Cellular structure of bone

Question 1

Hydroxyapatite

Answer 1

The principle mineral component of bone, composed of calcium and phosphate (Ca10(PO4)6(OH)2). Responsible for the hardness of bone.

Question 2

Intramembranous ossification

Answer 2

The process by which flat bones develop as osteoblasts differentiate directly from mesenchyme precursors within mesenchymal connective tissue.

Question 3

Endochondral ossification

Answer 3

The process by which long bones develop via the conversion of a cartilage model into bone

Question 4

Chondrocyte

Answer 4

Mature cartilage cell. Produces cartilage matrix. Undergoes hypertrophic differentiation in the growth plate to allow linear growth

Question 5

Growth plate

Answer 5

Specialised cartilage structure found at the metaphysis of long bones between the ossification centers. Allows linear growth.

Question 6

Osteocyte

Answer 6

Terminally differentiated osteoblasts encased within mature bone. They are connected via a network of dendritic processes that allow them to have a mechanosensory role. Thought to be the master regulators of bone remodeling.

• Embedded in lacunae in mature 
bone
• Connected via processes 
through canalicular channels
• Form a mechanosensory 
network throughout bone

Question 7

Osteoblast

Answer 7

Bone cells responsible for secreting osteoid which is then mineralized to become mature bone.

• Secretes osteoid; the organic 
component of new bone
• Osteoid is mineralised over 
time to become mature bone
• Some osteoblasts are 
embedded in the new bone and 
differentiate into osteocytes
• Secretes osteoid; the organic 
component of new bone
• Osteoid is mineralised over 
time to become mature bone
• Some osteoblasts are 
embedded in the new bone and 
differentiate into osteocytes

Question 8

Osteoclast

Answer 8

Large, multi-nucleate cells responsible for resorption of bone.

• Giant multinuclear cells formed from 
the fusion of macrophages
• They seal off a portion of bone 
beneath them
• They secrete acids and enzymes to 
‘resorb’ the sealed off bone

Question 9

Osteoid

Answer 9

The organic matrix of bone, consisting mainly of collagen and other protein. Secreted by osteoblasts, which line the surface of newly-formed bone.

Question 10

Bone remodeling

Answer 10

The skeleton is continually renewed throughout life via the bone remodeling cycle, a process whereby old or damaged bone is removed through osteoclastic bone resorption, and an equal amount of new bone is formed by the osteoblasts

Question 11

What is bone?

Answer 11

• INORGANIC - 65%
– calcium hydroxyapatite (Ca10(PO4)6(OH)2)
– is storehouse for 99% of Calcium in the body
– 85% of the Phosphorus, 65% Sodium,
Magnesium

• ORGANIC - 35%
– bone cells and protein matrix

Question 12

Bone types and classifications

Answer 12

• Anatomical bones
–  Flat, long, short/cuboid, irregular, sesamoid
• Macroscopic structure
– trabecular/cancellous/spongy
– cortical/compact
• Microscopic structure
– Woven bone  (immature) 
– Lamellar bone (mature)
• Anatomical bones
–  Flat, long, short/cuboid, irregular, sesamoid
• Macroscopic structure
– trabecular/cancellous/spongy
– cortical/compact
• Microscopic structure
– Woven bone  (immature) 
– Lamellar bone (mature)

Question 13

Bone types and classifications cortical and trabecular

Answer 13

CORTICAL
• long bones
• 80% of skeleton
• appendicular
• 80-90% calcified
• mainly structural, 
mechanical, and 
protective

TRABECULAR
• vertebrae & pelvis
• 20% of skeleton
• axial
• 15-25% calcified
• mainly metabolic
• large surface area

Question 14

The Geography of a Bone

Answer 14

diagram

Question 15

Bone Development

Answer 15

• Bones develop throughout growth and into adulthood
– Growth plate fusion and ossification completes development
– Clavicles growth plates fuse at ~20 years old
• Intramembranous ossification
– Direct differentiation of osteoblasts from connective tissue
– Flat bones
• Endochondral ossification
– Bones form from a cartilage model
– Long bones

Question 16

Endochondral ossification

Answer 16

diagrams

Question 17

what does the growth plate do?

Answer 17

Growth plate allows rapid linear growth

diagrams

Question 18

The specialised bone cells

Answer 18

Osteocytes - mechanosensory network
embedded in mature bone

Osteoclasts - multinuclear cells that
resorb/remove bone

Osteoblasts - produce osteoid to form
new bone

Question 19

why do we need a remodelling cycle?

Answer 19

• The skeleton is not a fixed or stagnant organ, it is dynamically
regulated to maintain health
• Small portions of bone are constantly being removed and
replaced
– Your whole skeleton has been replaced after 7 years
• This is done by the specialised bone cells via the bone
remodelling cycle

Question 20

The Bone Remodelling Cycle

Answer 20

diagrams

Question 21

Control of bone remodelling

Answer 21

Endocrine
• Estrogen, Thyroid hormone, PTH

Paracrine
• RANKL, Wnt signalling

Question 22

Osteoclast differentiation and RANKL

Answer 22

• RANK receptor – Activation required 
for osteoclast differentiation and 
survival.
• RANK Ligand – produced by 
osteocytes and osteoblasts
• OPG (osteoprotegerin) – decoy 
receptor for RANKL also produced by 
osteocytes and osteoblasts.

Question 23

Wnt signalling and bone formation

Answer 23

• Highly complex pathway 
involved in many different 
organ systems
• Stimulates osteoblast 
differentiation
• Inhibited by Sclerostin and 
Dkk-1

Question 24

Osteocytes as key regulators of remodelling

Answer 24

diagram

Question 25

Osteocytes as key regulators of remodelling

Answer 25

diagram

Question 26

Bone disorders - Osteopetrosis

Answer 26

• LRP5 activating mutations
• van Buchem’s and SOST
• Defined as having a bone mass greater than 2.5 SD below average
peak bone mass
• Estimated that it will effect 1:2 women and 1:5 men.
– Causes 500,000 low impact bone fractures every year in the UK
• Can be primary (menopause, aging), or secondary (drugs, disease,
lifestyle)
diagrams