

Question 3

What happens during this stage of ossification?
1) Growth of blood vessels into the perichondrium

Answer 3

• The perichondrium turns into vascularised periosteum
• Osteoprogenitor cells differentiate into osteoblasts

Question 4

What happens during this stage of ossification?
2) Osteoblasts secrete osteoid

Answer 4

• Osteoid is an unmineralised bone matrix
• Osteoid ossifies to form a colar of bone around the diaphysis
• This prevents diffusion to chondrocytes in diaphysis

Question 5

What happens during this stage of ossification?
3) Chondrocytes enlarge (hypertrophy)

Answer 5

• Chondrocytes stop secreting collagen and proteoglycans and start secreting alkaline phosphatase
• This causes calcium phosphate crystals to precipitate
• This calcifies the cartilage matrix.

Question 6

What happens during this stage of ossification?
4) Even less diffusion to chondrocytes

Answer 6

• Chondrocytes die, leaving holes in calcified cartilage
• This means there is space for blood vessels to invade

Question 7

What happens during this stage of ossification?
5) Blood vessels bring osteoblasts

Answer 7

• Osteoblasts replace calcified cartilage with bone
• This establishes a primary ossification centre in the diaphysis

Question 8

What happens during this stage of ossification?
6) Progression of ossification to either end of the bone

Answer 8

• Cartilage at each end allows the bone to keep lengthening
• Osteoclasts arrive in the blood and absorb the centre of the bone to form the meddulary cavity

Question 9

What happens during this stage of ossification?
7) Establishment of secondary ossification centres

Answer 9

• The matrix is calcified, chondrocytes die, blood vessels bring osteoprogenitor cells.

Question 10

What happens during this stage of ossification?
8) Ossification centres expand at their edges

Answer 10

• Ossification is near enough complete but there is a layer of cartilage that remains between the primary and secondary ossification centres
• This is called the physis/ growth plates

Question 11

What are the different layers of cartilage that can be seen under a microscope?

Answer 11

Top - bottom:
1) Reserve/ resting cartilage (normal cartilage)
2) Proliferating zone (chondrocytes dividing rapidly by mitosis)
3) Hypertrophic zone (Alkaline phosphate secretion)
4) Calcification zone (cells dying)
5) Ossification zone (Vessels bring osteoblasts)

Question 12

What happens during the closure of growth plates?

Answer 12

The growth plate ossifies until the diaphysis and the epiphysis unite.

Question 13

How do bones grow in diameter/ in width?

Answer 13

Bones grow width ways via intramembranous ossification.

Question 14

What happens when bones grow in diameter?
Describe the process)

Answer 14

• Osteoblasts in the perisoteum deposit bone around the periosteal vessels.
• New layers of bone are then added onto the outer surface.

Question 15

What is the role of osteoclasts when bones grow in diameter via intramembranous ossification?

Answer 15

Osteoclasts breakdown and reabsorb bone in the centre to make sure that as the bone grows in circumference it doesn’t become to heavy.

Question 16

What are the two different types of bone?

Answer 16

Lamellar bone and woven bone

Question 17

Describe the structure of lamellar bone

Answer 17

• Has collagen fibres in organised, parallel layers
• Stronger and more dense type of bone.

Question 18

Describe the structure of woven bone

Answer 18

• Irregular arrangement of collagen fibres
• Weaker type of bone (usually replaced by lamellar bone later on)
• Predominantly seen during rapid growth and during healing of broken bones

Question 19

What minerals are involved in bone growth?

Answer 19

• Calcium and phosphate
• Calcium and phosphate are filtered in the kidney and stored in the bone marrow

Question 20

What hormones affect bone growth?

Answer 20

Calcitriol, calcitonin, and parathyroid hormone (PTH)

Question 21

What are the 3 functions of calcitriol?

Answer 21

• Increases calcium and phosphate absorption from the small intestine
• Enhances reabsorption of calcium and phosphate by the kidney
• Stimulates the release of calcium from bones

Question 22

When is PTH released?

Answer 22

PTH released when blood calcium levels fall

Question 23

How does PTH work to increase the concentration of calcium in the blood?

Answer 23

• Increases reabsorption of Ca2+ and phosphate from the bone
• Increases reabsorption of Ca2+ from the kidney
• Reduces the absorption of phosphate from the kidney
• Increases the conversion of vitamin D into calcitriol

Question 24

When is calcitonin released?

Answer 24

Calcitonin is released when blood calcium levels are high

Question 25

How does calcitonin act to decrease the level of calcium in the blood?

Answer 25

• Inhibits the reabsorption of Ca2+ from the bone
• Inhibits the reabsorption of Ca2+ and phosphate from the kidney

Question 26

What hormones can affect bone growth?

Answer 26

• Growth hormone
• Sex hormones (oestrogen and testosterone affect the closure of the physis)
• Glucocorticoids (stress hormones inhibit osteoblasts so inhibit bone loss)
• Thyroid hormone (increases reabsorption of bone)

Question 27

Why does bone remodeling need to occur?

Answer 27

• To meet mechanical demands placed on it
• To maintain the tissue
• For mineral homeostasis