1.04 - Bone & Cartilage

Question 1

Describe Bone

Answer 1

A specialised form of connective tissue

Cells in an abundant extracellular matrix

Question 2

Describe the Extracellular Matrix of Bone

Answer 2

The Extracellular matrix is made up of fibres (Type I collage of tensile strength) in ground substance (called osteoid) that becomes mineralised by the deposition of calcium hydroxyapatite (hardness)

Question 3

What are the types of cells involved in bone formation and regulation?

Answer 3

Osteoprogenitor cells
Osteoblasts
Osteocytes
Osteoclasts

Question 4

Describe Osteoprogenitor cells

Answer 4

Precursor cells that self replicate or differentiate into bone-forming cells

Question 5

Describe Osteoblasts

Answer 5

Bone forming cells that deposit osteoid and control subsequent mineralisation

Question 6

Describe Osteocytes

Answer 6

Modified osteoblasts that become surrounded by newly formed bone - regulate homeostasis

Question 7

Describe Osteoclasts

Answer 7

Large multinucleate phagocytic cells that resorb bone and break down calcified matrices
Originate from haemopoietic stem cells

Question 8

Describe the process of ECM calcification in bone

Answer 8

The matrix comprises: 70% inorganic salts (calcium and phosphate) and 30% organic matrix (90% Type I collagen)
Calcification begins a few days after the deposition of osteoid by the osteoblasts
Calcium phosphate precipitates on the collagen molecules.
About 75% of the calcium hydroxyapatite is deposited in the first few days but complete calcification may take several months.

Question 9

What are the two types of bone and their respective subtypes?

Answer 9

Woven (or immature bone)
Lamellar (orderly strata of bone matrix)
- Compact Bone
- Cancellous Bone (Trabecular & spongy)

Question 10

Describe Woven or immature bone

Answer 10

Randomly arranged collagen fibres in the osteoid
Produced when osteoid is produced rapidly
Eventually remodelled to form Lamellar Bone

Question 11

Under what conditions is Woven (or immature) bone produced?

Answer 11

When osteoid is produced rapidly, such as in foetal bone, healing of fractures and in bone diseases such as Paget’s disease

Question 12

Describe Lamellar bone

Answer 12

Mature bone
Regular parallel bands of collagen fibres in the osteoid
Stronger and more resilient than woven bone

Question 13

What structures is compact bone made up of?

Answer 13

Lamellae
Haversian canals
Haversian systems (osteons)
Lacunae
Canaliuli

Question 14

What is Lamellae?

Answer 14

Concentric layers of mineralised bone matrix

Question 15

What are Haversian Canals?

Answer 15

Neurovascular channels surrounded by lamellae. Provide nutrients to the tissue.
Canals run parallel to the surface along the axis of the bone

Question 16

What are Haversian System (or osteons)?

Answer 16

Lamellae + Haversian canals

Question 17

What are Lacunae?

Answer 17

Small hollows in the matrix in which osteoblasts become encased

Question 18

What are canaliculi?

Answer 18

Small channels within the bone matrix formed by the thin processes of osteocytes that extend from the lacunae.
Canaliculi rising from one lacuna man anastomose with those of other lacunae and then eventually with larger, vessel containing canals within the bone

Question 19

What is the function of Canaliculi?

Answer 19

Provide the means for the osteocytes to communicate with each other and to exchange substances by diffusion

Question 20

Describe Cancellous bone

Answer 20

Also called Spongy or Trabecular bone
Mature bone with irregular lamellae
Bony trabeculae separated by interconnecting spaces containing bone marrow
Lamellae in cancellous bone do not form aversion systems

Question 21

List the steps in bone remodelling

Answer 21

Damage repaired
Ion homeostasis maintained
Bone reinforced for increased stress
Bone growth

Question 22

What are some important factors influencing bone remodelling?

Answer 22

Nutrition
Hormones (Vitamin D, Parathyroid Hormone - PTH)
Physical Activity
Age
Growth Factors

Question 23

What are the two types of bone development and growth, and where do they occur?

