Histological Organization of Cartilage and Bone

Question 1

What is cartilage?

Answer 1

Specialized connective tissue that is avascular and has a highly hydrated matrix (hence nutrition is by diffusion).

Question 2

What is perichondrium?

Answer 2

A layer of dense connective tissue that is firmly attached to and surrounds elastic cartilage and all types of hyaline cartilage except articular type of hyaline cartilage. It is absent in fibrocartilage.

It contains blood vessels that supply nutrient molecules which eventually diffuse into the cartilage.

Question 3

State the locations of elastic, fibrocartilage and hyaline cartilages. (3 each)

Answer 3

Elastic cartilage: pinna of the ear, epiglottis, external acoustic meatus, Eustachian tube

Fibrocartilage: intervertebral disc, pubic symphysis, intra-articular discs (knee joint menisci)

Hyaline cartilage: costal cartilage, articular cartilage, epiphyseal growth plates

Question 4

List the cell types in cartilage.

Answer 4

1. Chondrocytes - located in lacunae and secrete and maintain the cartilage ECM
2. Chondroblast - located just under the perichondrium and transforms into a chondrocyte
3. Chondroclast - found where calcified cartilage is degraded
4. NB: Fibrocartilage contains fibroblasts and chondrocytes.

Question 5

The extracellular matrix in cartilage has ground substance and fibres. Collagen __a___ polymerizes into fibres and is found in fibrocartilage. The other collagens exist as _____b______. _______c______ fibres are found in elastic cartilage. (Hint: the collagen type here is not found in any other cartilage.)

Answer 5

a. I
b. fibrils
c. Elastic

Question 6

A. List glycosaminoglycans generally found in cartilage.
B. What is a proteoglycan?
C. ________________ usually does not form PGs because it is huge hence exists freely.

Answer 6

A. Chondroitin 4-sulphate, chondroitin 6-sulphate, hyaluronic acid, glucosamine, keratan sulphate

B. It is composed of GAGs covalently attached to core protein either as a single GAG e.g. decorin or multiple as in aggrecan

C. Hyaluronic acid

Further notes:
~ Glycosaminoglycans (GAGs), also known as mucopolysaccharides, are long, linear polysaccharides consisting of repeating disaccharide units (i.e., two-sugar units). Each GAG unit consists of a uronic sugar and an amino sugar (except for keratan sulfate, which contains galactose instead of uronic sugar). GAGs are highly polar and attract water, making them useful as lubricants and shock absorbers in the body.
Other than contributing to the structure and organization of tissues, GAGs also participate in cell signaling, cell adhesion, and growth factor regulation. In cartilage, GAGs maintain joint health by retaining water and providing cushioning.

Question 7

List 2 proteoglycans in cartilage that belong to the Lectican family.

Answer 7

Aggrecan, Versican

Further notes:
Lecticans, also known as hyalectans, are a family of proteoglycans that play essential roles in the extracellular matrix. Other examples include brevican and neurocan.

Question 8

1. In cartilage, hyaluronic acid is attached to PG to form a _____________.
2. What’s the anatomical basis of cartilage hydration?

Answer 8

1. proteoglycan aggregate
2. Presence of proteoglycan aggregates that attract and retain water (imbibition of water)

Further notes:
In cartilage, proteoglycan aggregates are formed when multiple proteoglycan molecules attach to a single long hyaluronic acid chain. Each proteoglycan molecule consists of a core protein to which several glycosaminoglycan chains, such as chondroitin sulphate and keratan sulphate are attached.

These aggregates create a highly hydrated gel-like matrix in cartilage, which allows it to resist compression and act as a shock absorber in joints. The negative charges on the GAG chains attrat water molecules, contributing to the resilience of cartilage under mechanical stress. This structure is crucial for the proper function of cartilage in distributing loads across joints and providing smooth articulation between bones.

Question 9

What makes the cartilage matrix special?

