Development and growth of bone

Question 1

What are some of the functions of bone?

Answer 1

• Support of the body shape
• Systems of levers for muscle action
• Protection of internal organs
• Site of blood cell formation
• Mineral storage pool

Question 2

What are the two mechanical properties of bone and what is responsible for these properties?

Answer 2

Cable-like flexibility – and resistance to tension because the framework is collagen and other bone proteins (osteoid) Pillar-like stiffness – and resistance to compression conferred by impregnation of collagen with crystalline mineral (hydroxyapatite – a complex calcium hydroxyphosphate)

Question 3

What are the two main types of bone tissue?

Answer 3

Woven (immature) Lamellar (mature)

Question 4

When is woven bone found in adults?

Answer 4

It is only found in repairing fractures

Question 5

What is the technical term for the shaft of a bone?

Answer 5

Diaphysis

Question 6

What is the technical term for the head of a bone?

Answer 6

Epiphysis

Question 7

What type of bone is found around the outside of the bone?

Answer 7

Compact lamella bone (cortical bone)

Question 8

What is the term given to the cavity within the bone?

Answer 8

Medullary cavity (contains bone marrow)

Question 9

What type of bone is found within this cavity?

Answer 9

Trabecular/spongy bone (this is still lamellar bone)

Question 10

What units is lamellar bone in the compact bone arranged into?

Answer 10

Osteons/Haversian Systems They have a central canal containing vessels and around the vessels you have concentric layers of bone maintained by concentrically arranged osteocytes

Question 11

In what planes are these units arranged?

Answer 11

They are usually organised along pressure planes

Question 12

What provides the main blood supply to the bone marrow?

Answer 12

Nutrient artery (passing through the nutrient foramen)

Question 13

What are the two layers of the periosteum?

Answer 13

Fibrous Cellular

Question 14

What are the main roles of the periosteum?

Answer 14

Bone growth and repair It is vascular It has a good sensory nerve supply

Question 15

What do osteogenic cells give rise to?

Answer 15

Osteogenic cells  Osteoblasts  Osteocytes

Question 16

What lineage do osteoclasts come from?

Answer 16

They are derived from the immune cell lineage – they are modified macrophages

Question 17

At what point in fetal life does the skeleton start to develop?

Answer 17

6 weeks

Question 18

What are the two types of ossification?

Answer 18

Intramembranous Endochondral

Question 19

Describe intramembranous ossification.

Answer 19

Occurs within existing vascular connective tissue Bone matrix (ostein) is deposited around the collagen This mineralises to form woven bone It then remodels to form lamellar bone

Question 20

Describe endochondral ossification.

Answer 20

Occurs within existing fetal cartilage models Cartilage calcifies and the chondrocytes die Periosteal osteoclasts cut channels for sprouting vessels Osteoblasts enter with the vessels to build bone around them

Question 21

List some bones in which intramembranous ossification takes place.

Answer 21

Skull bones Jaw bones Part of the clavicle

Question 22

At what point do the epiphyses ossify in a child’s wrist?

Answer 22

2 years

Question 23

Until what point do epiphyseal plates remain cartilaginous?

Answer 23

Until you stop growing – then the epiphyseal plates will calcify

Question 24

What is the mechanism of appositional growth of bones?

Answer 24

Apposition – addition to exterior at periosteum Osteoblasts and osteoclasts create ridges and grooves on bone surface Blood vessels align in grooves Osteoblasts build new osteons round vessels Osteoclasts remove bone from endosteal surface

Question 25

What is the problem and solution of long bone growth?

Answer 25

Most long bones must support large forces while growing

These would disrupt terminal appositional growth

Solution?

Shaft ossifies first, followed by epiphyses

Growth continues by ossification at growing cartilage plate between them

Growth cessation when cartilage growth ceases and plate is over-run by ossification

Question 26

How can bone be remodelled?

Answer 26

Blood Ca2+ levels high:

Calcitonin released by parafollicular thyroid cells;

breakdown of bone matrix by osteoclasts inhibited;

uptake of Ca2+ into the bone matrix is promoted.

Blood Ca2+ levels low:

Parathyroid Hormone (PTH) released by chief cells of parathyroid gland;

Bind to receptors on osteoblasts (osteoclasts don’t have PTH receptors)

Osteoblasts release osteoclasts activating factor

osteoclast bone resorption activity promoted - more calcium released into the bloodstream

increases Ca2+ re-absorption by the kidneys.

Question 27

How is a fracture healed?

Answer 27

1. Bleeding is an important part of the process.
2. If the bone is stable this heamatoma will become infiltrated by fibrous matrix and invaded by cartilage/bone progenitors and becomes cartilaginous to form a callus - a connective tissue structure
3. callus forms spongy bone
4. spongy bone remodelling

Woven bone forms first then becomes lamella bone