Answer 23

Intramembranous ossification: direct replacement of primitive mesenchymal tissue by bone (vault of the skull, maxilla and most of the mandible)
Endochondral ossification: Cartilage bone is replaced by normal bone (long bones, vertebrae, pelvis)

Question 24

What are the major functions of bone?

Answer 24

Largely structural (movement, protective)
Source of calcium and phosphate
Haemopoiesis (marrow)
Gives the skeleton the necessary rigidity to function as an attachment and lever for muscles and supports the body against gravity

Question 25

Describe Cartilage

Answer 25

Few cells with abundant extracellular matrix (ground substance)
Cells include: chondroblasts, chondrocytes
Does not contain vessels or nerves. Depend on the diffusion of nutrients which limits the thickness of cartilage
Surrounded by a layer of dense connective tissue, the perichondrium
Contains about 75% water

Question 26

What are chondroblasts and chondrocytes?

Answer 26

Chondroblasts are precursor cells in cartilage

Chondrocytes are mature cells in cartilage

Question 27

Describe the ECM in Cartilage

Answer 27

A gel like matrix with firers (collagen and elastin fibres) in ground substance (sulphated proteoglycans and hyaluronic acid)

Question 28

What are the functions of cartilage?

Answer 28

Provides semi-rigid and bone rigid structural support
Supports soft tissues (ear, nose, epiglottis, trachea)
Shock absorption in joints
Important in development, growth and healing of bones

Question 29

What are the types of Cartilage?

Answer 29

Hyaline cartilage (glassy, blue)
Fibrocartilage (white)
Elastic cartilage (yellow)

Question 30

Describe Hyaline Cartilage

Answer 30

Most common type
Fibres - Type II Collagen
Ground Substance - Sulphated GAGs
Found in nasal septum, larynx, tracheal rings, most articular surfaces and sternal ends of ribs
Forms the precursor of bone

Question 31

Describe Articular Cartilage

Answer 31

A specialised form of hyaline cartilage
Transforms the articulating ends of bones into well lubricated, wear-proff, slightly compressible surfaces
Partly vascularised
Components (including collagen fibres) are arranged in such a way as to maximise biomechanics functions
Composition:
 - 70% water
 - 15% Proteoglycans
 - 15% Collagen (Type II)
 - 1% Chondrocytes

Question 32

Describe Fibrocartilage

Answer 32

Fibres: Collagen
Ground Substance: Sulphated GAGs
Found in intervertebral discs, some articular cartilage, pubic symphysis
Appearance: alternating layers of hyaline cartilage and thick layers of dense collagen fibres orientated in one direction

Question 33

Describe Elastic Cartilage

Answer 33

Fibres: Elastic and Collagen
Ground Substance: Sulphated GAGs
Found in external ear, auditory canals, epiglottis, part of laryngeal cartilages and walls of eustachian tubes

Question 34

Describe the Formation of Cartilage

Answer 34

Precursor cells become round and form densely packed cellular masses –> centres of chondrification
The cartilage forming cells, chondroblasts, begin to secrete the components of the ECM of cartilage
As the amount of matrix increases, chondroblasts become separated from each other
Chondroblasts become isolated in small cavities within the matrix –> Lacunae
Chondroblasts differentiate into mature cartilage cells –> chondrocytes

Question 35

What are the two mechanisms by which cartilage can grow?

Answer 35

Interstitial Growth

Appositional Growth

Question 36

Describe Interstitial Growth

Answer 36

Chondroblasts within existing cartilage divide and form small groups of cells, which produce matrix to become separated from each other by a thin partition of matrix
Interstitial growth occurs mainly in immature cartilage

Question 37

Describe Appositional Growth

Answer 37

Mesenchymal cells surrounding cartilage in the deep part of perichondrium (or the chondrogenic layer) differentiate in to Chondroblasts. Appositional growth also occurs in immature and mature cartilage.