Lecture 2

What are the only cells present in cartilage?

Chondrocytes

Where do chondrocytes sit in cartilage?

In the lacunae of the extracellular matrix.

What do chondrocytes secrete?

They secrete the cartilage extracellular matrix

From what type of cells are chondrocytes derived?

Mesenchymal stem cells.

What is the term used to refer to cartilage cells with high proliferative behavior?

Chondroblast

What types of collagen can be found in the cartilage matrix?

Type I or type II collagen, or elastin fibres.

What are the main components of the amorphous extracellular material in cartilage?

Proteoglycans and hyaluronan.

What is the structure of aggrecans (proteoglycans)?

Large molecules consisting of a core protein and numerous sulphated glycosaminoglycan (GAG) side chains.

What is the result of there being multiple negative charges on GAGs in relation to the characteristics of aggrecan aggregates?

Aggrecans aggregate are extremely hydrophilic, absorbing high quantities of water.

What counteracts the swelling pressure in cartilage caused by aggrecan aggregates?

Collagen fibres that resist stretching (tension).

Where does cartilage get its nutrients and oxygen?

It relies on diffusion from nearby capillaries due to its avascular, alymphatic and aneural properties

What is the component and characteristic of hyaline cartilage?

It has a glassy appearance and consists of type II collagen.

What is unique about the cartilage structure of articular cartilage?

It doesn’t have perichondrium; instead, it has synovial fluid.

Give 3 examples of where hyaline cartilage can be found

• respiratory tract (nose, larynx, trachea, bronchi)

• ventral ends of ribs

• articular cartilage (smooth, resilient, frictionless surface)

• embryonic cartilaginous anlagen of bones

• growth plates of growing long bones

Where is elastic cartilage found?

In the external ear and epiglottis.

What are the components of elastic cartilage?

Consists of type II collagen and elastic fibres

What type of collagen is present in fibrocartilage?

Type I collagen fibres.

Where would you find fibrocartilage

• intervertebral discs

• menisci

• points of attachment of tendons and ligaments to bone

• cardiac skeleton

What makes fibrocartilage different to elastic and hyaline cartilage

Fibrocartilage has no perichondrium and has a low ratio of GAGs/collagen

What is the difference between appositional and interstitial growth in cartilage?

Appositional growth expands from existing surfaces causing them to grow wider; interstitial growth occurs within cartilage causing it grow in length

What happens to chondrocytes during endochondral ossification?

Chondrocytes hypertrophy, become closer, and their matrix becomes thinner.

What is the role of VEGF in endochondral ossification?

It stimulates angiogenesis.

What is defined as the layer of vascular tissue surrounding cartilage?

Perichondrium

What is formed between the epiphysis and metaphysis during the formation of secondary ossifications?

A cartilaginous growth plate (physis).

What stimulates chondroblast proliferation?

Growth hormones.

What stimulates chondrocyte hypertrophy?

Thyroid hormones.

What is the role of BMPs in ossification

BMPs stimulate osteoblast differentiation from bone marrow stromal cells

What is the role of RANKL in ossification

Receptor activator of NFκB ligand (RANKL) stimulates osteoclast differentiation

What is the brief endochrondral ossification pathway ?

1. Cartilage model is formed

2. Angiogenesis

3. Perichondrium turns into periosteum

4. The periosteal collar forms

5. Chondrocytes hypertrophy

6. Chondrocytes replaced by osteoblasts

7. Osteoclasts arrive and resorb cartilage matrix

8. Osteoblasts secrete osteiod

9. Formation of the primary centre of ossification

What forms as a result of secondary centres of ossification

The ossification of epiphyses occurs along with a physis