Cartilage & Bone在线视频

Hi everyone,

welcome to the world of clinical histology.

In the last session,

we mentioned that cartilage

and bone are specific connective tissues.

Like others,

they consist of cells

and extracellular matrix.

The matrix is solid,

whose firm consistency allows the tissue

to bear mechanical stress.

Today we will discuss cartilage and bone,

mainly cartilage.

In this session,

you are asked to list 3 types of cartilages

and their respective contributions.

Then describe the components

of cartilage matrix.

You are also expected to

list the characteristics

of the cartilage structure,

then analyze the most likely pathogenesis

of osteoarthritis.

Osteoarthritis (OA) is mostly found

in people older than 65 years

and is associated with trauma,

history of repetitive joint use,

and obesity

Joints that are weight-bearing

or heavily used

are most prone to cartilage degeneration.

Fragments are released by wear-and-tear

of the articular cartilage.

This process triggers the secretion

of matrix metalloproteinases

and other factors from macrophages

in adjacent tissues,

which exacerbate damage and cause pain

and inflammation within the joint.

OA of the knee primarily

affects the cartilage,

but ends up damaging the bone surface,

synovium, meniscus,

and ligaments.

It involves the gradual loss

or physical property changes

of the hyaline cartilage

that lines the articular ends

of the knee joint bones.

Are you curious about why OA

can affect the bone surface and joints?

To answer this question,

you need to know the structures

of cartilage.

Cartilage is a specialized connective tissue

that provides

for both tenacity and flexibility.

It is mainly found

in the form of hyaline cartilage

which is so named because of its smooth,

glassy bluish-white appearance when fresh.

Cartilage forms the precursors

for the skeletal structures in the fetus,

which is replaced by bone in the new born.

The exception is the articular surfaces

of bones

involved in joints

and the ventral ends of the ribs.

Hyaline cartilage also makes up the cartilage

in the nose,

bronchi, larynx and trachea.

The cartilage cells are called chondrocytes

and are found in spaces (called lacunae) surrounded

by the extracellular cartilage matrix.

Chondrocytes differentiate

from the fibroblasts

and surround the cartilage area.

So first,

do you know which part of our body

is composed of cartilage?

The surface of most cartilage

is covered by a layer of special

connective tissue,

the fibrous perichondrium.

The exception is the articular cartilage

which lacks perichondrium.

Nutrients for the articular cartilage come

from the synovial fluid.

Deep to this layer is cartilage tissue.

The main cell type is chondrocyte.

And in the outer-most layer of the cartilage,

just below the perichondrium,

there are active matrix-producing chondroblasts.

There are some special structures

in the cartilage.

As chondroblasts

and chondrocytes produce matrix,

the cell becomes surrounded by matrix

and then locates in a small "room"

or lacuna.

The lacuna is only visible

when the cell shrinks

during tissue preparation

and is pulled away from the edge

of the matrix wall.

Each lacuna is surrounded

by the more darkly stained cartilage capsule.

As we go deeper,

we can see a few lacunae gathered to

form a "cell nest".

These chondrocytes are very mature

and are less metabolically active.

So now you realize that the nutrients

of the cartilage are only supplied

by the surrounding connective tissue，

the perichondrium.

Cartilage can grow from the outside

and inside.

Repair or replacement

of injured cartilage is very slow

and ineffective,

due in part to the tissue's avascularity

and low metabolic rate.

Hyaline cartilage contains chondrocytes,

embedded in a unique matrix

that gives the tissue both tenacity

and flexibility.

Look at the chondrocytes housed in lacunae.

Note that some lacunae contain

more than one chondrocyte.

These are daughter cells

formed after division.

The cartilage in the ear contains

many elastic fibers

and is therefore called "elastic cartilage".

On slide preparation,

elastic cartilage can be distinguished

by the stain for elastin that brings

out the dense bundles.

The slide shows fibrocartilage

from an intervertebral disc.

It is distinguished by very scattered,

infrequent chondrocytes

and collagen fibers running

in the matrix.

Within an intervertebral disc,

collagen loss or other degenerative changes

in the annulus fibrosus

are often accompanied by displacement

of the nucleus pulposus,

a condition called a slipped

or herniated disc.

This occurs most frequently

on the posterior region

of the intervertebral disc

with fewer collagen bundles.

The affected disc frequently dislocates

or shifts slightly

from its normal position.

If it moves toward nerve plexuses,

it can compress the nerves

and result in severe pain,

and other neurologic disturbances.

The pain accompanying a slipped disc

may be felt in areas innervated

by the compressed nerve fibers,

usually the lower lumbar region.

In the later stage of knee OA,

patients often suffer from severe pain

due to the damage

of articular ends of bones

in the knee joints.

The articular ends may merge

and form a bone-like structure.

Compared with the original articular tissue,

bone is a type of connective tissue

with a calcified extracellular matrix.

It is specialized to support the body,

protect many internal organs,

and act as the body's calcium reservoir.

High concentrations of calcium

and phosphate ions cause formation

of hydroxyapatite crystals,

whose growth gradually calcifies

the entire matrix.

More information is available

in other learning materials.

So in this session,

we have mainly introduced three type

forms of cartilage:

hyaline cartilage, elastic cartilage,

and fibrocartilage.

We have also talked about the structure

of hyaline cartilage.

Hope you can understand the features

of hyaline cartilage

by relating to the clinical case of OA.

See you next time.