Answer 9

1. It has no blood vessels to deliver oxygen and nutrients
2. It is therefore well hydrated to facilitate diffusion of nutrients and oxygen to the cells

Question 10

Growth of cartilage is by two methods: List them.

Answer 10

Interstitial growth: pre-existing chondrocytes divide within lacunae and as each daughter cell secretes matrix, there is increase in volume of the matrix (occurs especially in epiphyseal growth plate and articular cartilage)

Appositional growth: from differentiation of perichondrial chondroblasts to chondrocytes which in turn secrete matrix (responsible for the growth in girth of cartilage)

[Histological slide]

Question 11

State the histological features of hyaline cartilage.

Answer 11

The chondrocytes are in lacunae and exist in isogenous groups.

The extracellular matrix is homogenous giving cartilage a glassy appearance.

[Histological slide 1] [Histological slide 2]

Question 12

What are the territories in the extracellular matrix of hyaline cartilage? (They come about because the distribution in extracellular matrix components is not uniform.)

Answer 12

Capsular/pericellular matrix
- ring of more densely staining matrix located immediately around the chondrocyte
- composed of mainly type VI and IX collagens

Territorial matrix
- region a little farther from the chondrocytes, surrounding mainly isogenous groups
- contains predominantly type II collagen fibrils with smaller quantities of type IX

Interterritorial matrix: surrounds the territorial matrix

[Diagram 1] [Diagram 2] [Histological slide]

Question 13

What collagen type is mainly found in hyaline cartilage?

Answer 13

Type II, provides resistance to intermittent pressure

Question 14

What GAGs are found in hyaline cartilage?

Answer 14

Hyaluronic acid
Chondroitin 4 and 6 sulphate
Keratan sulphate

Question 15

The proteoglycan found in hyaline cartilage is ________.

Answer 15

aggrecan

Question 16

What are the functions of hyaline cartilage?

Answer 16

1. At articular surfaces: to transfer forces between articulating bones, to distribute forces in joints and to provide a nearly frictionless surface for joint movement
2. At the epiphyseal plate and foetal skeleton it faciliates growth.
3. It forms the respiratory cartilage, enabling support with minimal load.

Question 17

What is the function of perichondrium of respiratory cartilage?

Answer 17

provides adequate nutrition and stem cell pool which is needed to survive a life time

Question 18

State the zones of the epiphyseal plate. (5)
(NB: the epiphyseal plate separates the cartilaginous epiphysis from the bony diaphysis)

Answer 18

1. Resting zone: has stem cells
2. Zone of proliferation: chondrocytes multiply and line up in rows of small and flattened lacunae; causes longitudinal growth of a long bone
3. Zone of hypertrophy: cessation of mitosis, shift from aerobic to anaerobic metabolism, enlargement of chondrocytes and thinning of lacuna walls and finally the cells die by apoptosis.
4. Zone of calcified cartilage: characterized by mineralization of the matrix and chondrocyte apoptosis
5. Zone of ossification: characterized by osteoblastic bone deposition within empty lacuna spaces left behind by dead chondrocytes, forming cancellous bone of the metaphysis.
6. [Histological slide 1] [Histological slide 2]

Question 19

With respect to the epiphyseal growth plate:
(a) ________ is associated with failure of calcification and accumulation of cells in the zone of hypertrophy.
(b) ________ fractures occur in the zone of hypertrophy because it is the weakest.
(c) Why is the zone of hypertrophy the weakest zone?

Answer 19

(a) Rickets
(b) Salter Harris
(c) Because it lacks both collagen and calcified tissue.

Question 20

Which hormone stimulates closure of the epiphyseal plate?

Answer 20

Oestrogens

Question 21

A. Since the articular cartilage lacks perichondrium, how does it get its nourishment?
B. The articular cartilage has poor regeneration capacity when injured hence when injured, it is replaced by _______________.

Answer 21

A. Synovial fluid and subchondral bone
B. fibrocartilage (by fibrosis)

Question 22

What forms of regression do hyaline cartilage undergo?

Answer 22

Ossification: commonly growth plate and margins of articular cartilage
Calcification: costal cartilage
Fibrosis: damaged articular cartilage

Question 23

Location of fibrocartilage?

Answer 23

1. Knee menisci
2. Intervertebral disc (annulus fibrosus)
3. Fibrocartilaginous disk of pubic symphysis
4. Sternoclavicular and temporomandibular joints
5. Acetabular and glenoid labra
6. Lining of tendon grooves

Question 24

State the function of fibrocartilage.

Answer 24

absorbs shock by resisting compression and shear forces
[Histological slide 1: comparing the various types of cartilage] [Histological slide 2: Fibrocartilage]

Question 25

State three histological features of fibrocartilage.

Answer 25

• some cells are in lacunae (chondrocytes-round nuclei) and some are not (fibrocytes-flat nuclei)
• fibrous appearing extracellular matrix due to collagen I fibres
• no perichondrium
• [Histological slide 1]

Question 26

Under fibrocartilage, state the:
1. Cell types
2. Collagen type of fibres
3. GAGs
4. PGs

Answer 26

Cell types: chondrocytes and fibroblasts
Collagen type of fibres:
Mainly Collagen I also Collagen II and III
Collagen II imparts resistance to compression
GAGs: same as hyaline
PGs: Versican (produced by fibroblasts) and some aggrecan (produced by chondrocytes)

Question 27

Functional adaptations of fibrocartilage.

Answer 27

The knee menisci have a superficial layer of hyaline cartilage and a deep layer of fibrocartilage. The fibres are arranged circumferentially and radially to absorb shock from all directions.

IV disc annulus have more collagen I fibres on the periphery of the annulus and increasing collagen II in annulus towards the nucleus (equal in younger people, becomes disproportionate in older ones). The fibres are arranged in orthogonal arrays (like how feathers are arranged)

Question 28

Osteogenesis imperfecta is a genetic disorder of connective tissues caused by an abnormality in the synthesis of processing of _______.

Answer 28

type I collagen

Question 29

Name the stain used to identify elastic fibers in connective tissue.

Answer 29

Weigert’s elastic stain

Question 30

Where is elastic cartilage found?

Answer 30

1. External ear
2. Eustachian tube
3. Epiglottis

Question 31

What is osteopetrosis?

Answer 31

This refers to an inherited disorder marked by increased bone density due to a defect in bone resorption by osteoclasts.

Question 32

The periosteum has two layers, the outer fibrous and the inner cellular. The outer layer is rich in ____(1)____. The inner cellular region is rich in ____(2)____ cells which transform into ____(3)____, and is responsible for increase in ____(4)____ of the bone.

Answer 32

1. collagen type I
2. osteoprogenitor cells
3. osteoblasts
4. girth

Question 33

Describe the light microscopic features of compact bone (6 marks)

Answer 33

Bone matrix (4 marks)
 Highly mineralized and is laid down in layers (lamellae) namely
 Harversian system – with a central Harversian canal and concentric lamellae
 Interstitial system between Harversian systems
 Inner and outer circumferential lamellae adjacent to endosteum and periosteum
respectively
Bone cells (2 marks)
 Osteoblasts, osteocytes (in lacunae, with processes), osteoprogenitor and osteoclasts

Question 34

Compare endochondral and intramembranous ossification.

Answer 34

~ Endochondral ossification proceeds through the formation of intermediate cartilage, generally hyaline cartilage (the cartilage only serves as a template); in intramembranous ossification, the bone forms directly from the mesenchyme.
~ Endochondral ossification is involved in the formation of long bones as well as the bones at the base of the skull; Intramembranous ossification is involved in the formation of flat bones i.e. the flat bones of the face, most of the cranial bones, and the clavicles.